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A Blurring Of The Lines Is - PowerPoint PPT Presentation

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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
Digital Elevation Models (DEM) / DTM

Fogo Island, Cape Verde RepublicASTER DEM / image

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

dem data sources
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

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
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
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
DEM sources: global GTOPO30 ~1993 1km


SRTM  (Shuttle Radar Topographic Mission) Feb 2000

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

dem layers
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


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
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

4 aspect
4. Aspect

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


5. Incidence

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)


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

directed high res sensors ikonos 1999 4 m
Directed high res sensors- Ikonos 1999 4 m

dem products
DEM products

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
Can DEMs Enhance Digital Image Classification?

SPOT image data

dem summary
DEM summary
  • 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
LiDAR DEM 2009 – UNBC area: (Light Detection And Ranging)

Resolution 50 cm

Flooded PG bowl image

by Shane ~ 10,000 BP




In lab

this week