Digital Elevation Models _DEM_ Digital Terrain Models _DTM_ Brief

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Digital Elevation Models _DEM_ Digital Terrain Models _DTM_ Brief Powered By Docstoc
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Digital Elevation Models (DEM)
                            Digital Terrain Models
                            (DTM)


                            Digital Surface Models
                            (DSM)


                            All DEMs are created
                            from remote sensing !
Applications in digital image processing:

• Inclusion in queries and analysis
• Inclusion as layers in classification
• Feature extraction via topographic effects
• 3D modelling and visualisation
           Extended list of DEM uses

Extracting terrain parameters
Modeling water flow or mass movement
Creation of relief maps and models
Rendering of 3D visualizations
Rectification of aerial photography or satellite imagery
Reduction (terrain correction)
Terrain analyses in geomorphology and physical geography

http://www.satimagingcorp.com/svc/dem.html
    Early DEM generation:
pick regular points or digitise contours
                     DEM data formats
A. Discrete elevation data

 Contour lines – from maps or digital files

 Mass points     …. and break lines

        These are interpolated into GRIDS (PCI etc..)


B. Continuous DEM data

Triangulated Irregular Networks (TIN)


 Raster grids – for remote sensing … ideally ~same pixel size
Evolution of DEM creation


1950s Generation of contours from stereo photos

1980s Mass points from stereo photos

1990s Automated generation of masspoints

2000s Direct generation of grids from stereo-imagery
      (and expansion of LiDAR)
DEM sources: global            GTOPO30 ~1993 1km
  digitising of 1:1,000,000 world maps – contours -> grid




    http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.html
                            DSM sources 2000+
   Shuttle Radar Topographic Mission (SRTM) Feb 2000 30-90m




http://srtm.csi.cgiar.org      download by 5 x 5 degree lat/long tile     60N – 56S


http://www.cgiar-csi.org/data/elevation/item/45-srtm-90m-digital-elevation-database-v41
           ASTER 1999- 30m

0.78-0.86 microns
    ASTER Global DEM (2009) data from 1999-2008




http://asterweb.jpl.nasa.gov/gdem.asp 30m   download   1 x 1 degree
ASTER Global DEM : Kananaskis Park area DEM and #scenes used
              BC TRIM DEM 25m raster grid
Created from mass points and break lines Aerial Photography 1979-89 (1:70,000)
   TRIM DEM – masspoints ~70m spacing
captured onscreen from stereo-photography ‘soft copy’
        (prior to fully automated image matching)
    TRIM DEM 1998


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

Vertical accuracy to within 10metres
UTM pixel coordinates to even 25m,
edges to round 100m



Continuous surface
16 bit codes,
 -9999 for invalid numbers
e.g. across border into AB
      Federal v Provincial DEM sources

BC TRIM: 25m grid 1980s


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

BC: is TRIM DEM resampled to 50m pixels


Canada NTDB: www.geogratis.ca (by E / W 1:50,000 map sheet)

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



Data are mostly stored / downloaded in Geographic (lat/long)
     DEM data processing – PCI or ArcGIS

Data are usually stored in Geographic tiles (lat/long)

 Download or transfer data tile(s)

 Mosaic as needed (and maybe clip)

 Reproject e.g. Albers, UTM … and ….. resample to match imagery
       (avoid nearest neighbour for resampling)

 Clip to area / imagery

 Generate shaded relief to assess quality / problems
       DEM grid
(.tif format or ESRI grid)
   displayed as grayscale
    elevation in metres

      Hillshading
(NW light source – 315, 45)
                 DEM products
Elevation     0- ~9,000 (earth)             16 bit (0-63,356)

Shaded relief 0-255

Slope         0-90     (if in degrees)

Aspect       0-360     16 bit (flat = ?)

Incidence     0-90    requires orbital metadata



Visual: Anaglyphs and 3D perspectives
 SPOT DEM: 2002                    5-15m




http://www.spot.com/web/SICORP/1809-spot-3d.php
 Directed high res sensors- Ikonos 1999 2-5m




http://www.satimagingcorp.com/galleryimages/ikonos-high-resolution-dem-eritrea.jpg
Photogrammetry: Tatras, Slovakia 2m
              LiDAR 10cm – 10m (Mt.St. Helens)




http://www.nasa.gov/centers/goddard/images/content/67409main_dh2.gif

				
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