Check that resampled DTM cover the entire image
From http://tully.ups-tlse.fr/grizonnet/let-it-snow/issues/38
If the input DTM (or resampled DTM) does not covert the image footh
In the resampling stage, pixels on the image ROI without DTM info will be affected with the value -32768.
It leads to error in the computation of the snow line (ZS) and then error in the
Spotted on LS8 with an image over Norway