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Digital Elevation Models (DEM)

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					     Digital Elevation Models




The topographic surface continuously varies
           Digital Elevation Models (DEM)




DEM: A digital representation of a topographic surface
    http://en.wikipedia.org/wiki/Digital_elevation_model




> Recreation and landscape planning   Timberline
DEM: A digital representation of a topographic surface
A DEM is a continuous grid of elevation values - one height per pixel (grid cell)

DTM: Digital Terrain Model - technically more correct

DSM: Digital Surface Model             e.g. includes vegetation, buildings etc..
Beetle kill in BC, shown as ‘elevation’




                            http://www.for.gov.bc.ca/hre/bcmpb/Annual.gif
DEM (= DSM): New York (using LiDAR)
             DEM creation
A. by digitising contours (e.g. NTS maps -> NTDB layer).
This is 'second hand' digital, as contours are abstract



B. Digital stereo photogrammetry: (e.g. BC TRIM)
This is a better option, captured directly from aerial photographs


C. Direct image grid DEM (new millenium)

D. LiDAR
C. Direct image grid DEM (new millenium)
Satellite sensors creating DEM grids directly have multiplied since 2000
                      DEM data versus DEMs
Mass points or Contour layers can be used to:

store elevation data and to create DEMs

…. but they are not DEMs themselves




      Lines                   Points        Raster GRID (DEM)
Contour lines can be interpolated from DEM heights or pre-exist from digitised maps, and
      are a standard layer in digital databases and online web mapping for display.
                      DEM data formats
A. Discrete data

 Contour lines – from maps or digital files

 Mass points

       These are interpolated into GRIDS (topo to raster)

B. Continuous data


 Raster grids – standard pixel size (1m 25m 100m etc..)

 Triangulated Irregular Networks (TIN)
            Triangulated Irregular Networks (TIN): vectors




a series of triangles capturing the topography .. x, y, z at nodes
Each triangle has a defined slope and direction (aspect)
The development of the TIN model is often attributed to Dr. Tom Poiker (SFU)

Advantages: variable data density depending on landscape
           significant points or lines can be encoded e.g. peaks, ridges, valleys

Disadvantages: more complex, needs more processing to generate
              triangle facets are often evident in processing

Summary: most DEM data today are stored and processed as raster GRIDS
              BC TRIM DEM 25m raster grid
Created from mass points   Aerial Photography 1979-89 (1:70,000)
                           BC TRIM DEM


Interpolated to 25m grid
by 1:250,000 map sheets
(100 tiles assembled)

Vertical accuracy to within 10metres


Continuous surface -lakes /ocean
-9999 for invalid numbers
e.g. across border into AB
                             DEM
                (.tif format or ESRI grid)
displayed as grayscale or colour ramp - elevation values in metres
Hillshading /
shaded relief

(NW light source –
    315. 45)



 Values = 0-255

  (dark to light)
Slopes

         •   Hydrology
         •   Route Planning
         •   Slope Stability
         •   Avalanche
             Prediction
         •   Habitat Use
         •   Fire Behaviour
         •   others …


         Slope is calculated in

         percent % or degrees °

         [ 45° = 100% ]

         Bright = high slope
            DEM derived layers


Elevation    0- ~10,000 (earth)

Shaded relief 0-255

Slope        0-90     (if in degrees)

Aspect       0-360    (flat = ?)


Visual:      3D perspectives
      Federal v Provincial DEM sources

BC TRIM: 25m grid 1980s

Federal: www.geobase.ca mostly from digitised contours except BC

BC: TRIM resampled to 50m pixels (by E / W 1:50,000 map sheet)


Canada NTDB: www.geogratis.ca

               1:50,000 1950-90        50m pixels
               1:250,000 1950-90       250m


Often stored / downloaded in Geographic (lat/long)
Data MUST be projected for analysis / display
   e.g. UTM, Albers etc..
DEM sources: global              GTOPO30 ~1993 1km




    http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.html
SRTM   (Shuttle Radar Topographic Mission): 90 metre pixels, 60ºS-60ºN
                      DEM data availability
DEMs have been created at a variety of scales by different agencies.
Some can be downloaded free


                                    TYPICAL
  AGENCY           SCALE          RESOLUTION
                                    (metres)

Municipal       1: 5,000        1 (usually contours)
Provincial      1: 20,000               25             BC TRIM (see lrdw.ca)

Federal         1: 50,000               50               -> www.geobase.ca
Federal         1: 250,000             200
International   1: 1,000,000        500-1000



SRTM             1: 100,000              90
Prince George LiDAR DEM - 2009




                            UNBC
                          3D perspectives and fly-throughs
                           e.g. Google Earth and ArcGIS ArcScene


User specifies:

> viewer position

> azimuth / vertical angle

> vertical exaggeration

'draped' layers
   e.g. imagery, map scan,
        vector layers


terrainmap and
 virtual terrain project
http://www.google.com/mars/
            Sample midterm questions
         Multiple choice and short answers
Multiple Choice:

The first operational GIS in the world was developed in which country:
a. USA
b. Australia
c. Canada
d. France
e. Great Britain

Short answers:

What is the main reason that a projected system such as UTM is a
more appropriate method of georeferencing than the geographic
graticule (latitude / longitude) in a GIS ?

Review: lecture notes, plus associated links and lab topic notes (not software)

				
Jun Wang Jun Wang Dr
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