map and compass part 2

Transcription

,1
~
Compasses-their choice and use.
Page 1 of 5
You may have seen in school science experiments the old trick of floating a magnetised needle on a
bowl of water -slowly it swings around, supported by surface tension, until it points North/South. This
is the simplest form of compass, aligning itself with the Earth's magnetic field. The next step up, still
fairly primitive, is the type where the needle is suspendedon a pivot, which allows it to swing freely,
and is usually housed in a circular case. This is often marked only with the Cardinal Points -North,
South, East and West. This kind of compass will let you determine directions only very roughly -for
more precise navigation a more sophisticated device is needed.
The first thing your compass should enable you to do is to sight a landmark and take its bearing. A
bearing is defined as 'the angle between a line heading North from your position and a line heading
towards the landmark.' (The North line can be defined as either 'True', 'Grid' or 'Magnetic' North -more
of this later!) There are 360 degrees in a complete circle, with zero degreesbeing conventionally placed
at the 120' clock position (North). When counting degreesyou always start at this zero point and count
clockwise around the dial. Therefore North = 0 degrees, East = 90 degrees, South = 180 degrees and
West = 270 degrees. To get a visual idea of this c;lndwhat a bearing signifies, see the three drawings
below.
N
N
I
A
~ ...'.
,
Your
position
Your
position
Your
Posi1on
The second thing that your compass should enable you to do is to measure angles on a map. Again, the
angle that you require will be the one between a North-running line and a line which runs through your
position and a mapped landmark. This angle is usually known as a course, since it will normally be used
to pick the course that you will travel. In order to measurethis angle, what's really neededis a protractor
-a device for measuring angles. Fortunately, compass makers have handily combined the compass and
protractor into the (surprise surprise!) protractor compass,more commonly called a baseplatecompass.
This type of compass is fundamental to most navigation concerning map and compass across country. It
generally has a clear rectangular baseplate, with the compass housing -almost always circular -at one
end. This housing, known as the capsule, can be rotated in relation to the baseplate.The outer rim of the
capsule is marked with North, South, East and West and also with degrees -which increase as you move
around the capsule clockwise. The capsule is usually marked with a series of parallel lines, called
North/South lines, which lie parallel to a line running through the North and South points on the capsule.
http://www.countrywalkers.co.
uk/compass.html
7/11/2003
Page2 of 5
Most baseplate compassesalso have an arrow, usually called the direction of travel arrow, marked on
the base. It's called this because it shows the way you want to go when the capsule is set to a course
you've taken off the map. If you want to measurea bearing in the field, this is the arrow you point at the
landmark you've chosen. In line with this auow is the index mark, scribed next to the capsule. This line
makes it easier to read off the number of degrees to which the capsule is set -the capsule is turned to
indicate the couect number of degreesrequired. This all sounds horribly complicated. Take a look at the
sketch below to see the main components of the baseplate compass -things will become a bit clearer,hopefully!
When the compass is held level, with the needle free to
rotate, said needle will come to rest with the North end
(usually red) pointing to Magnetic North. A lot of map and
compass operations include rotating the capsule until the
North end of the needle aligns with the North mark on the
capsule. To make this lining-up easier -and more accurate
-the capsule has an orienting arrow inscribed, again Index
usually in red, on its bottom surface. Another name for this Mark
arrow is the gate. For the sake of my typing-numbed
fingers, from now on I'll say 'put the needle in the gate'capsule
instead of 'align the North end of the compass needle with
the orienting arrow, making sure that red lines with red.' Luminous
OK? Right.
blObS
When used together, the three working parts -baseplate,
capsule and needle -enable you to take the bearing of a
landmark when out in the field and measure courses on a
map -more of that later, though. For now, a few more
points regarding compasses.
Diredion 01
travel arrow
Orienting
arrow
Needle
(red~N)
NorthI&Juth
Lines
Base
plate
3600
Dial
A refinement of the baseplatecompasshas a folding mirror attached to the baseplate.This allows you to
sight both the landmark you're using to take a bearing and the capsule at the same time. This enables you
to take a more accurate reading as well as startling yourself with the reflection of the wind-blown
apparition staring back at you! Although they are initially a little tricky to use, if you need to take really
accuratebearings, this is the kind of compass for you.
Typical
indications
of variation
on a map.
Now for the three kinds of North. Bet you thought there was only one
North, eh? Wrong. There are three, namely True, Magnetic and Grid
Grid
North.
North. Wait! Don't run away in horror! All will be explained. You see,
compass needles hardly ever point to True North -they point to Magnetic
North, the difference being known in the UK as the variation. In Britain
this
can be
and five degrees,
though which
it is only
absolutely
accurate
for between
the year two
of publication
on OS maps,
usually
show a
,
j
~agne~c
I'
orth \
True
North
diagrammatic sketch of the three Norths in the margin and also give the
increase (or decrease) in variation over the next few years so a calculation
can readily be made of the variation as of the present moment. (Have a
look at the sketch to see what I mean.) It follows that map and compass
work will involve some simple addition or subtraction when using a
'AtOlecentreofthis sheetTrueNorthstandard baseplate compass. Why? Well, when you take a bearing from a
is O()35'eastof Grx.North.Magneticlandmark, you have measured an angle between Magnetic North, using the
t.1~"'"I- ~n.I_~."'" ~. '="~.~~n \Aj~...
http://www .countrywalkers.co.uk/compass.html
7/11/2003
Page3 of 5
compass needle, and that landmark. A course taken from a map, on the
other hand, measures the angle between Grid North and your landmark -hence the need for the
arithmetic. More of that later!
If you can't be bothered with all that, you can buy a 'set-and-forget' compass.These have a gate that can
be moved in relation to the North mark on the capsule. You simply set the gate at an angle to Grid North
which is equal to the variation and that's it until you travel to a region where the variation, er, varies
from where you now are. It's much simpler than it sounds! When you have set this type of compass,the
needle, when in the gate, points to Magnetic North whilst the North mark on the capsule indicates Grid
North. No sums needed! This may sound gimmicky, but believe me, get out across open country on a
cold day and the mind can become a bit sluggish without you realising it. It may not be life or death but
you could face a much longer walk home than you'd intended if you forget to correct for variation
!
Do ensure that, whatever model compass you buy, it is fluid-damped. This means that the capsule is
filled with fluid which damps the needle'smovement, without which the needle spins and wobbles like a
nervous breakdancer. Decent compasseswill operate through a range of temperatures which most of you
will be out in -if you are out in weather below -40 degrees or above +40 degrees centigrade, which is
the range I mention, you really don't need my help -you need professional help, and quickly! (Or your
name is Fiennes). Do avoid buying a compass if you see an air bubble present in the capsule -it
probably has a leak. Extremely high temperatures-the car's parcel shelf is a classic -can causethe fluid
to expand to a degree where it bursts the capsule. Take heed from one who knows
Also a good idea is to buy a compass in the region of the World you intend to use it. Why? Well, North
of the Equator needles dip to the North, south of it they dip South. The manufacturers compensate for
this by weighting the needle so that it can move freely when held flat. So if your great-aunt from Sydney
sends you a compass to use here, chances are that it will bind and not work properly. You can always
move Down Under, but that does seema bit drastic!
Compassesare affected by other magnetic fields than the Earth's. Don't lean on a metal gate when using
one, or stand under high-voltage power lines. Even small items -penknives, buckles etc. -can throw the
needle out, so keep them well out of the way when using it.
Lastly, once you have learned the art of reading a compass,taking bearings and plotting courses and are
confident of the absenceof magnetic interference -trust it. It may only appear a simple tool -and so it is
-but it is also a highly accurate precision instrument that you can -and some day you may have to -trust
with your life.
I mentioned earlier about 'taking a bearing' from a landmark. So what use is it? Well, a bearing enables
you to do a very important thing: walk in a straight line! This may sound funny but, if you're out in open
country, such as moorland, it's very easyto -yes! -lose your bearings. Go on long enough and you
might lose your marbles as well! Seriously, this is where that well-known phrase originates. It is a fact
that trying to walk a straight line over any distance outdoors is well-nigh impossible without the aid of a
compass.It's no good saying 'well it's midday, the sun's in the south so 111take my bearing from that.'
As the day goes on, the sun's apparentmovement will fool you but good. You '11end up on a remote
http://www.countrywalkers.co.uk/compass.html
7/11/2003
.
Page 4 of 5
heath and be devoured by bears. (OK so I'm given to exaggeration here.) Most people do tend to wander
in circles when unguided, though. Whether this is due to one leg being a bit stronger than the other, the
terrain, or the phasesof the moon doesn't matter -that's what happens.So by sighting a landmark, taking
a bearing and walking the bearing, you will avoid all this grief and frustration. Here's how to do it.
Suppose you possessa standardbaseplatecompass.You're out on a flat, open heath and have decided
from studying the map that you need to head West to get where you want to be. The map shows a hill
with a lone tree on top over to the West. (This is a really good map, by the way!). First, find which way
North lies. Hold the compass flat with the orienting arrow turned so that it is in line with the direction of
travel arrow. then turn yourself around slowly until the North (red) end of the needle is in the gate.
Presto! You are now facing magnetic North! You will notice that West lies to your left. Have a lookcan you see the tree? Yes! Now to take the bearing.Turn to face the tree, this time ensuring that the
direction of travel arrow (the big broad one etched into the top of the baseplate)is pointing at your
landmark (the tree). The North end of the needle is still pointing North, but becauseyou've turned is no
longer in the gate. Turn the capsule until the needle rests in the gate, whilst still pointing the direction of
travel arrow at the landmark. You have now taken your bearing, which, if the tree is exactly to the West,
is 270 degreesMagnetic. Remember, degreesare read clockwise from North, so West is 270, OK? The
'Magnetic' bit merely means that you have oriented your compass initially to Magnetic North, not Grid
or True North. As you are only taking a reading from the compass, without relating to any bearings on a
map, this causesno problem. To walk the bearing, set off West and keep walking the way the dirction of
travel arrow is pointing when the needle is in the gate -you'll have to check more often than you think!
'Why?' I hear you say 'I can seethe landmark anyway!' OK -suppose a mist rolls in, or it gets too dark to
see very far, or the land dips or rises, or the Forestry Comrnision race up and chop the tree down. Then
where are you? LOST! However, as long as you follow the bearing, you will reach the hill, pass the tree
and gain the pub on the other side. Easy, isn't it? Well, maybe not quite that easy -try it on your local
park a few times before venturing out on the Scottish plateaux in a Winter blizzard
The sketches
below will give you a better idea. Sorry they're only basic but that compassabove was murder to
construct in Serif Draw Plus!
You will soon discover that in the real world you will need to take regular bearings in order to stay in a
straight line -the land usually rises and dips too much for you to sight your final destination. The
principle is the same, though -take sightings on landmarks you can see are on your way of travel, and
set and walk the bearing. Here is where knowing how to read and interpret a contour map comes in -you
can identify landmarks along the way from the map and then take your sightings from them.
7/11/2003
Compasses-their choiceanduse.
Page5 of 5
Walking in a straight line is fine -but what if there is something in the way? If you can see across it,
simply walk around and then follow your original bearing. If you can't, a different approachis needed. It
could be a lake, quarry or just an impenetrable thicket. Well, first thing to decide is which way you wish
to go to skirt the obstacle. Let's say it's a lake, with a horrid quaking bog to the left. Don't know about
you but I have a deep and abiding horror of quaking bogs -comes from walking on Dartmoor where
they lie in wait for the unwary! So we'll go right, over the pleasant flowery meadow. Make a 90 degree
turn right (yes you can use your compass for this!) and start walking as straight a line as you can until
you have gone far enough to pass the obstacle. When you do this, count the number of double paces you
take (every time your right foot hits the ground is one double pace). When you've gone far enough to
clear the lake, turn 90 degrees to the left and walk a line until you have again cleared the obstacle (on
this leg there is no need to count paces). Now turn 90 degrees left again and walk the same number of
double paces as you did on the outward leg. If you now turn right, you should see your landmark again
in front of you. Off you go! The sketch below will hopefully make this a bit clearer.
Readingandunderstandinga contourma~.
Grid references -how the~ work and how to read them.
How to use the ma~ and com~asstogether.
7/11/2003
teading
1,
andunderstandinga contourmap.
Page1 of 3
TheBas~
It was proved beyond reasonable doubt some time ago that the World is round. Strangely, road maps
still show the World as if it were completely flat -few, if any, indications are given as to the rise and fall
of the ground which we are all aware of in the natural course of events. This is becauseroad maps are
what are termed Planimetric Maps', showing the ground as if the onlooker were directly overhead.
These maps are fine for driving, as all you need to know is which road to follow -gradients, valleys and
other geographical features being largely irrelevant. On foot it is quite a different story. You may be
crossing ground which undulates gently, only to be suddenly confronted with a vertical ascent which is
beyond your ability to surmount -and it could run for miles in either direction! It follows that some
method of knowing what sort of terrain lies in front of you is is beneficial -and in some casesessential.
Contour maps show the ups and downs of the terrain by means of contour lines. A contour line
representsthe same height (or altitude) above sea level throughout its length. If one were drawn around
a lake, it would exactly match the shoreline -water finds its own level, therefore the shore is always at
the same altitude.One of the easiestways to understand contour lines is to imagine a perfectly conical
hill -say like a child's drawing of a volcano. Seen from the side it looks like a perfect triangle, yet seen
from the top it's a perfect circle. (See Fig.l, complete with cute little arrows.). Each of the lines running
.4.
around it are the contour lines and are at the same height above sea
-i,4\
FIG.1
~evel.The height change from one line t? the other is calle.dthe vert~c~l
SlOE
VIEW
,1.--1-1-\
interval or more commonly the contour interval. On any gIven map It IS
-i
~
TOP VlE'N
'"/
f t/:~."
I"
...-
, ( .r
I.
,I , I\'-./
11/
.
,
'\
i-UP
of equal value, usually 10 metres on the 1:50,000 and 1:25,000 scale
maps used by most walkers. That means that the gain (or loss) in height
between any contour line on these maps is 10 metres. It follows from
this that if lines are widely spaced,the gradient (how steep the hill is!)
will be gentle -close together and it will be steep. Very closely gathered
contour lines indicate that a cliff is present and are probably best
avoided! Lines that are multiples of 50 metres above sea level (called
index contours) are slightly heavier and usually numbered (150, 200,
250 etc.), which helps you to figure out which way the gradient is going
or down.
\\ \;~~:~J;;I
This is really the basic and first principle of understanding contour maps
"" ,.._~-- .../ ","
-the fact that concentric, unbroken contour lines form complete paths,
"" ===:::-/
more often than not irregular in shape, that describe the shape of hills
and mountains at the height at which the contour is mapped. Given that most mountains are anything
but regular, the contour lines can be of the most tortuous shapesimaginable -yet a little practise will
enable you to begin to 'see' in your mind the physical shapedescribed by this ingenious system.
Once you have grasped the above principle, you can
move on to the next one -how contour lines relate to
valleys. Simply put, contour lines in valleys form 'Y'
shapes (typically narrowing and steep valleys), which
can spread to wide U' shapes (typically broad, shallow
valleys) -but the effect is the same -the closed end
points to higher ground (seeFig. 2).
uk/Contourmap.html
7/11/2003
.
,Reading
andunderstandinga contourmap.
Page2 of 3
Ridge contours can be confused with valley contours as
they, too, form U' shapes-the difference is that the closed
end of the U' points to lower ground. (See Fig.3)
Streams, indicated by blue lines, are a sure sign of a valley -though not all valleys have them! By the
way, it can be a good idea to follow a stream down a broad, open valley if you're not sure where you are
and wish to reach lower ground but even this can get you mired in boggy sections. Never, ever try this
on steep slopes, or those heavily wooded. You can quickly find yourself trying to negotiate very steep,
wet ground -water will always find the quickest way downhill!
To be really certain if a group of contour lines indicate a ridge or a valley, you need to look for the
heights marked along the contour lines in order to determine in which direction the land slopes. Index
contours (mentioned above) have the heights above
Index
mean (average) sea level printed on them, every fifth
line being an index contour. If the two nearest index
contours are located nearest any given spot, then traced
along until you come to the height (altitude), it will
0
readily be seen which direction the land slopes at that
point and the 'V' or 'U' can be identified as a ridge or
valley (See Fig.4).
(j
Very often the closed ends of two U's' can be seen
pointing toward each other. This is a sure indication of a
pass between two areas of higher ground and, when
thought about, makes perfect sense. If you started on
either section of higher ground and walked downhill in
the direction that the closed end of the U's' were
pointing, you would be bound to end up at the pass. (See
Fig. 5)
It's interesting to note that some features cannot be accurately representedon a contour map. The classic
is the Inaccessible Pinnacle (The InPin) in the Skye Cuillins range -being wider at the top than at the
bottom, map-makers are stumped when it comes to indicating a contour! Similarly, severely
overhanging cliffs are a cartographer'snightmare -but we'll avoid those anyway, eh?
2. Scale -what is it gnd which oneis_~~
uk/Contounnap.html
7/11/2003
Readingandunderstandinga contourmap.
Page3 of 3
In the UK, the Ordnance Survey is the primary source of maps, though others are available. One piece of
terminology that needs to be understood from the start is small scale versus large scale. On a smallscale map, landscape features are shown relatively small, the map itself covering a much larger
geographical area than a large-scale map where features are, of course, shown much larger. For example
a I :250,000 scale map such as the OS Routemaster series, shows a tract of land 1 kilometre long
covering a mere 4 mm. on the page. On an OS Pathfinder series 1:25,000 scale map, the same tract of
land covers 4 cm. (For those of you who, like myself, grew up on miles and yards that means 2Y2inches
= 1 Mile.) It can be seen from this that the small-scale map isn't going to be of much use to walkers -it
simply doesn't show enough detail. An easy way to remember what scale a map represents is -the
bigger the number, the smaller the scale.
The OS maps most useful to walkers are the 1:50,000 scale Landranger and 1:25,000 Pathfinder (now
being replaced with the Outdoor Leisure series). I find the Pathfinder most useful for the kind of walking
I tend to do -i.e. no great distance! -but, if you plan to do lengthy linear walks, the Landranger, with its
greater geographical coverage, may prove more useful, though at the expense of slightly less detail. It
should be noted that on these maps all contour lines are in red and there is also a wealth of other
information present in the form of map symbols. These symbols represent many features found on the
ground such as camp sites, public telephones, parking sites, mountain rescue posts(!) and, of course,
footpaths. The explanation ('legend') for these symbols can be found on every OS map, usually on the
back cover and these help to bring the map to life. With a little practise you will be able to identify
features that will aid your navigation -lines of electricity pylons and church spires always being a good
bet -and the whole process of getting from A to B will become much easier and enjoyable. One thing
must be remembered about maps, though -they are only as accurate as when they were drawn or
revised. Although major features -mountains, lakes and the like -don't tend to move around too much,
things like hedges, minor roads and field boundaries can and do. Small trees become thick woods, ponds
silt up -I think you get the picture.
Reading and understanding a contour map is the first and perhaps most essential part of the skill of
navigation across country. The ability to interpret what is on the map and translate it into physical
features around you -or vice versa -could be all you need to get around an afternoon's walk in the
Cotswolds -but to identify where things are with precision you need to be able to understand grid
references, which is what I'll go into next.
Grid references -how the~ work and how to read them.
How to choosea com~ass
Using maQand comQasstogether.
http://www .countrywalkers.co.uk/Contourrnap.html
7/11/2003
Using mapandcompasstogether
Page1 of 7
This is where the preceding sections come together and will tell you how to orient a map as well as how
to correct .for variation. At present, this is as far as I'm going to go on the subject of navigation, as to
continue will take us into realms usually only needed if serious wilderness hiking is to be undertaken. If
that is what you intend to engage in at some time in the future, I suggest that you firstly practice with
map and compass until you are absolutely confident of your understanding of the techniques involved
and, secondly, you try to undertake some 'hands-on' training with an outdoor centre or better still
Outward Bound. The sort of countryside my walks describe on this site is gentle, undulating stuff in the
main and map and compass work are fairly simple -indeed, in most casesa Pathfinder map is all you11
need. Please, though, be under no illusions that taking a swift hike in the Cotswolds prepares you to
jaunt off on a three-day trek in the Scottish highlands. Get lost there and you may stay there for good!
So follow the rules -and never underestimatethe terrain, or overestimate your skill and ability, and you
won't go far wrong.
The best place for your first map check is the start of your walk. This has the obvious advantage of
telling you that at least you're starting in the right place! First, find your location on the map. Usually
this is easy: you are at the marked point where the path leaves the road or wherever your chosen path
starts. Next, orient the map. In other words, place the map on some flat surface so that directions on the
map correspond to directions in the field. It is always easiestto visualize what you aredoing if the map is
oriented. Sometimes you can orient it by eye -if there is a lake straight ahead of you, and a prominent
hill to your right, turn the map until the lake lies straight ahead of your position on the map and the hill
is to the right of that position. In similar fashion, all other directions on the map will also correspond to
reality. Sometimes, you can't orient by eye -if the landscape lacks any obvious mapped features, you
need to use your compass to orient the map.Here, and for the rest of this section, we are going to assume
that magnetic North and grid North lie in exactly the samedirection, which should enable you to grasp it
a bit more easily. In the next section we'll find out how to compensate for the difference that (usually)
exists between grid and magnetic North. On almost all maps, North is at the top of the sheetso, to orient
the map roughly, hold the compass horizontal and glance at the needle which points North, of course.
Now turn the map so the top also points North. Note that, when the map is oriented correctly, the left
and right margins represent lines running north and south.To orient the map more accurately, set the
capsule to zero degrees.Place the compass on the map so one long edge of the baseplatelies along either
the left or right margin. Now rotate map and compass together as a unit until you have placed the needle
in the gate, so the North end of the needle points to the North mark on the capsule. The map is now
oriented and directions on the map corresponds to directions in reality! Have a look at ~
which
should make this a bit clearer.
Now that you have found your position and oriented the map, take a look around and identify some
nearby landmarks that are marked on the map. Determine your general direction-of -travel. Are you
heading North? West? Try to develop a feel for the relationship between the cardinal points and major
terrain features. You might note, for example, that the valley you will be walking up runs eastand west,
while the tallest hill is basically to the north. Knowing where you started and in what direction you are
travelling will help prevent silly mistakes such as placing the south end of the compass needle in the
north end of the gate which erm, some of us have been known to do! While you have the map out, try to
uk/mapandcompass.html
7/11/2003
Page2 of 7
create a mental image of the terrain you will be travelling through. This is where you'11get the benefit of
having learned to read a contour map -it will make creating this mental image a far easiertask.
Make a point of getting out your map and compass every hour or so to locate your position on the map. I
know that on a short country ramble this is probably not necessarybut it is good practice! Note the time
you started and the time it takes you to reach various points along your route. It will give you a senseof
your pace that day, which will help you to keep track of your location and assist you if and when you
come to plan your own routes and walks. Let's say your goal for the day is the summit of some hill or
mountain. You reach it at lunchtime (becauseyou are horribly fit, unlike me who will of course take far
longer!) and sit down to admire the panorama spread out before you. Orienting your map will give you a
rough idea which distant landmark is which. If you want to know exactly which one each is, I'm afraid
you11have to get a bit more technical
First, take a bearing on the peak you are interested in, using the technique described Rreviously. That
bearing, you will recall, is just the angle betweena line heading north and a line leading to the landmark,
the angle being measured clockwise from the North line. Although you used the direction-of -travel
arrow (or notch in the mirror) to sight the object, note that you would have arrived at exactly the same
result if you had sighted along one long edge of the baseplate(this is an important clue, Watson!).
Now you are going to transfer that angle to the map. One line of the angle, represented by the compass
gate, will point North; the other line, represented by one long edge of the baseplate, will run right
through your position and towards the landmark you are looking at. To make the logic of the next step
easier, orient the map before continuing, then reset the compass to the bearing you have just measured.
Now place the compass on the map so that the compass gate points north, towards the top of the map,
and one long edge of the baseplatesits on top of your position. Note that the long edges of the gate (and
the North/South lines in the capsule) run parallel to the right and left margins of the map. Don't twist the
capsule in relation to the baseplateand ignore the compassneedle. You are simply using the compassas
a protractor now, so the needle is irrelevant (even though I have shown it!) The long edge of the
baseplate that is sitting on top of your position now points directly at the landmark you sighted and took
a bearing on. ~below
shows a compass placed correctly on the map. Note -this is important ~
that you must make sure you follow the long edge of the baseplate in the direction indicated by the
direction of travel arrow. With the map oriented correctly, you will see that the long edge of the
baseplate not only points to the landmark shown on the map -it also points to the real one you can see.
Often the edge of the baseplate isn't long enough to reach from your location to the landmark shown on
your map. You need to be able to extend the line of the baseplateand for this a clear plastic ruler is ideal
-one of the six -inch ones usually does nicely. Don't use a metal one! (Try it sometime and seejust how
far it throws the needle out!) Place one edge of the ruler alongside the edge of the baseplate that runs
through your position. The ruler will now extend towards the mapped landmark, making it easy to
identify the correct one.
Your map and compass will also help you to solve the opposite problem -which way to go, that is!.
Once again, let's assume that you know where you are on the map. This time, you want to use the map to
tell you which way to go. Suppose you are at Roadbend and want to know the direction to Topofthehill.
To solve this problem, you need to measurean angle on the map and transfer it to the teuain. The angle,
called a course, will be the angle between a line heading North and a line heading to your destination,
with your position as the point of the angle. Sounds very confusing, but stick with it, all will be
revealed!
Start by laying one of the long edges of the baseplatealong an imaginary line connecting your position
and your destination. Sometimes, as in V,
the baseplate will be long enough to extend between the
7/11/2003
'~
Page3 of 7
two; other times you will need to get out your ruler again to position the baseplate accurately. Be sure
the direction of-travel arrow points at your destination. Now twist the capsule until the gate points north,
as shown in ~.
Both the long edges of the gate, and the North/South lines, will run parallel to the left
and right margins of the map. Once again, you are using the compass as a protractor, so the needle's
position is irrelevant. Forget it whilst you set the capsule! Read the course (the angle) at the index mark,
where the direction-of -travel arrow abuts the compass dial -in the examples shown, it would be about
315 degrees(measured from North clockwise, remember?)
To transfer that angle to the field, pick up your compass and rotate it as a unit, without moving the
capsule in relation to the baseplate, until you have placed the needle in the gate. When you have done
this, you will see that the direction-of -travel arrow now points in the direction you want to go. Pick a
landmark, walk to it, pick another landmark, and so on. This is one of the basic methods of crosscountry navigation.
I
(
'"
"...
""
'"
\
\
~
/
--
/"
r m~'
I
su
,
'-'
\
\.
'" "" ~
)
)
/
v
(
~
/'
,.,--,
J
/
~
~
,\
/1
--/
I
I
Your
position
Your
position
0 \
~0"
~
Q'
You can use a variation of this technique to find your position along a track, ridge, stream -any
http://www .countrywalkers.co.uk/mapandcompass.htrnl
7/11/2003
Page4 of 7
prominent linear feature in fact. First orient your map North/South, then lay one long edge of the
baseplate along a line intersecting your rough position and a prominent landmark. Rotate the capsule
until the gate aligns with the North/South lines on the map and the needle rests in the gate. If one end of
the baseplate rests on the landmark, your position must lie along the line of the long edge of the
baseplate. ~will
make this clear, I hope! If you can identify two landmarks, you don't even need to
be following a terrain feature. Simply take a bearing off one, and pencil in your line of position on the
map. Then take a bearing off the second, and pencil in that line of position. Your location is the
intersection of the two lines. ~
shows how this works and is far less confusing than my
descriptions! If you can take a bearing off a third landmark and pencil in that line of position, so much
the better. Your true position should lie somewhere inside the triangle formed by the three lines of
position (becauseunless you are a truly whizzo navigator, there will always be a slight, er, discrepancy!)
Once you have identified your location with lines of position, perform a 'reality check'. If the lines cross
at a stream, and you are standing on a ridge, something is definitely wrong! A cardinal rule of navigation
and map reading is this -never make the map fit your readings or vice versa! If in doubt, take your
readings again -and again -until you are sure of them -and only then follow them!
How to correct for variation.
Sometimes reading explanations of how to reconcile grid north and magnetic north is, to say the least,
confusing. Fear not! There is a simple, easy to understand way to solve variation problems. You just
need to remember some straightforward logic. You can forget silly rhymes such as 'grid to mag, add;
mag to grid, get rid'. This was once taught by the armed services, but it only works if the variation is
West -a bit unfortunate if you're a soldier on exercise in China
Anyway -British maps are oriented to the National Grid. If you use maps without the crossed lines of
the grid, check in the margin that the map has been printed with its side edges aligned True, or
geographic, North and South, and for 'grid North' below read 'true North'.
True North is the direction to the geographic North Pole -one end of the earth's axis of rotation -but
in this section we shall use grid North. Magnetic North (with reservations explained later) is the
direction in which a compass needle points. Weare calling variation the difference in direction between
grid North and magnetic North. Variation, therefore, is an angle, measured with grid North as the
http://www .countrywalkers.co.uk/mapandcompass.htrnl
7/11/2003
Page5 of 7
starting point. In Britain, magnetic North lies to the West of grid North (to the left, or anti-clockwise, as
we look at a map) and we say that the variation is West. West variation is measured anti-clockwise.
(This is in contrast to bearings and courses, which are always measuredclockwise -an important point
to remember.) Therefore, if magnetic North lies 6 degreesWest of grid North, the variation is 6 degrees
West, not 354 degreesEast.
Ignoring the correction for variation can lead you seriously astray, and raise the awful possibility of
eating apple twigs and beetles for dinner as you bask in the warmth of your torch. For each degree that
your course is in error and each kilometre (0.6 miles) that you travel, you will be off by about 18mtr.
(60ft). If the variation is 6 degrees,you will be off by over 200m (650ft) after walking just 2km (1 mile).
Need I go on? One thing's for certain -if you go on, we may not see you again for quite sometime!
Strictly speaking, it's wrong to say that 'the compass needle points to the magnetic North pole'. What
the needle actually does is align itself with the earth's magnetic field. Compass needles mayor may not
actually point at the magnetic north pole itself, which, in 1981, lay just north of Bathurst Island in
Canada's Northwest Territories. For reasons not well understood, the magnetic poles move slowly, over
periods of many years, through circular paths with a diameter of about 160km (100 miles).
For a rambler or hill-walker, all that doesn't matter. If you are using the most recent OS map available,
the variation it gives will be accurate to within a degree or two. Although variation changes slowly as
you travel East or West, you can assume that the needle always points in the same direction within the
bounds of the area you cover in a typical human-powered trip. For the sake of simplicity, let us just say
the compassneedle points to magnetic North.
Let's assume for the moment that you didn't spend the extra money to buy a set-and-forget compass.
Before you can understand how to correct for variation with a standard baseplate compass, you need to
rmember the Two Facts. First, every angle that you measure on a map is measuredclockwise, with grid
North as the starting point. Forget the needle -you are simply using the compass as a protractor. Second,
every angle that you measure in the field by placing the needle in the gate of the compass is also
measuredclockwise, but the starting point is magnetic North.
We'll refer to angles with grid North as the starting point as grid bearings or grid courses. (They are
essentially the same. A bearing is just a direction to a landmark; a course is a direction you will follow remember?) All angles measured with a compass, using magnetic North as the starting point, will be
magnetic North bearings or magnetic North courses.
Now let's say that you are walking in the Cairngorms where the variation is always West. Take a look at
figure 4.2. Grid north is marked zero degrees.Magnetic North is marked 6 degrees -note that it lies to
the Westof grid North. In other words, the variation is 6 degreesWest. Still with me? OK -now say you
measurea course on the map to a landmark and find it to be 90 degrees.That is a grid North course; you
started measuring at grid North, just as the diagram shows. Now if you take a bearing on the landmark
with your compass, you will find the bearing to be -hey presto! -96 degrees. That is a magnetic North
bearing, since you started measuring at magnetic North.
The grid north angle, measured on the map, is less than the magnetic north angle measured in the field
with the compass by placing the needle in the gate. Furthermore, the difference is 6 degrees -exactly
the amount of the variation. And that leads to our first conclusion: when the variation is West, as it
always is in Britain, grid North angles (bearings and courses)are always going to be less than magnetic
North angles (bearings and courses). If you measure an angle on the map and want to transfer it to your
compass, you must add the variation to the grid North angle becausemagnetic North angles are always
7/11/2003
Page 6 of 7
greater than true North angles when the variation is West. If you measure an angle with your compass
and want to transfer it to the map, you must subtract the variation, becausegrid North angles are always
less than magnetic North angles when the variation is West.
You don't need to memorize these rules (huge sighs of relief all round!). Just remember the logic behind
them. If you need to jog your memory, look at the variation diagram in the margin of your map.
Fig. 6 gives another example of the relationship between magnetic and true North when the declination
is West, this time for a true north angle of 270 degrees.One picture is woth a thousand words, yes?
If you are using a set-aDd-forgetcompass, you can forget about adding and subtracting once you have
set the variation and double-checked that you set it in the right direction. With most such compasses,
setting the variation is idiot-proof. (Depends on the idiot, of course!) Usually you turn a small screw or
perform some other simple operation to adjust the compass gate so that it points to the angle
representing the variation. If the variation is 6 degreeswest, for example in the Cairngorms, the compass
gate (orienting arrow) would point to 354 degrees(360 -6 = 354). It's that easy.
When you are measuring an angle on the map with a set-and-forget compass, you ignore the needle (as
always) and the compass gate (which no longer points to North on the capsule). Instead, you always use
the capsule's North/South lines when orienting the capsule North and South, making sure North on the
capsule points to North on the map. I hate to labour this point but it is in fact quite easy to have the map
upside down, especially when your head is full of angles and you've just recued the map from a thorn
bush
To transfer that angle to your compass, simply place the needle in the gate. Angles measured with your
compass can be transferred directly to the map, again using the North/South lines in the capsule, not the
gate. As you can see, set-and-forget compasseslet you avoid mental gymnastics when you are cold, wet
and would much rather think about steak pie and beer than compass courses. They also let you avoid
spending lots of time drawing lines on your maps. In the time you save, you can easily earn the money
to buy the better compass.
Avoid trying to learn compassuse by rule-of-thumb methods. Some books attempt to make the problem
too simple. Believe me, it is not simple. Straightforward, yes. Simple, no. Learn the logic of this section
and you will be able to cope almost anywhere.
Now I know you're probably thinking 'well he's obviously a smart fellow and know everything about
maps and compassesand how to use them.' Alas, if it were only so. It's a bit like riding a bike -you
never really forget but you do get rusty! If I'm planning a walk where I know I will need to navigate especially if there's a bit of compass work involved -I read up on it again and take my handy crib sheets
with me! There is no mileage (wrong -there may be a great deal of it if you make a mistake!) in thinking
you know how it's done. The essenceis practice. Seriously, a last few rules to mull over:1. If in any doubt as to your readings, check again.
2. Don't makethe map fit your readingsor vice versa
3. If you go wrong, retrace your stepsuntil you're sure you're on the correct path
4. Trust your compass -like a computer, it doesn'tmake mistakes -you do!
http://www.countrywalkers.co.uk/mapandcompass.html
7/11/2003
Page7 of 7
5. DONT PANIC!
Well, that's it. I do hope that this article will enable some of you to have more fun on your trips in the
countryside and, should you wish to attain expert status in navigation, hopefully join Outward Bound
courses or something equally heroic.Oh, and one last thing -if you're planning a Polar expedition this or
any other year -don't call me!
HappyWalking!
Readingandunderstandinga contourmap.
Grid references -how they work and how to read them.
How to choose and usd,a coffiQass.
7/11/2003
Grid References -how they work and how to use them.
Page1 of 2
I mentioned at the start of this article the six-figure numbers that are at the start of my walks. These are
Grid References. Every place on a map has one, and, what's more, it's unique. Modern electronic
navigation devices, which rely on communications satellites in geosynchronous orbit, can plot your
location to the nearestmetre square of ground -but what use is knowing that your Grid Reference ('Grid
Ref') is 8532643434 if you can't find it on the map! For practical purposes, most Grid Refs are given as
six-figure numbers. This will place you in an area 100 metres square-accurate enough for our purposes.
The whole of the British Isles is divided into squares100,000 metres on a side (100 kilometres). Each of
these large squares is given a reference comprising a letter pair -ST for example. Landranger and
Pathfinder maps are divided further into areas 1,000 metres (1 kilometre) on a side. To identify anyone
of these squares, two things are needed. The first is the letters (ST). The second is a four-figure
reference, say 8534. (These aren't real references, they're just examples, but the principle is exactly the
same on any map.) So what do they mean?Well, the extremely large square that encompassesthe whole
country has as its starting point for reference its bottom left-hand corner, or its South-West extremity.
The six-figure Grid Ref for this point would read 000000. It follows that if you could stand on this
point, the only way you could move within the giant square is East or North. Map references start with
Eastings (moving Eastwards from the start point), then Northings (moving North from the base line). An
easy way to remember this is that E' comes before N' in the alphabet. As you begin to move East from
the corner, every 100 kilometres you will encounter an Easting line, which runs South -North up the
map. Similarly, Northings run West -East across the map. If this sounds all horribly confusing, have a
look at Fig. 1 which (hopefully!) will make things clear. I know it's animated but I thought I'd try to kill
two birds with one stone here. Notice that the reference 'ST' appears on the map and that the 'town' (the
blue square)has the number 2215. This means that as you travel East (to the right), the last Easting line
is, in this case, Number 22. Stop there! Next look for the last Northing line before the town -15, right?
So you now know that the 'town' is to the EaSJ;__of
line 22 and to the ~
of line 15. Its Grid Ref. is
therefore ST2215. It always works this way -the Grid Ref. is always the square to the East and North of
the figures given. The second reference -ST248166 -refines the area that the square indicates. How?
Well, the larger square is subdivided into smaller squares,ten on a side. It follows that if the large square
is one kilometre square, the smaller squares must be 100 metres square (1,000 divided by 10 = 100).
First count to the last major Eastings line -24. Then count across until the last 'imaginary' fractional
Easting before the red blob -(lets say its a hilltop) which is not shown as a line on an OS map -is
reached. That figure is 8, thus giving an Easting of 248. Now perform a similar operation with the
Northings. Last major line = 16, then up to the last line before the 'hilltop' = 6. This give a Grid Ref. of
ST248166, the square in question being to the East of '248' and the North of '166'. And that's all there is
to it! Try getting hold of a Pathfinder map and plotting Grid Refs for points of your choice -honestly,
you'll very soon get the hang of it!
uk/gridref.html
7/11/2003
Grid References -how they work and how to use them.
Page2 of 2
Readingandunderstandinga contourmaR.
How to chooseandusea com~ass.
Using ma~andcom~asstogether.
http://www .countrywalkers.co. uk/gridref.html
7/11/2003
Land NavigationHandout
Page 1 of 12
CI BOLA
f~£~=~
searchand rescue
Location: St. Chad's Episcopal Church and Bear Canyon Trailhead
Instructors: Paul Donovan, Tom Russo
General Description:
Approximately two hours of classroomtime will be devoted to explanation of map features, compassuse, and the use
of map and compass together for wilderness navigation. Following some exercisesto practice use of these skills, we
will proceed to the field. Participants will be shown a topographical map with waypoints marked on it, and will be
expected to copy the waypoints onto their own copies of the map and plan a route which will allow them to visit the
waypoints and use their map and compass skills to navigate along this route.
Required map: Pleasecomepreparedwith a copyof the SandiaCrestUSGSquadranglemap.
Map Skills
Map symbols
The following table of map symbols shows you some of the features which USGS topographic maps may contain. A
fuller list is available in the US Geological Survey pamphlet "Topographic Map Symbols," which may be obtained for
free wherever USGS maps are sold.
http://www.swcp.com/csar/naY_lesson/nay_handout3.html
10/26/2004
~
."...'
-p
Page2 of 12
~.""'~ feItU"",,;St;own
..~~,..- ~C,..
.
"
~y~.or
_..~i;;,u,",_u~_e;;C~
-~..r_.~_C.
Co
!
n~~".:""""ft'A"'"
1.fW""",.,a4"..,,~._.,.
t"...
V¥\ ..,.
..
.
r "~;"c l
r
t, c.1".,;
'
.,+1""
~
,
.,.. '0.. 0.'0 ji
"""
"'iiL4
"""~A
n
06,f
"A
,
I1""
"'~""~:;:;
,..",.1;1,."," q~ "U"""~'~~"'!""I,'.C,;1"""ff'f"'
"t ;il" ~"'
1..1_/',,'
~Sy~
$,
J
I tTaC~rairN~.."..".."
" ..' I J
iP"e
.CC',
,.."..,{ C
~ W_-Fqtllf..-alu.
L'k.orp~~q,
oc,
BuUdifl~$;.fbarn,
warehaU$'iettJ,
, , ",i,j ~c:::t..
-..#
SUi
fdlngs
...,.(dntling..:pfaceoyemp1~,.t...J.
.c.
P"ll!I:lJ!alstre.m
..".,
i. I' Ii,
School;
Church
i
,.1, ..~~ - am"
"",lrSIl~SVl;,.
.,...
"
4i$~:~:'
(:;ell1,1_1:1
',
,
"
""
..~
c...,...~
Map
~-bOIj".,.
'"
..c ~".
c
~ ". "'._~".Ccc' r"~Hi1
,
Telephone.
r.
t .,_0...," v
rIJ.W8rfan$mISSlcnrJO..
" "
Wooos-hrU$flWOOd
Open
http://www.swcp.com/csar/naY_lesson/naY_handout3.html
10/26/2004
Page 3 of 12
Recogniiing Topographical Features
Elevation features are described on maps by use of contour lines. A contour line on a map is the line you would trace
out on the terrain if you were to walk along a path of constantelevation. Making the mental translation from contour
lines on a map to the terrain around you takes practice, and we will spend some time today doing just such translation
practice.
Put simply, here'show somecommonlyobservedterrainfeaturestranslateinto contourlines:
.Peaks -closed curves of decreasing size. Inner curves are at higher elevation than outer curves, and the peak is
within the innermost curve.
.Valleys -these appear as a series of "V" shapedcurves. The point of the "V" always toward the head of the
valley, and when there are streamsin the valleys the "V" points upstream.
.Saddles -The space betweenpeaks can be recognized by its hourglass shaped contour lines.
.Gentle slopes -slow rises in elevation over a long horizontal distance are representedby widely-spaced contour
lines.
.Cliffs -since they representdramatic changesin elevation over a short horizontal distance, cliffs are represented
by closely-spaced contour lines.
Hereare someexamplesof how terrainfeaturesaretranslatedinto contours:
~
Now that you've seenhow contours relate to terrain features, try this self-test. Match the contours on the left with the
terrain on the right.
http://www.swcp.com/csar/nav_lesson/nav_handout3.html
10/26/2004
Page4 of 12
Land Navigation Handout
A
B
2
c
D
4
e
F
It is important to use all the infomlation provided by the topo map in order to correctly identify the features. Note that
the figures 1, 3 and 6 on the right above all have two peaks, but by using the shapeof the contour lines and their
relative elevations you can detemline which contour figure goes with which side view. This becomesan important skill
to learn when trying to identify features in the real world; matching figure 3 on a map with profile A in the real world
would be a big mistake if you were counting on the identification to tell you where you are!
Map coordinate systemsand grids
USGS quad maps all contain grids of one sort or another. Perhapsthe most well-known outside SAR circles is the
Latitude/Longitude grid (Lat/Lon), but more often we use the Universal Transverse Mercator system (UTM) in SAR
work.
The Lat/Lon system
In the Lat/Lon system the features on the surface of the earth are mapped onto a sphere, and a pair of angles is used to
identify the points on the earth. The meridians of longitude are 360 equally spacedgreat circle arcs connecting the
north and south poles. The meridian which passesthrough Greenwich, England is arbitrarily called "0" longitude, and
meridians to the east or west of this meridian are measured in degrees east or west. Here in Albuquerque we are at
approximately 106 degrees west longitude. Parallels of latitude start at the equator, which is 0 degreeslatitude, and
10/26/2004
Page 5 of 12
are basically slices parallel to the equator; they are also measured in degrees,and the angle referred to is the one
between a line connecting the center of the earth to the surface of the earth at the equator and another line connecting
the center of the earth to the surface of the earth at the point in question. Here in Albuquerque we are near the parallel
of latitude designated 35 degreesnorth.
The latllon systemis cumbersometo usefor SARwork. Thereare360 degreesusedfor latitude(0-180 Eastand0-180
West),andthereare 180degreesusedfor longitude(0-90 North and0-90 South).Eachdegreeis divided into 60
"minutes"and eachminuteinto 60 "seconds."The biggestproblemfor the mapuseris that lines of longitudeconverge
at the polesand also a differenceof "3 minutes"betweentwo points cannotreadilybe convertedto a distance,since
this distancedependscrucially on the distancefrom the equator.
The UTM system
The UTM systemis a rectangularcoordinatesystem.The globeis divided up into "zones"of 6 degreeslongitudewith
the fust zonerunning from 180degreeswestlongitudeto 174degreeswestlongitude.The centralmeridian in each
zoneis assignedthe arbitrary "Easting"coordinateof 500 kilometers,andall points within the zoneareassigned
coordinatesbasedon their distancefrom the equator("Northing") and from the hypothetical0 point of Easting
coordinate;so at the equatorand at the centralmeridianthe coordinateis (500.0,0).Sincezonesarelessthan 1000
kilometerswide thereis no point which is actuallygiventhe coordinate0,0, andall UTM coordinatesarepositive.
Zonesare alsodivided into sectionsdesignatedby a letter. Here in Albuquerquethat is whatthe "s" standsfor before
our UTM coordinates,but this is redundantinformation: it merelydenotesthe rangein which the northingcoordinate
falls.
Different ways of reporting UTM coordinates
Most GPSunits reportUTMs in metersratherthankilometers,and it is commonto seeon your GPSdisplaysomething
like this:
138 0360639
3885020
This is to be readasa locationin UTM zone 13, sectionS, with eastingcoordinateof 360369metersand a northing
coordinateof3885020 meters;that meansthatthe given positionis 139631meterswestof the centralmeridian (which
hascoordinate500000)of zone 13,and 3885020metersnorthof the equator.In kilometersthis would be 360.369
eastingand 3885.020northing.
Even though only the digits down to 100 meters are significant, when reading UTM coordinates from a GPS
display to basecamp, rattle off every digit that is displayed,regardlessof precision. This is to ensure that no
errors are introduced into the coordinates by teamswho interpret their own coordinatesand round them off.
Leave issuesof precision to basecamp, and instead concernyourself with getting information to them without
introducing error into it yourself. This [eastingfollowed by northing] is the preferred method for reporting
UTMs to basecamp,eventhough someother organizations, notably the military, usea different, abbreviated
method, the Military Grid ReferenceSystem(MGRS), describedbelow.
A quick glance at a map shows that [even the xxx.x kilometer format] is sending more information than is typically
necessary. In general, on a 7.5 minute quad map only the kilometer and tens of kilometer digits change, and so one
could abbreviate the reported figure even further by leaving off the hundred and thousand kilometer digits. This is a
technique routinely used in the military, and in this (MGRS) reporting system one would report the location above as
"606850" ---the first three digits being the ten kilometer, one kilometer and one hundred meter digits of easting, and
the second three digits being the same figures for the northing coordinate. Figuring out what digits to use is fairly easy
when you look at a topo map: the UTM coordinates are printed along the edge of the map like this:
10/26/2004
~
Page6 of 12
I
3583000mN
~':1!O'
",,;;>$
?QO'.
5f
The UTM coordinates shown are 779.0krn easting and 3683.0krn northing. As you can see,the leading digits, which
would be dropped in this format, are printed smaller than the digits you would report. Unfortunately, if you use the six
digit format in reporting to base camp you might find yourself having to explain why you're only reporting one number
instead of two; this format is not widely used on missions, and you're not saving any time if you use a format that
requires you to explain yourself. Stick to the eastinginorthing format, add verbiage to make it clear what you're
reporting (" easting zero-three-six-zero-six-three-niner, northing three-eight-eight- five-zero-two-zero 'I).
One last point: when reporting UTM coordinates, one reports the easting first, then the northing. This is easyto
remember when reading them offGPS units, becausethe format displayed is the correct format to read off to base
camp for most GPS units (I have seensome low-end units do it backwards, though). To remember this when using a
map, just Read Right Up (i.e. Read left to Right along the horizontal edge to get easting, then Up along a vertical edge
to get northing).
Geodetic Data
The Earth is not actually spherical, and this createsa problem in mapmaking. The Lat/lon and UTM coordinates of a
point on the surface of the earth are actually dependentupon the way that the Earth differs from a sphere. This is only a
problem when there are multiple measurementsfor how the Earth is shaped,and naturally there are mutiple
measurements.
Until very recently, all USGS maps were made with coordinates based on some measurementsof the shapeof the Earth
made in 1866. This resulted in what was known as the North American Datum of 1927, or NAD27. But recently the
USGS has updated their data, and now usesthe North American Datum of 1983 (NAD83) on its maps. It is not
possible to "mix-and-match" UTM coordinates taken off of maps with different data. An NAD27 map might
show a particular pair of coordinates corresponding to a point on the earth some 200 yards away from where the same
coordinates would be on a map of the same area with NAD83. This point was hammered home to us last year when we
tried to work in an area which straddled two USGS quads and we used quads that had beenmade with different data --and we were puzzled about why UTM coordinates read from the maps were not working out the way we expected them
to. Always check your map datum when comparing coordinates obtained from two different sources(GPS/Map,
Map/Map, Map/team-in-the-field-reporting-position, etc.).
The mixing-and-matchingof map datumis most oftena problemwhenusingmapsalong with GPSreceivers.Most
GPSreceiversusethe WGS84datum(neitherNAD27 nor NAD83!) out of the box, andhaveto be resetthroughthe
menusystemto usea differentdatum.BEFOREYOU LEAVE BASE CAMP, you shouldmakesurethat you areusing
the samedatumthat is usedon incidentbase'smaps!This hasbeena problemin recentmissions,and you must
absolutelybe awareof it.
One last thought: while it could be considered unnecessaryradio chatter, you might consider reporting your geodetic
datum along with your UTM coordinates when calling in a position to base camp. This reducesthe possibility that they
not be aware of the difference between your datum and the one on their map; it doesn't eliminate it, of course, but it
makes sure that the mistake of transcribing an NAD27 UTM coordinate onto a map with NAD83 grid lines without a
conversion isn't your mistake.
Using a Compass
Parts of a compass
10/26/2004
Page 7 of 12
There are'several different kinds of compasses,but they have many common features. The baseof an orienteering
compass is a rectangular piece of transparentplastic. On the ends and sides there are often scalesof inches, miles, etc.
that relate to the common scales on maps. A certain distance on the map is equivalent to an actual distance on land as
determined by the scales. On the base is an arrow, called the "direction of travel" arrow or DOT. The DOT is used to
depict where you are going or where you are pointing the compass.
The bezel is a raised circular transparent mechanismhaving marks on the edge representing the number of degrees.
Inside its perimeter are a set of parallel lines. The middle line among these usually has some sort of arrow, pointing to
the north mark on the edge. Let's call the middle arrow the "northward" arrow.
Inside the bezel is the magnetic needle, with one end which will point to magnetic north. It is suspendedat the center
and is usually balanced so it doesn't rub againstthe bezel. The bezel is also usually filled with a liquid to damp the
motion of the needle, so that it settles quickly after some disturbance. The needle is usually colored red and white or
red and black. The important point is that the red part of the needle always points toward the north pole of the local
magnetic field. Note that this is not the sameas saying that the needle always points toward the Earth's magnetic north
pole. The difference is that due to perturbations in the Earth's magnetic field, it does not look like a simple dipole or bar
magnet, with North at one end and South at the other. The Earth's magnetic field has curvature. We'll talk more about
this when we discuss declination.
Determining the bearing to a landmark
The proper technique for holding a compassdependsupon what type of compass you have. For an orienteering
compass without fold-up mirror or any other sort of sighting mechanism, the best method is to place your elbows
comfortably at your sides, and keep them against your sides. To obtain a bearing to a landmark, face the landmark
squarely with your feet comfortably apart. Hold the compass in front of you with your elbows close to your sides, with
the compass level and the direction of travel arrow pointing directly away from you, perpendicular to the plane of your
shoulders. In order to get consistent readings from the compass, it is important to re-create this position faithfully. Turn
your whole body to modify the direction you are pointing, rather than moving your hands or arms. Holding the
compass in this manner will result in more repeatable measurementsand help to decreaseerrors in your bearings. Now
rotate the bezel of your compass until the "north" (red or luminous) part of the needle is within the orienting marks.
You can now read the magnetic bearing to the landmark off of the bezel at the direction of travel arrow.
A sighting compass must be held up to your eye so that you may look through it. Some of these have a folding cover
with a mirror on the inside. When used, the cover is openedto tilt above the bezel, and there is a notch on the cover for
sighting. The idea is to look at your target through the sighting notch and use the mirror to see when the magnetic
needle is properly in place. Make sure to hold it as level as possible so the needle doesn't drag, and that any alignment
marks such as lines on the mirror or notches on the bezel are properly lined up.
To obtain the bearing to a landmark, simply sight toward the landmark and rotate the bezel until the north-pointing end
of the needle lines up with the alignment marks in the bezel. Then read the bearing to the landmark off the edge of the
bezel.
Sometimes it is useful to know the "back bearing" from a landmark to your current location. The easiestway to do this
is to find the bearing of the landmark, then turn the compassaround and read the back bearing off of the bezel at the tail
end of the direction of travel arrow. The back bearing is also easily determined from your bearing by simply adding or
subtracting 180 degrees.Depending on what's comfortable for you, another way to determine back bearing is to simply
use the bezel. Twist the bezel until the southward -pointing end of the magnetic needle (usually black or white) is lined
up with the northward arrow of the bezel. The reading which is now indicated by the arrow or tick-mark on the bezel is
the back bearing.
Exercise:finding bearingsto local landmarks
Once we get to the practice areayou'll see that we have laid out markers pointing at prominent features nearby. Go to
each one in turn and determine the bearing from the marker to the landmark.
http://www.swcp.corn/csar/nav_lesson/nav_handout3.html
10/26/2004
Page8 of 12
Walking a bearing
It sounds simple, but there are some practical considerations when you decide to walk toward a landmark you have
chosen. For example, how can you make sure that you stay on course? What if there are some obstacles in the way?
You could walk with your compass out in front of you setto the desired direction of travel, and keep looking down at it
to stay on course. A better way is to pick some distant object that you can see that is in the direction that you want to
go, and walk toward it. Keep looking at the object frequently, since its appearancemay change as you get closer, or you
may lose sight of it if you drop into a low area. When you get to the object, repeatthis exercise until you get where you
want to go. If there are obstacles (streams, cliffs, rocks, etc.) in the way, you can walk around them to get to the object
you picked out from your last point. Then go to the other side of the object and repeatthis process.
Exercise: The Three-PointCompassWalk
This is a simple field exercise we will do to practice walking bearings. We'll fmd an area that's open enough to work in,
but wooded enough for it to be a challenge. Mark your starting position by dropping a coin (the value of the coin
should be proportional to your confidence that you can find it again). Pick a random bearing, set your compassto that
bearing, and walk it for a random distance, say 100 feet. Rememberthat distance. Stopping after this distance, add 120
to the bearing you've beenwalking, then set your compassto the new bearing and walk for the samedistance as before.
Stop, add 120 to the bearing again, and walk the samedistance once more. You should be no more than a few paces
away from the spot where you dropped your coin.
Magnetic anomalies
Since compassneedles are really just lightweight magnets, compassmeasurementscan be thrown off by nearby metal
objects. Be sure to keep the compasswell away from things like your radio, your car, that barbed-wire fence you're
standing next to, railroad tracks, the power lines nearby, etc. You also need to keep metal objects such as belt buckles,
knives, and pens away from the compass.
Thereare otherphenomenaassociatedwith terrainthat can affectcompassreadings,too. Tailings from mineswhere
iron or othermagneticoreswere gatheredcan affectcompassreadings.Therearealsogeologicalfeaturesthat are
magnetic,suchasthe Malpaisvolcanicdepositssouthof Grantsand northwestof Ruidoso,New Mexico.
Navigation with a map and a compass
Magnetic declination
The map and compasscan be used together to tell you precisely where you are and how to get where you want to be
But without keeping a few things in mind you might as well not have either.
As we mentioned earlier, the compassneedle does not actually point at the northern end of the Earth's axis, that is "true
north," but rather at the north pole of the local magnetic field. Most maps, however, are drawn in a projection which
puts meridians of longitude parallel to the sides of the map ---that is, the vertical edgesof the map point to true north.
The angle between a line drawn from any point on the map to the north pole, and a line drawn from that same point to
the local magnetic north is known as the magnetic declination, and is currently about 10.5 degrees East in our area. In
other words, when your compassis reading zero degreesalong your direction of travel, you're actually traveling on a
true heading of 10.5 degreestrue. Similarly, if you measure on your map that you have to follow a true heading of 10.5
degrees, you must know that when you use your compassto follow that path you have to set it for a heading of zero
degrees!
Many peoplehavecomeup with mnemonicsto help rememberwhetherto add or subtractdeclinationto convertfrom
magneticto true bearings.Onereliabletool that you can'tforgetis the declinationdiagramprinted on the map.Here's
an exampleof one, takenfrom a USGStraining website (NOT a mapof ourarea!):
10/26/2004
Page9 of 12
~
What this diagram tells you is that true north (the line with the star) lies at the top of the page. Magnetic north (the line
with the half-arrowhead) is 13 degreesto the right of true north, and the UTM grid ("GN") north is two minutes to the
left of true north. So, when your compassis reading 0, it is pointing thirteen degreesto the right of true north, so 0
magnetic = 13 true. In the case of this declination ("east declination"), MAG+DEC=TRUE. If you memorize formulas
better than you can read declination diagrams, remember that formula, becauseit's the one that's appropriate for areas
with east declination such as ours.
One approach for dealing with declination is to draw magnetic north lines onto your map. To do this, set your compass
to the declination ---thirteen degrees in the case above, and set it on the map with the north line of the bezel parallel to
a true north line on the map (ignore the needle for this, just use the markings on the case). Now your direction of travel
arrow points along magnetic north. Using the edge of your compassas a straightedge, draw a magnetic north line. It is
best to draw several of these lines, acrossthe entire map. Now you can read magnetic bearings directly off of the map
by making measurementsrelative to your magnetic north lines instead of the true north lines, obviating the need for any
formulas at all. But be mindful of one thing: magnetic declinations change over time, and the declination printed on the
map might not be the declination which is actually affecting your compasstoday; the change is small over a year, but
some maps were printed 10 years ago or more. The declination you must take into account is today's declination,
becausethat's the one your compasssees.So if you draw in magnetic north lines, make sure you're doing so with the
right declination.
Another caution which can be important in other parts of the country: the declination diagram is not always to scale,
especially if it is depicting small angles. In the caseof small angles the figure might be exaggerated, but the numbers
printed nearby will be correct. Sometimes map users are told to extend the magnetic north line on the declination
diagram to obtain magnetic north lines on the map, and most of the time that's OK, but watch out for printed statements
nearby that the diagram is "for obtaining numerical values only." And remember, too, that the declination diagram
might be outdated. For these two reasonsit's probably better not to use the diagram directly to draw your magnetic
north lines.
Exercise:finding true bearings
With eachof the magneticbearingsyou obtainedin the bearingexerciseabove,determinethe true bearingby applying
declination.
Resectionto locate position on the map
In order to determine what direction to go in order to get where you want to be, you must first know where you are.
Sometimes this is easy, such as when you can unambiguously identify a feature on the map, and you know that you are
standing right next to it. In other casesthe map and compasscan be used together to locate your current position on the
map. This process is sometimes referred to as "triangulation" but is more precisely called "resection."
In order to locate yourself on the map by performing resection, the basic idea is to compare your topographic map to
what you are looking at, and identify terrain features that you are sure you can both seeand associatewith a feature on
the map. Just by looking at the map and the terrain, you should have a general idea of your location. Terrain recognition
is important to pinpoint your location more accurately. Now you determine bearings to these features, and draw lines
on the map corresponding to the bearing to those features. The use of at leastthree lines is recommended, and they
http://www.swcp.com/csar/naY_lesson/nay_handout3.html
10/26/2004
Page 11 of 12
out objects that you will be hiking to, and see how long it takes you to get there. Stop and look around while hiking,
and see if you can pick out near and far objects on your topographical map.
Walking a bearing taken from a map
Ok, you've marked two points on your map, one representing your starting point and the other representing the place
you want to be. What now?
The easiestway to set yourself up to walk to your destination is to set your compassto the right bearing. Draw a
straight line between starting point and destination, set your compassso that the DOT arrow points from starting point
to destination. Now rotate the bezel of your compassso that the parallel lines inside are lined up with the magnetic
north lines you've drawn onto your map. Your compassis now set so that if you turn yourself until the north-pointing
part of the compassneedle is lined up with the alignment marks on the bezel, then you will be walking the correct
magnetic bearing to your destination.
Onceyou have setyour compassto the correctbearing,you can forgetthe mapagainandjust follow the bearingaswe
discussedabove:pick out a landmarkthat lies along your intendeddirectionof travel and walk towardsit.
Route-finding strategies
The High Road or the Low Road?
After determiningwhereyou areandwhereyou wantto go, you mustthen considerhow to getthere.You could walk
in a straightline, following a bearinguntil you getto your landmark.The shortestdistancebetweentwo points is a
straightline, but only on a perfectlyflat surfaceor if you can fly there! Evenin real terrain,the directrouteis not
alwaysthe fastestor the safest.
..
Sample Routes for hypothetical leg
In the picture above, assumethat we want to go from point A to point B. We could go by route 1,2, or 3. Route 1 is the
straightest, but goes through heavy vegetation and you might have trouble navigating once in there. Route 2 is also
fairly straight and less tree-covered, but goes over two hills that might take a lot of time and energy to climb. Route 3 is
the longest, but has little vegetation and a gradual slope. You must consider tradeoffs such as distance, navigation
ability, and how strong you feel in order to decide the best route for you. There is no right or wrong answer.
Locating a nearby "handrail"
http://www.swcp.corn/csar/nav_lesson/nav_handout3.html
10/26/2004
~
References
~//www.swcp.com/csar/naY_lesson/naY_handout3.html
10/26/2004

map and compass part 2

Transcription

Similar documents

Heritage Category: Listing List Entry No : 1227165 Grade: II County

a brochure on BBVA Compass for your cause.

BEACONSFIELD ASBESTOS PROSPECT

KRS Bearing

personal - Yayasan Khazanah

Time Marches On and So Does CALAOMS

Elite XL - Barudan embroidery machines

COMPASSES Final as of March 19, 2006

SEIKO 6105-8119 THE BEST SEIKO DIVE MODEL?

machine needles Choosing the Right Machine Needle

“The Strategic Compass Model” Defined