Digital Elevation Model (DEMs) from Topographic Maps

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The process of Digital Elevation Model (DEM) creation begins with the scanned and geo-referenced topographic map (or nautical chart in the case of bathymetry). Contour Lines, spot elevations, coastlines and bodies of water from the raster image are extracted, converted to digital vectors and given elevation values.

Once a digital image has been fully vectorized and attributed, a raster representation of the map is created based on those vector features. After editing and testing the resulting file is then output based on the clients file format and projection specifications.

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  Azerbaijan DEM (Digital Elevation Model) custom built from over 300 1:50,000 topographic map sheets

Azerbaijan 20m Digital Elevation Model (DEM) custom built from over 300 1:50,000 topographic map sheets.

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File Format File Sizes Delivery Options
• USGS ASCII (.dem)
• Digital Terrain Elevation Data (.dted)
• Generic ASCII
• Generic BIL
• ERDAS Imagine (.img)
• ER Mapper (.ers)
DEM file sizes will largely depend on the scale and contour interval of the source map, file format, and the spatial resolution of the DEM. DEMs generally compress quite nicely due to the file format and structure of the data. Land Info provides several different delivery options:
• USB flash drive
• External hard drive (USB 2.0/USB 3.0/FireWire 400/FireWire 800/eSata)

Quality Control

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Each DEM goes through a variety of steps to achieve the desired quality that Land Info has delivered time and time again.
  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
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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.)

Scale Contour Interval Geographic UTM
1:24,000 30 Feet 1/3 Arc Second 10 Meters
1:50,000 20 Meters 1 Arc Second 30 Meters
1:100,000 40 Meters 2 Arc Second 60 Meters
1:200,000 50 Meters 3 Arc Second 90 Meters
1:250,000 100 Meters 3 Arc Second 90 Meters

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Related Pages:
Examples of Digital Elevation Models (DEMs)
Digital Elevation Models (DEMs) Information
Digital Elevation Models (DEMs) from Satellite Imagery

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