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DEM – Digital Elevation Models


source: US Army

source: Microsoft Virtual Earth 2D

source: Microsoft Virtual Earth 3D

What is a DEM?  DEM stands for Digital Elevation Model.  It is a raster (bitmap) image who’s pixel values are representative of elevation; usually it is a gray-scale image.  There are a possible 256 color values in a most gray-scale images; those colors are mapped to elevation where black represents the lowest elevation value, white represents the highest elevation value, and likewise for everything in between.  Actually, Virtual Earth 3D uses 16 bit DEM pixel values, so their potential elevation resolution is much much better than 256; I use 256 as an example.

Microsoft Virtual Earth map tiles are 256 pixels by 256 pixels.  Each tile has an accompanying DEM tile, which is used to bump out the elevation in Virtual Earth 3D.
256 is a nice number in computer science because it is a power of 2 (a developer told me that -I don’t fully understand, so you can take it for whatever its worth).

The relationship between pixel values and elevation is not linear.  More attention is paid to lower elevations, so the darker end of the pixel range (nearer sea level) gets a bigger share of the grayscale range of the DEM than do areas of higher elevation.  That is probably because that’s where most of mankind’s stuff is, so lower elevations steal some of the resolution away from higher elevations.  That sounds fair -but I don’t live in Tibet.  Really, 0 – 5 feet above sea level is much more important than 5,000 – 5,005 feet above sea level.

I don’t know what algorithm Microsoft is using to map the DEM pixel values to elevation, but in general it might look something like this…


The series of images on the left illustrate how DEMs can be applied to maps or aerial/satellite imagery to connote a 3D environment.  Microsoft Virtual Earth 3D uses a series of DEMs to give form to their terrain model.  If you haven’t played with this yet, you should -it is amazing.

How does Virtual Earth 3D translate DEM pixel values into a terrain model?  Virtual Earth 3D leverages hardware rendering for its 3D modeling.  The DEM elevation values are used to build an elevation wireframe, or series of vertices that make up a triangular network.  Virtual Earth map tiles are then draped upon this surface.  That surface is then shaded to look more convincing, but that is another story altogether.

Here are some cool places to hit in VE 3D with spectacular elevation:

The Grand Canyon
Crater Lake
Zion National Park
The Golden Gate Bridge

John Nelson / IDV Solutions / john.nelson@idvsolutions.com



4 responses

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