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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

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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

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Download Canadian data from geobase.ca (1:50,000 / 1:250,000) = BC provincial data (25m) resampled for NTDB

BC: federal and provincial data – TRIM DEM (25m)

After 2000 methods

c. Direct image grid DEM data from imagery RADAR e.g. Shuttle Radar Topographic Mission (SRTM)

Stereo digital satellite imagery (adjacent satellite tracks)

Issues: cloud cover and missing data (from space)

ASTER SPOT

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SRTM (Shuttle Radar Topographic Mission) Feb 2000

90 metre pixels, 56ºS - 60ºN latitude e.g. Google Earth

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

ASTER DEM <- global DEM 30m pixels

Global DEM (ASTER) http://asterweb.jpl.nasa.gov/gdem.asp

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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)

S

Manicouagan

Crater, Quebec

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DEM from Photogrammetry: Tatras, Slovakia 2m

Directed high res sensors- Ikonos 1999 4 m

http://www.satimagingcorp.com/galleryimages/ikonos-high-resolution-dem-eritrea.jpg

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DEMs in Digital Image Classification

Possible Input channels: Raw bands e.g. Thematic Mapper 3,4,5

Ratios / Indices / Components (e.g. NDVI, Tassel)

Elevation

Slope gradient

Incidence (not aspect)

DEMs in Digital Image Classification

http://www.geocarto.com.hk/cgi-bin/pages1/june05/33_Saha.pdf

1 Green 2 Red 3 NIR 4 MIR 5 NDVI 6 DEM

Land Cover Classification Using IRS LISS III Image and DEM in a Rugged Terrain: A Case Study in Himalayas