[OSM-dev] Zeroing or Extending a Hillshade-Tiff

[Peter Körner]

Hi

I have a Hillshading-Tiffs (a GeoTIFF with a single Greyscale-Band) in 
the correct projection and correctly aligned.

My problem is, that it's slightly smaller then the area I'd like to 
render using mapnik:

http://imageshack.us/photo/my-images/202/bildschirmfotovom201303.png/

There are options i'm aware of:
  a) set the water level (its values 180-182 in the greyscale-band) to
     transparent, so there's no visible border at the end of the image
  b) extend the image with value 181 at the borders to fit the size of
     the image i'm going to render

My question is:
what way would you choose and wich tools could be used to go that way?

Regards, Peter