Resolve "zs computation method return -1 when error occur, whereas "-1" is a valid zs value"

Closes #66 (closed)

src/histo_utils.cxx

@@ -254,7 +254,7 @@ short compute_snowline_internal(const itk::VectorImage<short, 2>::Pointer compos
     for (int i=histogram->GetSize()[0]-1; i>=0; i--)
       {
       snowline = get_elev_snowline_from_bin(histogram, i, fsnow_lim, fclear_lim, offset, center_offset);
-      if(snowline != -1)
+      if(snowline != -1000)
         return snowline;
       }
     }
@@ -263,7 +263,7 @@ short compute_snowline_internal(const itk::VectorImage<short, 2>::Pointer compos
     for (unsigned int i=0; i<histogram->GetSize()[0]; ++i)
       {
       snowline = get_elev_snowline_from_bin(histogram, i, fsnow_lim, fclear_lim, offset, center_offset);
-      if(snowline != -1)
+      if(snowline != -1000)
         return snowline;
       }
     }
@@ -302,7 +302,7 @@ short get_elev_snowline_from_bin(const itk::Statistics::ImageToHistogramFilter<i
   // Ensure that the current elevation is in the validity range. (nodata and/or inconsistent elevation)
   if ( ( histogram->GetMeasurementVector(idx1)[0] < -413 ) && (histogram->GetMeasurementVector(idx1)[0] < 8850) )
     {
-      return -1
+      return -1000;
     }
 
   // Compute the total number of pixels on the altitude bin
@@ -331,7 +331,7 @@ short get_elev_snowline_from_bin(const itk::Statistics::ImageToHistogramFilter<i
     }
   else
     {
-      return -1;
+      return -1000;
     }  
 }