About DEM File Data

DEM files typically contain land XYZ information at a regular grid-spaced interval to represent ground relief.

In general, the DEM source data is contained in one section of the DEM file (Logical record type `A'), and the actual data (XYZ) information is contained in another part of the DEM file (Logical record type `B').

The following data, based on logical record type `A', is displayed for each DEM file that you add to a surface:

Element Name	Corresponding Data Element Number in Logical Record Type ’A’	Details
Description	1	The first 140 characters of a DEM file, which may include the file name and location.
Estimated Point Total	N/A	The estimated number of points in the DEM file, rounded up to three significant digits.
Coordinate System Type	5	The ground planimetric reference system: UTM, State Plane, or Lat/Long.
Zone	6	Code that defines the zone in ground planimetric reference system. These can be found in Appendixes 2-E and 2-F of Standards for Digital Elevation Models. The Zone is meaningful only if Coordinate System Type is UTM or State Plane.
Horizontal Datum	27	The horizontal datum of the ground planimetric reference system, such as NAD27.
Vertical Datum	26	1 = local mean sea level 2 = National Geodetic Vertical Datum 1929 (NGVD 29) 3 = North American Vertical Datum 1988 (NAVD 88) For more information, see Appendix 2-H of the Standards for Digital Elevation Models.
Vertical Datum Shift	31	The value is the average shift value for the four quadrangle corners obtained from program VERTCON. AutoCAD Civil 3D adds this value to the vertical datum to convert to NAVD88. If the vertical datum is NAVD88, then the vertical datum shift value should be zero.
DEM Level	3	There are three possible DEM Levels: Level 1 DEMs are elevation data sets in a standardized format. This level includes 7.5-minute DEMs or an equivalent that is derived from stereo profiling or image correlation of National High Altitude Photography Program, National Aerial Photography Program, or equivalent photographs. Level 2 DEMs are elevation data sets that were processed or smoothed for consistency and edited to remove identifiable systematic errors. DEM data derived from hypsographic and hydrographic data digitizing, either photogrammetrically or from existing maps, are entered into the Level 2 category after review on a DEM editing system. Level 3 DEMs are derived from DLG data by using selected elements from both hypsography (contours and spot elevations) and hydrography (lakes, shorelines, and drainage). If necessary, ridge lines and hypsographic effects of major transportation features are also included in the derivation.
X-Y Units	8	The unit of measure for ground planimetric coordinates throughout the file, such as radians, feet, meters, or arc-seconds.
Elevation Units	9	The unit of measure for elevation coordinates throughout the file, such as feet or meters.
Minimum Elevation	12	The minimum elevation for the DEM. The value is the units of measure defined by Elevation Units.
Maximum Elevation	12	The maximum elevation for the DEM. The value is the units of measure defined by Elevation Units.
X Spacing	15	DEM spatial resolution for X values. The value is the units of measure defined by the XY Units.
Y Spacing	15	DEM spatial resolution for Y values. The value is the units of measure defined by the XY Units.