USGS DEM files are organized into these record types:

• Type A records contain information about the DEM, including name, boundaries, and units of measure.
• Type B records contain elevation data arranged in “profiles” from south to north, with the profiles organized from west to east.
• Type C records contain statistical information on the accuracy of the DEM.

There is one Type A and one Type C record for each DEM. There is one Type B record for each south-north profile.

DEMs are classified by the method with which they were prepared and the corresponding accuracy standard. Accuracy is measured as the root mean square error (RMSE) of linearly interpolated elevations from the DEM compared to known elevations. The levels of accuracy, from least accurate to most accurate, are described as follows:

• Level One DEMs are based on high altitude photography and have a vertical RMSE of 7 meters and a maximum permitted RMSE of 15 meters.
• Level Two DEMs are based on hypsographic and hydrographic digitizing with editing to remove identifiable errors. The maximum permitted RMSE is one-half of the contour interval.
• Level Three DEMs are based on digital line graphs (DLG) and have a maximum RMSE of one-third of the contour interval.

DEMs will not replace elevation data obtained from field-run surveys, high-quality global positioning systems, or even well-calibrated altimeters. They can be used to avoid potential for error which can be involved in manually interpolating points.