Converting the DEM datum

The elevation reference in the digital elevation model (DEM) must match that of the imagery you want to orthorectify. Elevation values can be referenced to a number of different surfaces, but with mapping operations, you typically use elevations above mean sea level (MSL), which is based on the geoid.

With most math models, the model is based on ground control points (GCP), which are also based on the geoid.

With OrthoEngine, there are two math models that are based on orbital information instead of GCPs: the radar-specific model, and the rational-functions model when used with the IKONOS GEO Ortho Kit product, CARTOSAT, KOMPSAT-2, KOMPSAT-3, PALSAR, PRISM, QuickBird, OrbView, and WorldView imagery. Orbit information is always referenced to an ellipsoid, and the ellipsoid number is taken from the projection information in the file.

A DEM extracted from satellite imagery using the preceding math models is based on an ellipsoidal model of the Earth, not the geoid. The difference between elevations relative to the ellipsoid and those relative to the geoid can be significant; in some areas, up to 107 meters. Therefore, you must ensure that the elevation reference of the DEM matches that of the imagery.

To convert between ellipsoid and MSL, OrthoEngine calculates the difference between the geoid and the ellipsoid at the point in question, and then applies the difference to compute the transformed elevation.

In the following table, the key aspects of converting a DEM datum are summarized.

Item	Key information
Input DEM file	The file containing the channel to convert. The file must be a GDB-supported type and contain at least one DEM channel (data to process).
Input DEM channel	The channel in the DEM file to convert.
Output DEM file	If an output file of the name you specify exists already, it must be of the same size, projection, and resolution as the input file. You can, if necessary, specify the input file as the name of the output file.
Output DEM channel	If you select New, the output channel will be of the same bit depth as the input channel. If you select an existing channel, it also must be of the same bit depth as the input DEM channel.
Input DEM datum	Typically, the elevation datum is read from the ELEVATION_DATUM metadata tag, and, in the Convert DEM Datum window, the corresponding Input DEM datum type is selected by default. For example, when the value of the tag is ellipsoid, Ellipsoid is selected.
Elevation unit	Like Input DEM datum, the elevation unit is read typically from the ELEVATION_DATUM metadata tag, and, in the Convert DEM Datum window, the corresponding Elevation unit type is selected by default. For example, when the value of the tag is meter, Meter is selected.
Background value	Typically, the background value is read from the NO_DATA_VALUE metadata tag, and, in the Convert DEM Datum window, the corresponding value appears in the Background value box. If the tag does not exist, a default value of `-32768.0000` is displayed.
Failure value	Like Background value, the failure value is read typically from the FAILED_VALUE metadata tag from the input DEM channel, and, in the Convert DEM Datum window, the corresponding value appears in the Background value box. If the tag does not exist, a default value of `-32768.0000` is displayed.
Copy all layers from the input file to the output file	If you are converting to an existing file, and the number of layers in the input file does not match the number of layers in the specified output file, the check box is unavailable.

To convert the DEM datum

If you do not already have the Convert DEM Datum window open on your desktop, in the OrthoEngine window, on the Tools menu, click Convert DEM Datum; otherwise, proceed to the next step.
The Convert DEM Datum window appears.
In the Input DEM file box, type the path and file name of the DEM or, to select a file, click Browse.
In the Input DEM channel list, select the channel you want to convert.
In the Output DEM file box, type the path and file name to which to write the converted DEM or, to select a file, click Browse.
In the Output DEM channel list, select the channel to which to write the conversion or, to write to a new channel, select New.
Do the following, as applicable:
- Beside Input DEM datum, do one of the following:
  - To convert the ellipsoid elevations to MSL, click Mean Sea Level.
  - To convert the MSL elevations to ellipsoid, click Ellipsoid.
- Beside Elevation unit, do one of the following:
  - To convert the elevation to meters, click Meter.
  - To convert the elevation to feet, click Feet.
- In the Background value box, enter a new value, if necessary.
- In the Failure value box, enter a new value, if necessary.
- To copy all layers from the input file to the output, select the Copy all layers from the input file to the output file check box.
Click Convert.
The DEM datum is converted.