Digital Elevation Models (DEMs)

Transcription

1 Digital Elevation Models (DEM) - Terrain Models (DTM) How has relief depiction on maps and online changed with digital mapping/ GIS?.. Perhaps more than the other map elements / layers Digital Elevation Models (DEMs) 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 or points 1

2 Traditional method : stereo photos > contours -> shaded relief Digital method: digital photos > DEM > contours/shaded relief and also DEM <-> Contours ( topo-raster ) Air Photo - DEM - Shaded relief DEM creation methods A. by digitising contours (e.g. NTS maps -> NTDB layer). [This is 'second hand' digital, as contours are abstracted from the terrain] stereo photos -> contour lines -> digitised lines -> convert to raster GRID (topo to raster) 2

3 DEM creation methods B. Digital stereo-photogrammetry: (e.g. BC TRIM) captured directly from aerial photographs stereo photos -> mass points -> convert to raster GRID (topo to raster) DEM creation methods: C. Direct image grid DEM (> 2000) Stereo digital satellite raster imagery Ikonos 5m 3

4 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 TYPICAL RESOLUTION (metres) B. Municipal 1: 5,000 1 B. Provincial 1: 20, A. Federal 1: 50, A. Federal 1: 250, e.g City of PG BC TRIM -> Canadian Digital Elevation Data (CDED) A. Global 1: 1,000, C. Global-SRTM 1: 100, Four DEMs (North Vancouver): 1km (GTOPO30), SRTM (90m); CDED 1:250,000 (100m); CDED 1:50,000 (25m) 4

5 How have DEMs impacted relief depiction? Sugar-loafs and hachures added graphically (not a software option) Contours Hypsometric Tints Shaded relief (hillshading) Tanaka illuminated contours Slope zones 3D perspectives / Fly throughs True 3D Sugar loaf technique: still used to show rough location of mountains (or for old look effect ) added graphically in GIS / graphics software 5

6 Hachures - also added graphically or advanced research (not a routine software option) Contours a routine layer, digitised from maps The National Topographic DataBase (NTDB) is the digital equivalent of the printed maps - download from 6

7 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. Default contour display: ArcMap How can we make contours look more as they should on a map? Change colour to brown Darker/thicker for index contours Label online instead of above line Label only index contours Label each contour only once 7

8 Hypsometric Tints Selection of hues, chromas from colour sequences <- legend labels 8

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10 Shaded relief (hillshading) Addition of shading from an imaginary Northwest Light source: No longer does the practitioner require artistic ability. Shaded relief (hillshading) : The user selects azimuth and zenith, 315 and 45 standard to match the NW light source. It is easily generated from the DEM (elevation) but may not be as good as from a skilled artist 10

11 Using the transparency option to combine shading and tints JLC Geomatique addition of shaded relief to topographic maps For BC TRIM maps, see: 11

12 Tanaka relief contours not a common software option Tanaka Contours Forests for the World (applying hillshading to contours) oops! 12

13 Slope and aspect layers (mostly for GIS analysis) Slope:Gradient (increasing saturation) Aspect: direction slopes are facing (range of hues) Google Earth 3D perspectives (and flythroughs) Easily generated from DEM / overlay layers in this case, satellite images 13

14 ArcScene The user selects: Viewer location viewing angle vertical exaggeration and overlay e.g. aerial photograph, scanned map or map layers. Animations: This may not work in class try this one instead: 14

15 Physical Models (traditional) : Challenger map ( ) 25 x 25 m ~1:50,000 True 3D physical models (40 x 74 feet 1:99,000) Digitally created by laser 15

16 3D perspectives: Rocky Mountain trench Castle Creek Glacier The UNBC Geowall In lab this week True 3D Summary: How have DEMs impacted relief depiction? Sugar-loafs and hachures Contours Hypsometric Tints Shaded relief (hillshading) Tanaka illuminated contours Slope zones 3D perspectives /fly throughs True 3D x y y yyy x y (GIS) yyy y 16