How Accurate Is Your DEM Extracted From Topographic Maps?

As accurate as the source maps, or other materials

The positional accuracy, vertical and horizontal, of a DEM can be no better than the source materials. When topographic maps are used as source materials, their horizontal and vertical accuracy determines the accuracy of the derived DEMs. Please refer to national mapping standards for the accuracy statement.

Data Quality Checks

Certain features that are vectorized are checked for horizontal accuracy relative to their locations on the maps (or digitizing accuracy). These include the four corner points of each map, all benchmark and spot elevations, and elevation contour lines. Acceptable accuracy is defined as all line features falling within the bounds of the same line as scanned. This is verified by viewing the original raster scanned image overlaid with the vector feature. All lines which end near the edge of the source map are allowed to continue to the edge of the digitized map, defined by the four corner points. The same check is applied to point features.

Checks are made on the vectorized features to catch mislabeled contour lines and misalignment in contours between map sheets.

All benchmark and spot heights are verified by probing the resultant DEM and comparing elevation values with the original raster image or the vectorized features.

Checks are made on the resultant DEMs by examining a cross-section (or profile) to catch elevation values that are out-of-range anomalies.