Printer Friendly

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

Digital Elevation Model  - Germansen Lake (209 Columns X 333 Rows)- 25m resolution The elevational number 1023 appears many times because it is the lake surface.
Air Photo	Elevation Values	DEM (Elevation)	Shaded Relief	Landsat on DEM

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)

TOP

3. The impact of digital data on relief depiction

a. Contours

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/

c. Hill-shading

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 equally illuminated.      

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. Logan

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

Everest: http://www.photogrammetry.ethz.ch/research/mounteverest/

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)

MARS (ESA)

Google earth ...

Summary: 3D perspectives along with hill-shading have seen the greatest increase in use with automation. (WHY?)