Digital Elevation Models _DEM_ by linxiaoqin


									       Digital Elevation Models (DEM) - Terrain Models (DTM)

A DEM is a continuous grid of elevation
values - one height per pixel (grid cell)
They are the modern equivalent of
terrain previously stored in contours
Google Earth – 3D perspectives (and flythroughs)
           DEM creation
A. by digitising contours (e.g. NTS maps -> NTDB layer).
This is 'second hand' digital, as contours are abstract

stereo photos -> contour lines -> digitised lines -> convert to raster GRID (topo to raster)
                DEM creation

B. Digital stereo-photogrammetry:
   (e.g. BC TRIM)

This is a better option, captured directly from aerial

stereo photos -> mass points -> convert to raster
    GRID (topo to raster)
Traditional method : stereo photos –> contours -> shaded relief

Digital method: digital photos –> points –> DEM –> shaded relief
                     and DEM <-> Contours

       Air Photo        -        DEM           -    Shaded relief

                   This is the DEM – for data analysis
DEM creation:      C. Direct image grid DEM (> 2000)
Stereo digital satellite raster imagery

                                              Ikonos 5m
                                DEM data

DEMs have been created at a variety of scales by different agencies.
Some can be downloaded free – all except the top one in this list

  AGENCY            SCALE         RESOLUTION

B. Municipal     1: 5,000               1            e.g City of PG
B. Provincial    1: 20,000             25            BC TRIM
A. Federal       1: 50,000              50
A. Federal       1: 250,000            200            CDED:
                                                      Canadian Digital
                                                      Elevation Data
A. Global        1: 1,000,000       500-1000
C. Global-SRTM     1: 100,000            90
SRTM   (Shuttle Radar Topographic Mission): 90 metre pixels, 56ºS-60ºN
DEM sources: global              GTOPO30 ~1993 1km
LiDAR DEM - 2009
      How have DEMs impacted relief depiction ?

Sugar-loafs and hachures – added graphically (not a software option)


Hypsometric Tints

Shaded relief (hillshading)

Tanaka illuminated contours

Slope zones

3D perspectives

Fly throughs

True 3D
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.
               Hypsometric Tints
Selection of hues, chromas from colour sequences

                                          <- legend labels
              Shaded relief (hillshading)
Addition of shading from an imaginary Northwest Light source
Shaded relief (hillshading) :          No longer does the practitioner require artistic ability.
The user selects azimuth and zenith, 315 and 45 standard to match the NW light source.
It is easily generated but may not be as good as from a skilled artist
Using the transparency option for shading and tints
        JLC Geomatique – addition of shaded relief to topographic maps

For BC TRIM maps, see:
Tanaka relief contours – not a common software option
Tanaka Contours – Forests for the World (by applying hillshading to contours)
Slope and aspect layers (GIS analysis)
3D perspectives:   Rocky Mountain trench – Castle Creek Glacier
Trails – UNBC to
Peden Hill

Dan Abraham, 2005
                               3D perspectives
Perspectives are produced by GIS and visualization software. The user selects
parameters such as viewing angle, vertical exaggeration and what may overlay the
terrain, such as a 'draped' aerial photograph, scanned map or map layers.
Whistler – Brandywine Meadows
Whistler – Brandywine Meadows (terrain + shaded relief)
Four DEMs (North Vancouver): 1km (GTOPO30), SRTM (90m);
      CDED 1:250,000 (100m); CDED 1:50,000 (25m)
           CDED = Canadian Digital Elevation Data

Four DEMs (North Vancouver): 1km (GTOPO30), SRTM (90m);
      CDED 1:250,000 (100m); CDED 1:50,000 (25m)

       Pick your own map sheet in Lab 7 / assignment #2
Physical Models: Challenger map (1947-54) 25 x 25 m ~1:50,000
True 3D
physical models
(40 x 74 feet 1:99,000)



In lab
this week

‘True 3D’

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