Setting DEM file options

The elevation reference in the DEM must match the elevation reference of the imagery that you want to orthorectify. Most math models are based on Mean Sea Level. However, two math models are based on an ellipsoid: the Radar Specific model and the Rational Functions model. If you are using one of these models and your DEM is not based on an ellipsoid, see Converting the DEM datum.

In the DEM File window under Available images, select a DEM layer.
If you want to change the background elevation (NoData) value in the DEM layer, type a new value in the Background value box.
If you want to change the value used to scale the DEM pixel values, type the scale value in the Elevation scale box.
If you want to shift all of the elevation values in your DEM, type the offset value in the Elevation offset box.
Note: The elevation and offset are applied using the following formula:
```
  elevation_value = scale * (DEM_pixel_value + offset)
	
```
In the Elevation unit list, click Meter, Feet, or U.S. Feet to identify the unit that the elevation numbers in your DEM represent.
In the Elevation reference list, choose one of the following:
- Mean Sea Level: Elevations are referenced to Mean Sea Level. This is the default value.
- Ellipsoidal: Elevations are referenced to the ellipsoid. The coordinate system of the DEM raster file is examined to determine which ellipsoid and datum are to be used; for instance, if the DEM file is referenced to NAD83, the heights are assumed to be NAD83 ellipsoidal heights. If the DEM and math model are based on different coordinate systems, the heights will be automatically converted to the math model coordinate system before use.
If you want to see information about your DEM such as the DEM file name, DEM size, and georeferencing unit, click DEM Info. Click OK to close the information window.
Click OK in the DEM File window.