A digital elevation model (DEM) can sometimes contain pixels with failed or incorrect values. You can edit a DEM to smooth out such irregularities to create a more accurate model and, in turn, generate more accurate orthorectified images. For example, areas, such as lakes, often contain misleading elevation values. Applying a constant value to these areas improves the model and produces a more accurate representation of the lake in the ortho image. For more information, including suggestions on how to correct common irregularities, see Strategies for common situations with DEMs.
In many cases, you may need to convert a DEM from a digital surface model (DSM) to a digital terrain model (DTM). A DSM represents the elevation surface, including objects, such as buildings and trees, whereas a DTM represents a bare-Earth elevation surface. For this reason, using a DTM to perform orthorectification generally produces better results. However, a DEM extracted from overlapping images is a DSM; it first should be converted to a DTM, and then edited before using it for orthorectification.
You can use a bitmap or vector mask to identify specific areas of the DEM you want to edit. The mask itself does not change the values in the area that it covers: use the tools available in the DEM Editing window to modify the data under the mask.
After you finish editing your DEM, use this updated model to generate new ortho images.
Explore the subsequent topics of this section for more information:
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