Missing Data on Maps
Because of the importance of dealing with missing data, GIS programs have a special missing or no data category.
Dealing with missing elevations can use two approaches:
There is a DEM Holes submenu on the Edit menu with various ways to fill the holes.
Generally the first approach will be best with USGS 1:24K DEMs, because the missing data on the margins will almost always not be sea level. Setting it to sea level will cause map edge "cliffs".
Generally the second approach will be best with data sets like SRTM30, DTED Level 0, GLOBE, or GTOPO30, where you might be merging a great many small one degree cells, and any missing cells will almost certainly be water.
|Holes will have a greater impact on Reflectance maps or Slope maps, which require a neighborhood with eight neighbors surrounding the point (focal operations). For those maps, with a lot of holes, you might be better to use an Elevation map where a hole will affect only a single point.|
MICRODEM uses the MaxInt value (32767) to mark missing data, for both integer and floating point data sets. Other data sets have different startands; often a number of 9's marks mimssing data.
Last revision 11/23/2017