I use gdaldem to calculate the slope of a given region. This is the command I use:
gdaldem slope -s 111120 -p -compute_edges N34E069.hgt N34E069_slope.tif
(I've got the SRTM data from http://viewfinderpanoramas.org/Coverage%20map%20viewfinderpanoramas_org3.htm)
Some values in the raster band however have the noData value -9999. In the documentation I just came accross this line:
The value -9999 is used as the output nodata value.
But, what is the reason for getting a nodata value? When I create a color-relief map with the following color code:
-9999 255 0 0 0 255 255 255 90 0 0 0
It seems like the noData values occur on the peaks of mountains. But on the left in the flat area I also get some noData values.
How can I fix this problem?