DEM> add base heights question
Why? With some DEM files I am able to "add base heights", but on others I cannot. On the latter, when I try to "add base heights", the DEM display collapses to a vertical black line.
How can I convert the non-functioning DEMs so they operate like the other DEMs that do allow base heights to be added?
What is different about the two files formats?
An example of DEM products that do not allow base heights to be added are those obtained from the USGS Seamless Server website.
An example of DEMs that do allow base heights to be added are those obtained from the Ohio OGRIP website.
I am at a loss to explain why some DEMs allow the addition of base heights, while others do not. I would like to find out what the difference is between the two file types, and I would like to learn how to convert the reisistant DEMs to functioning DEMs.
Mapping Center Answer:
The DEM that you got from the USGS is in geographic coordinates while OGRIP DEM uses a projected coordinate system.
When you apply a base height, it should be in same units as your DEM. So in the case of your OGRIP data, the units are in feet so the height is also in feet. However, in the case of the USGS data, the heights in meters even though the DEM coordinates are in degrees.
So you can do one of two things -- either project the USGS datsa from geographic to projected coordinates (then the values will be in either feet or meters, depending on the projection), or provide conversion factor to "trick" the software into interpreting degrees as meters. The conversion factor is described in more detail in this online help document, which also has a table of conversion factors: Applying a z-factor.
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