A mask is a bitmap or polygon layer that identifies specific areas to edit. You can apply various operations to update the values of a digital elevation model (DEM) covered by the mask. Masks are stored on a separate mask layer that can be saved for future editing. In the DEM Editing window, you can work with polygon (vector) or bitmap masks.
Raster-bitmap masks are typically used to edit DEMs that contain many elevation failures.
To edit a DEM using a bitmap mask
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In the DEM Editing window, click the Edit via Bitmap tab.
The tab displays all the options available to work with bitmap masks.
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Use the Edit via Bitmap toolbar to perform editing tasks.
Button Label Description 
Edit Mask Layer Put the bitmap layer in editing mode, allowing you to create new bitmap masks. 
Mask Failed Pixels Create a bitmap mask identifying all failed pixels in the DEM. To use this option, on the Settings tab, the Failed box must contain a value. Focus automatically populates this field using the file's metadata.
New Mask Layer Create a new bitmap mask layer. 
Open Mask Layer Open an existing bitmap mask layer. 
Save Mask Layer Save the bitmap mask layer for future editing. 
Clear Mask Layer Clear all masks from the bitmap mask layer.