[GRASSLIST:8303] Setting Null Values

[Dave Kent]

Hi:

I have some scanned topographic maps in geotiff format.  When I try  
to import them, r.in.gdal tells me that they are not orthogonal and I  
need to straighten them with gdalwarp.   This works fine but as the  
map is rotated,  I get black slivers along the edges of the maps.

I want to patch the maps together.  If I set the black slivers to  
NULL with r.patch -z or r.null the valid black content in the map  
disappears as well.

Is there a way I can use the -dstnodata feature of gdalwarp to set  
the 'nodata' portions of the output map directly to a NULL value that  
grass will recognize on input?

Or is there some other way to solve this problem?

Thanks for your help.

Dave