What could lead to negative elevation values using ENVI's DEM Extraction Module?

The DEM Extraction Module was introduced in ENVI 4.2. When extracting a DEM from a dataset, such as an ASTER L1B file, some of the resulting elevation values may be negative. What are the possible causes of the negative values? What is the best way to avoid negative elevation values in the extracted DEM?
 
When extracting a DEM, you are given the option to extract an absolute or relative DEM. In an absolute DEM, elevation values are tied via GCPs to a planar map projection. If you choose not to select or enter GCPs, then the resulting elevation values are relative. In this scenario, ENVI will instead use 'pseudo GCPs' without elevation information, which may cause bias in the calculated RPCs .

There is no way to avoid negative elevation other than using GCPs. Further, it is not feasible for ENVI to force the elevation values to be positive, since that does not neccessarily guarantee more accurate elevation values. For example, if one has an estimated elevation of -50m (average) while the true elevation is 50m, ENVI could add an offset of 200m to make the average elevation of 150m, which ENVI recognizes based on the global DEM dataset. However, while all of the elevation values may now be positive, they may still be incorrect - which may even be worse than being negative.

In summary, it is beneficial to select GCPs to tie the DEM to a planar map projection and result in an absolute DEM.

Review on 12/31/2013 MM