10/9/2014 1 Digital Elevation Models (DEM) / DTM Fogo Island, Cape Verde Republic ASTER DEM / image Uses in remote sensing: queries and analysis, 3D visualisation, layers in classification Banks Peninsula, Christchurch, New Zealand DEM sources – where do they come from ? Almost all DEMs have been created from remote sensing: a. DEMs from digitising contours (e.g. NTDB) The earliest were created by digitising contours on maps into layers stereo photos -> contour lines -> digitised lines -> interpolate to raster grid Arcmap: topo 2 raster PCI geomatica: vdemint
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Digital Elevation Models (DEM) / DTM
Fogo Island, Cape Verde Republic ASTER DEM / image
Uses in remote sensing: queries and analysis, 3D visualisation, layers in classification
Banks Peninsula, Christchurch, New Zealand
DEM sources – where do they come from ?
Almost all DEMs have been created from remote sensing: a. DEMs from digitising contours (e.g. NTDB) The earliest were created by digitising contours on maps into layers stereo photos -> contour lines -> digitised lines -> interpolate to raster grid Arcmap: topo 2 raster PCI geomatica: vdemint
b. Digital Stereo photogrammetry: (e.g. BC TRIM) Mass points (70 metre spacing) captured from aerial photographs stereo photos -> mass points -> interpolate to raster grid
BC TRIM DEM
Interpolated to 25m grid By 1:250,000 map sheets (100 tiles assembled together) Elevation in metres = 16 bit DNs (not 8-bit !) 0-65,535 OR +/- 32,767 -9999 for invalid numbers e.g. across border into AB
ASTER image: Svalbard, Norway (15 metre resolution)
DEM availability
A DEM is a continuous grid of elevation values – one height value per pixel ….. in a channel (not a band) Resolutions and datasets available: NTDB 25m (Canada) TRIM 25m (BC only) ASTER 30m (global) – with holes … SRTM 90m (near global)
Download Canadian data from geobase.ca (1:50,000 / 1:25,000) * BC is provincial data (25m) resampled for NTDB
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DEM - layers a. Elevation DN = (metres, 16 bit): represented onscreen as grayscale or pseudocolour tints; DEMs are stored as integer elevations (metres) or 32 bit (after interpolation) some (NTS) DEM tiles in Canada may still be in feet …. conversion = .3048
b. Shaded relief (hillshade) A cartographic layer, DN= 0 - 255 (relative amount of light reflected), as grayscale; light source can be selected, usually from the NW. High values on NW facing slopes, low values on SE facing slopes.
Select light source azimuth and angle Default = 315, 45 useful to detect errors and assessing quality
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DEM grid displayed as grayscale
Hillshading (NW light source – 315, 45)
BC provincial DEM 1985 (1981-1989) – Homathko Icefield - mapplace.ca or lrdw.ca Errors from lack of contrast in snowfields = few data mass points
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c. Slope Calculated in degrees (0-90) or % (0 -> infinity) slope is rise/run = vertical change over the horizontal distance 8 bit results should be adequate for most purposes (not 16 or 32 bit)
Aspect
Displayed as grayscale pseudocolour
Darker for lower DN (0- ) , brighter for higher DN ( - 359)
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4. Aspect: the compass direction a slope is facing
A circular scale: N = 0 and 360
This raises three questions for analysis:
north facing slope has both extreme values, 0 and 360 ******
flat slopes have no value (they are given an arbitrary value, e.g. 510)
0-360 requires 16 bit data;
PCI converts to 8 bit by dividing by 2 (flat = 255)
5. Incidence
the angle (degree) of light incidence, based on the sun’s position
DN is related to the reflection based on sun angle
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Incidence
Image data look similar to shaded relief, but with values 0-90
the angle (degree) of light incidence, based on the sun position
Requires metadata for sun elevation and azimuth for the scene
6. 3D perspectives / fly-throughs e.g. Google Earth etc..
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7. Anaglyphs (and 3D onscreen)
St-Pierre – Miquelon
SRTM DEM
Klinakline Glacier, south Coast Mountains (ASTER DEM)
N
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Klinakline Glacier, south Coast Mountains (ASTER DEM)