LAND INFO's high quality DEMs and GIS Data can aid you in achieving your project goals in the most efficient manner. We may already have the data you need in our archive, or can custom produce it upon request to your specifications. LAND INFO's Project Managers will work closely with you and your staff to guarantee the quality of your project is at its highest standard and your desired results are achieved.

What are some common uses for DEMs? • Environmental Analysis • Civil Applications • Telecommunication Modeling • Simulation & Training – Military, Aviation, Urban Planning • Cartography	Advantages of using LAND INFO DEMs: • Accuracy and Quality • Extensive Geographic Coverage • Low Cost • Quick Turn-around Times
Also Available from Land Info: • Low-cost, off-the-shelf 30m seamless DEMs (NED -- US only) • SRTM (Shuttle Radar Topography Mission) 90m DEMs • New low-cost 1km (30 arc second) global DEM (SRTM30) • 30m DEMs from ASTER stereo pairs • 10m DEMs from TK-350 stereo pairs

The process of DEM creation begins with the scanned, geo-referenced Topographic Map or raster image. Contour Lines from the raster image are extracted, converted to digital vectors and given elevation values. Once a digital image has been fully vectorized, a raster representation of the map is created based on those vector features. Then the hydrographic vector features, such as rivers and lakes are added. The resulting file is then output based on the clients File Format Specifications.

3d Surface Representations

1. The vector data is directly compared to the Topographic Map to ensure the lines accurately represent the source map and to verify the vectors have the proper elevation values.
2. Benchmarks are used to check the accuracy of the attributes and to check the elevation values of contour lines. When coastlines exist, they are also used in this regard.
3. The DEM goes through an edge matching process that verifies a visibly smooth transition from one DEM (one map sheet) to another. This portion is done manually to assure the information from the source Topographic Maps are transferred to the DEM and there is no loss of accuracy.
4. Each DEM must pass a Root Mean Square Error (RMSE) check. An RMSE check assures that the DEM is accurate in comparison to the source Topographic Map by comparing a DEMs elevation values to estimated horizontal and vertical values and control points that have accurate elevation data associated to them. The information derived from the RMSE check is recorded in the “C” record, which is an element of the standard DEM output file format.

The control points are an important component of the DEM Quality Assurance check. Control points range from spot heights to points along index contours. A minimum of 30 control points selected throughout the entire file. Several different elevation values are used to spread the checked points evenly throughout the entire map. Several of the 30 points will be chosen near the edges and corners of the DEM to account for edge matching. Two RMSE tests and an average deviation test are performed to ensure the quality of the DEM. All three of the tests outcomes should be less than half the value of the contour interval of that specific map.

The following is an example of a typical RMSE:
Contour Interval = 10 meters
Number of Control Points Selected = 38
RMSE0 = 4.51
RMSE1 = 4.57
Average Deviation = 0.07

Map Scales

The Spatial Resolution (otherwise known as Grid Posting) of a Digital Elevation Model highly depends upon two different factors - contour interval and Topographic Map scale. The contour interval of a Topographic Map will vary depending on the lay of the land and the amount of detail that can be represented at any given Topographic Map scale. This chart shows some typical spatial resolutions from different Topographic Maps: (Note: Different cell sizes can be used for different scales of Topographic Maps, however this could result in a loss of accuracy.)