DIGITAL ELEVATION MODELS
1. What is a Digital Elevation Model (DEM)?
Digital elevation models contain heights which can be used to depict and analyze the terrain;
they are also known as DTMs (terrain). These
data can generate images that correspond to
some traditional relief depictions. A DEM most commonly is a continuous grid of elevation values - one height per pixel (grid cell).
a. DEMs from digitising contours
The earliest were created by digitising contours on maps into digital contour layers (eg NTS maps to NTDB layer). This is 'second hand' digital, as the lines
themselves are abstract and may
result in 'artifacts' in the model.
Process: stereo photos -> contour lines -> digitised lines -> convert to raster GRID
b. Digital Stereo photogrammetry: (e.g. BC TRIM)
This is a later and better option, as it goes back to the real surface, as captured from aerial photographs (mass points)
Process: stereo photos -> mass points -> convert to raster GRID
Summary: Data are collected as a 'lattice' of points or as contour lines.These however are 'discrete' - there are no values or data between the points or lines. In order to create a continuous surface of an elevation model, one must generate a raster Grid [or a vector 'Triangulated Irregular Network' (TIN) - GEOG300, not this course ]. Note that the DEM has become the layer from which other products are developed, whereas traditionally, other renditions came from contours.
c. Direct image grid DEM data (new millenium)
Newer techniques creating DEM grids directly have multiplied since 2000, these include:
RADAR (interferometry) e.g. Shuttle Radar Topographic Mission (SRTM)
LIDAR (LIght Detection And Ranging) examples: http://quake.wr.usgs.gov/research/geology/lidar: trade center
Stereo aerial photos and satellite imagery: ASTER SPOT IKONOS
Note: GPS can be used to collect elevation points, that can supplement a DEM (or create small ones)
2. DEM Availability
DEMs have been created at a variety of scales by different agencies. Some can be downloaded
free (see lecture on digital data)
TYPICAL RESOLUTION (metres)
||1 (usually contours)
3. The impact of digital data on relief depiction
Contour lines can be interpolated from DEM heights or pre-exist from digitised
maps, and are a standard layer in digital databases and online web mapping. ( e.g. pgmap mapplace lrdw-imap).
b. Hypsometric tints
Colour schemes can be selected from a variety of
menus: gray, chromatic or spectral ranges. Software defaults enable suitable colour schemes and intervals: legend bar- good example
Atlas of Canada: http://atlas.nrcan.gc.ca/site/english/maps/reference/national/
No longer does the practitioner require artistic
ability. The user selects azimuth and zenith, 315 and 45 to match the conventional
NW light source. It is easily generated but artifacts
appear if the data are flawed or sparse. Unless data are high
resolution, the results may not be as good as from a skilled manual
practitioner, who was able to manipulate the light source as needed,
for example when ridges ran NW to SE where the opposing sides would be
Mapplace shows the hillshaded DEM for BC:
For an excellent effect, tints can be combined with hillshading in GIS software using 'transparency' display options. example
BC interactive map has hillshading and combo with hypsometric tints: BC iMap
Canadian mapping companies using DEM hillshading: Gemtrek (Calgary) - [compare with this one produced manually ]
Cloverpoint (Victoria) JLC (Quebec)
Excellent websites on automated hillshading: shaded relief relief shading
DEMs can also be used to generate analytical layers, especially these two: slope = gradient, and aspect = direction
e. Tanaka relief contours
The technique is not often seen in GIS software,
more for special research topics: Mt.
f. 3D perspectives
Perspectives are produced by GIS and visualization software. The user selects parameters such
as viewing angle, vertical exaggeration and what may overlay the
terrain, such as a 'draped' aerial photograph, scanned map or map layers. These models have a role in
visualization of landscape, for managers, clients and in education. Many examples, some below:
Draped maps: mt.st.helens SFU: earthdetails.com
Geomantics: Scottish visualisation software ... (check out their address!)
Virtual terrain project....... Truflite
f+. 3D animation
Much of the same software that produces 3D perspectives also enables the generation of 3D movies and fly-throughs (the rapid generation of a sequence of perspectives).
3D visualization: http://www.visualizationsoftware.com/3dem.html
David Rumsey Collection: http://www.davidrumsey.com/GIS/3D.htm
Matterhorn on youtube: www.youtube.com/watch?v=5l0czNk2SC4 (needs anaglyph glasses)
Google earth ...
Summary: 3D perspectives along with hill-shading have seen the greatest increase in use with automation. (WHY?)