Well, I’ll be “DEM”ed!

By Larry Harwood

 

The Maine Office of Geographic Information Systems (MEGIS) has begun receiving new high resolution digital elevation models (DEM) from the US Geological Survey. DEMs can be used in the generation of three-dimensional graphics that display terrain, slope and aspect. This makes for more realistic and attractive maps and on-screen images by showing the earth’s surface as it actually would appear from a given height.

This graphic shows generated relief in the Standish area. The large water body in the center-left is Bonny Eagle Pond.

These images are often described as what you would see looking out the window of an airplane and are what graphics programs can use to “fly” over and around a virtual landscape. The angle and azimuth [1]of the “virtual sun” can be altered to mimic what the terrain would look like at different times of the day.   DEMs also have applications less graphic but quite useful. They are used in creating orthoimagery or aerial photographic images with the geometric qualities of a map. GIS technicians can use DEMs to model general patterns of drainage, map soil erosion and landslide hazards or calculate the area of proposed reservoirs. Because of these applications, DEMs are sometimes referred to as “gravity data”. They can also be used to model the propagation of radio waves to help site cell phone towers and do visibility analyses or “viewsheds”.

DEMs are digital files consisting of points of elevations, sampled systematically at equal intervals. GIS software converts DEMs into a raster layer. Rasters represent geographic features by dividing the world into discrete squares called cells. Cells, in this case 10 meters square, are laid out in a grid where each cell has a location relative to an origin and a value for elevation in feet. The fact that locations are arranged regularly permits the GIS software to infer associations among locations.

Questions?  For more information, contact: Larry Harwood by e-mailing larry.harwood@maine.gov.