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Digital Elevation Models (DEM) / DTM. Fogo Island, Cape Verde Republic ASTER DEM / image. Uses: queries and analysis, layers in classification, 3D visualisation. DEM data sources. Almost all DEMs have been created from remote sensing: a. DEMs from digitising contours (e.g. NTDB)

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Digital elevation models dem dtm l.jpg
Digital Elevation Models (DEM) / DTM

Fogo Island, Cape Verde RepublicASTER DEM / image

Uses: queries and analysis, layers in classification, 3D visualisation

Dem data sources l.jpg
DEM data sources

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 -> convert to raster

b. Digital Stereo photogrammetry: (e.g. BC TRIM)

Mass points captured from aerial photographs

stereo photos -> mass points -> convert to raster

Bc trim dem l.jpg

Interpolated to 25m grid

By 1:250,000 map sheets

(100 tiles assembled together)

Vertical accuracy to within 10metres

UTM pixel coordinates to even 25m

Resampled to BC Albers

16 bit codes, -9999 for invalid numbers

e.g. across border into AB

Post 2000 methods l.jpg
Post 2000 methods

  • c. Direct image grid DEM data from imagery

  • RADAR (interferometry) e.g. Shuttle Radar Topographic Mission (SRTM)

  • Stereo aerial photos, and optical satellite imagery from path overlap

Dem availability l.jpg
DEM availability

A DEM is a continuous grid of elevation values –

one height value per pixel ….. in a channel (not a band)


Municipal  1: 2,000  1 

Provincial  1: 20,000  25 

Federal  1: 50,000  50 

Federal  1: 250,000  200 

Global  1: 1,000,000  500-1000 

Download Canadian data from (1:50,000 / 1:25,000)

* BC is provincial data (25m) resampled for NTDB (50m)

Dem sources global gtopo30 1993 1km l.jpg
DEM sources: global GTOPO30 ~1993 1km

Slide7 l.jpg

SRTM  (Shuttle Radar Topographic Mission) Feb 2000

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

Aster dem global dem 30m l.jpg
ASTER DEM<- global DEM 30m

Aster image svalbard norway 15 metre resolution l.jpg

ASTER image: Svalbard, Norway (15 metre resolution)

Dem layers l.jpg
DEM layers

a. Elevation

DN = (metres, 16 bit):  represented as grayscale or pseudocolour tints;

Raster DEMs are stored as elevations (metres) or 32 bit (after interpolation)

some DEM tiles in Canada may be in feet (how would the values compare ?)

conversion = .3048

Slide11 l.jpg

b. Shaded relief

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.

Also useful for detecting errors and assessing quality

C slope l.jpg
c. Slope or

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

4 aspect l.jpg
4. Aspect or

The direction in which a slope is facing, measured by azimuth (0-360)

This raises three question for analysis:

a north facing slope has both extreme values, 0 and 360

flat slopes have no value (they are given an arbitrary numerical value, e.g. 555)

0-360 requires 16 bit data; some software (PCI) converts to 8 bit by dividing by 2

Slide16 l.jpg

5. Incidence or

Image data look similar to shaded relief, but with values 0-90

the angle (degree) of light incidence for each pixel, based on the sun position at time of scene capture. Requires knowledge of sun elevation and azimuth for the scene = metadata (file acquired with scene data)

Slide17 l.jpg

6. Second derivatives: or slope concavity and convexity, used in generating watersheds and also viewsheds using line of sight

Slide18 l.jpg

7. 3D perspectives / fly-throughs or lrdw.cae.g. Google Earth etc..

Castle glacier cariboo mountains l.jpg
Castle Glacier, Cariboo Mountains or

Directed high res sensors ikonos 1999 4 m l.jpg
Directed high res sensors- or lrdw.caIkonos 1999 4 m

8 anaglyphs malaspina glacier alaska l.jpg
8. Anaglyphs or Glacier, Alaska)

Slide25 l.jpg

Kamchatka, Russia or

Manicouagan crater quebec l.jpg
Manicouagan Crater, Quebec or

Dem products l.jpg
DEM products or

Elevation 0- ? 16 bit

Hillshading 0-255

Slope 0-90 (if in degrees)

Aspect 0-359 16 bit

Visual: Anaglyphs and 3D perspectives

Can dems enhance digital image classification l.jpg
Can DEMs Enhance Digital Image Classification? or

SPOT image data

Dem summary l.jpg
DEM summary or

  • Use of DEMs / DTMs:

  • queries and analysis

  • layers in classification

  • 3D modelling and visualisation

  • Resolutions and datasets available:

  • NTDB 50m

  • TRIM 25m (BC only)

  • ASTER 30m (global)

  • SRTM 90m (near global)

  • GTOPO30 1km (global)

Lidar dem 2009 unbc area light detection and ranging l.jpg
LiDAR DEM 2009 – UNBC area: or (Light Detection And Ranging)

Resolution 50 cm

Flooded PG bowl image

by Shane ~ 10,000 BP

Slide31 l.jpg

The UNBC or


In lab

this week