Digital (base) mapping data

1/22/2015
Digital (base) mapping data
Where do map data and layers come from ?
Most of our base data map layers have come from air photos
-> first to maps, which were then digitised post-1985
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Early aerial photography
San Francisco from a kite, 1906
(before planes):
balloons, pigeons and
mountain tops
Mapping the Rockies (photo-topography), 1890
Oblique photos from mountain peaks
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Since 1945, all our topographic mapping is done from aerial photography:
Air photos enabled a huge reduction in fieldwork costs, and an increase in how
quickly and accurately large areas could be mapped
Athabasca Glacier, 1958
Traditional surveying:
Theodolite
Triangulation
Triangulation station
Geodetic station
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Cadastral surveying / data
e.g. urban features
aboriginal lands
parks
http://clss.nrcan.gc.ca/data-donnees-eng.php
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Traditional aerial film photography
There are two distinct branches of aerial photography:
1. Photogrammetry:
"the science of obtaining reliable measurements from photos"
Correction of distortions due to radial and relief distortion
(see also next slide)
Done automatically with modern digital photography /photogrammetry
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Photos are not ‘planimetric’: there are distortions
“Tip, tilt and swing”
Digital photography: Terrasaurus aerial photography, Williams Lake
http://www.terrasaurus.ca/
Distortions can be corrected very quickly with digital imagery
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ORTHOPHOTOS
Once corrected, and georeferenced, photos can be used for mapping and
as a layer. A corrected photograph is 'planimetrically correct' and now
an orthophoto (e.g. google maps, pgmap or BC- imap ).
The digital process enables automatic generation of orthophotography
There are two distinct branches of aerial photography:
1. Photogrammetry:
"the science of obtaining reliable measurements from photos"
2. Photo interpretation:
"the identification and extraction of meaning of objects"
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PGmap: http://princegeorge.ca/cityservices/online/pgmap/Pages/Default.aspx
Digitizing – Terrain Resource Information Management (BC)
Onscreen from digital (stereo) photogrammetry since ~1990
http://geobc.gov.bc.ca/
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Errors are possible before or after digital era
Features are misinterpreted (e.g. the UNBC Agora, and false trails)
Changes make features become out of date
BC provincial photography is generally redone about every 10+ years
City photography more frequently: 3-5 years
PG 2003
2006
2010
http://princegeorge.ca/cityservices/online/pgmap/Pages/Default.aspx
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Digital mapping data
Where do the (lab) map layers come from ?
–
digitising of printed maps – vector layers
e.g. National Topographic DataBase (NTDB) – all 13,377 1:50,000 sheets
- onscreen
digitising from stereo photography
e.g. BC Terrain Resource Information Management (TRIM) – 7027 sheets at 1:20,000
- scanned
maps – creates raster image map (no map layers)
- orthophotos
and satellite images
- Digital photography and drones
-Global Positioning Systems (GPS) for new features (e.g. cutblocks, trails)
Mapping data can be either Raster or Vector:
Raster: Images, DEMs and continuous data:
one data value per grid cell. e.g. TIF, JPG, GRD
Vector: Feature data, based on linked (x,y) coordinates:
most common for mapping e.g. SHP, DXF
http://geogratis.ca
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Scanning (raster image): .tif or .pdf map scan or
geotiff (georeferenced)
Vector: points, lines and polygons: The National Topographic Database (NTDB)
Digitising:
All features
from this
printed map ->
digital data
$500 per map
sheet till 2001
Data Liberation
Initiative (DLI)
2001-07 :
half price for
universities
(wahoo ! )
free download
after 2007
[Not so for
some countries]
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Digital data for mapping
Digital data correspond directly to their analogue counterparts, and data are
available at these common scales:
Global:
1:1,000,000
largest scale for whole world
Federal:
1:250,000
medium scale for all Canada
Federal:
1: 50,000
largest scale for all Canada- provinces
Provincial: 1: 20,000
largest scale for all BC (+ some provinces)
Municipal: 1: 1,000 to 5,000
scale for cities / municipalities
Global data (small scale)
The largest scale for the whole world covered is 1:1,000,000.
The Digital Chart of the World (DCW) was completed in 1993.
Digitised from the printed maps
It is not suitable for mapping at larger scales.
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Digital era user generated data: http://www.openstreetmap.org
Data by country: http://freegisdata.rtwilson.com
Federal: NTS (analogue maps) -> NTDB (digital) - medium to large scale
Topographic data are available in digital form at: 1:50,000 and 1:250,000, from NTDB.
Since spring 2007 can now be freely downloaded from geogratis.ca
http://geogratis.cgdi.gc.ca/geogratis/en/product/search.do?id=8147
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Data; http://geobc.gov.bc.ca/
See also: http://maps.gov.bc.ca/ess/sv/imapbc/
Thematic and census data: Statistics Canada
http://library.unbc.ca/collections/data-statistics/canadian-data
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Abuse of digital data: ‘Geographic data’
The geographic graticule is good for data storage but not as a coordinate system for digital mapping
It is not rectangular - 1 degree longitude varies in size
example: pg-phonebook 2007
‘Projected’
pg-phonebook 2008
‘Geographic’
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