Digital Elevation Models (DEM)

Transcription

1 Digital Elevation Models (DEM) Digital representation of the terrain surface also referred to as Digital Terrain Models (DTM)

2 Digital Elevation Models (DEM) How has relief depiction changed with digital mapping /GIS software? - And who built that road ~~~~~~~~~??

3 Summary of common relief depiction methods TECHNIQUE COMPONENT FEATURES Sugar loafs shape Simple, stylistic Hachures slope much ink, no heights Spot Heights elevation non-visual data points Contours elevation heights, abstract Hyps. tints elevation Layer colours Created from contours Shaded relief aspect Visual, artistic Tanaka contours aspect visual but 'noisy' Slope maps slope uniform slope areas 3D perspectives shape visual, no fixed scale Physical models all true 3D takes up space

4 1. Sugar loafs: still used to show rough location of mountains or ye olde looke Ikonos 5m

5 2. Hachures - not a (common) software option show orientation of slope, and by their thickness and density provide a general sense of steepness Ikonos 5m

6 3. Spot heights and 4. Contours digitised from printed maps digital layers The National Topographic DataBase (NTDB) is the digital equivalent of the (13,350) printed maps - download from

7 Contour lines are a standard layer in digital topographic databases and web mapping for display. e.g. PGmap

8 Contours are not DEMS, but can be used to create them.. cannot create layers such as hypsometric tints, hillshading DEM generation Digital contours or points > interpolation to raster DEM ArcGIS: Topo to Raster The DEM is the modern source of relief layers (not contours)

9 Digital Elevation Models (DEMs) A DEM is a continuous grid of elevation values - one height per pixel They are the modern equivalent of terrain elevations previously stored in contours

10 DEM creation methods A> by digitising contours (e.g. NTS maps -> NTDB layer) - Done for all of Canada (~ ) stereo photos -> contour lines -> digitised lines -> interpolate

11 DEM creation B. Digital stereo-grammetry: (e.g. BC TRIM 1980s) This is a smoother option, captured directly from aerial photographs stereo photos -> mass points -> convert to raster GRID ( topo to raster )

12 TRIM DEM 25m raster grid (1996) Interpolated to 25m grid In 1:250,000 map sheets (100 x 1:20,000 tiles merged) Vertical accuracy ± 10metres (sub-metre precision is useless)

13 DEM creation: C. Direct image grid DEM (> 2000) satellite raster imagery used for NTS in the Territories Ikonos 5m

14 D. LiDAR DEM 2009 (PGmap) - most Canadian cities have a LiDAR DEM

15 DEM (raster GRID) data DEMs have been created at a variety of scales by different agencies. Many can be downloaded free all except maybe the top one in this list AGENCY SCALE TYPICAL RESOLUTION (metres) D. Municipal 1: 5,000 <1 B. Provincial 1: 20, A. Federal 1: 50, A. Federal 1: 250, e.g City of PG BC TRIM NTDB (Canada) C. Global 1: 100, SRTM (Radar) e.g. Google Earth

16 5. Hypsometric Tints (relief methods) Generated from DEM Selection of hues, chromas from colour sequences DEMs displayed as grayscale or a colour ramp -> tints elevation values (usually) in metres

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

18 6. Shaded relief (hillshade) Manual method : stereo photos > contours -> shaded relief Digital method: digital photos -> DEM > shaded relief Air Photo - DEM - Shaded relief

19 Shaded relief (hillshading) : No need for artistic ability. BC TRIM DEM Values = Based on relative reflectance with NW light source The user selects azimuth / zenith 315 and 45 standard to match NW light source.

20 NEVER show hillshade layer in legend the numbers are meaningless

21 Manual shading: 100 hours / square foot (trained specialist)

22 Shaded relief (hillshading) JLC Geomatique addition of shaded relief

23 Standard topographic map PLUS added hillshading available for all Canada NTS maps: at both scales 1:50,000 1:250,000

24 Using the GIS softwaretransparency option to combine shading and tints

25 Part of the BC wide hillshade layer Ikonos 5m

26 Using transparency to show terrain and forest cover Ikonos 5m

27 7. Tanaka relief contours not a common software option

28 Tanaka contours now an option in ArcMap ArcMap Terrain Tools v1.1 (2016)

29 8. Derived DEM layers: Slope used in GIS analysis, not often as map layer Values = 0-90 (degrees) or also in % Bright = steep Dark = flat

30 . 3D (2.5D) perspectives (and flythroughs) Google Earth, ArcScene User selects viewpoint, angle, vertical exaggeration, drape layers, e.g. labels, roads

31 3D perspectives (and flythroughs) Esri software ArcMap: 2D mapping ArcScene: 3D perspectives (next two labs) Since 2017 ArcGIS Pro: 2D + 3D - try in next week s lab Use the better one in Lab 8

32 Perspectives for ski hills e.g. Whistler-Blackcomb The user selects: viewing angle, vertical exaggeration and overlay such as a 'draped' aerial photograph, scanned map or map layers. Google Earth as background to overlay ski map layers

33 9. 3D Perspectives Whistler Brandywine Meadows (terrain + shaded relief)

34 Whistler Brandywine Meadows with draped NTS map

35 (2.5D) Animations:

36 Note: Jack Challenger s BC wood map is 25 x 25m Manually carved / created , now in storage 10. True 3D physical models NOT! x 74 1:99,000) -Solid Terrain Modelling Cut by laser

37 3d printer, Filaprint, Tumbler Ridge Jody Mitchell

38 How have DEMs impacted relief depiction? Sugar-loafs and hachures can be added graphically Contours digitised inventory, but a DEM is more useful Hypsometric Tints easily applied colour ramp from DEM Shaded relief (hillshading) - greatest impact 3D perspectives 2 nd greatest impact True 3D models: