[GRASS-user] v.surf.rst generates negative value

[Hamish]

temiz wrote:
> I created DEM using spline and tension.
> 
> But although contour map does not have any negative value, v.surf.rst 
> generates negative altitudes.
> 
> can you tell me why ?


The spline is a multi-dimensional curve fit to your data. If there is a
sharp corner (e.g. cliff ending at a beach) the curve's momentum may
continue the surface beyond the extent of the given data (ringing). 
Also look for two very closely spaced input points which have
significant divergence in value, and/or adjust the tension.

see  http://en.wikipedia.org/wiki/Gibbs_phenomenon
     http://en.wikipedia.org/wiki/Image:Synthesis_square.gif



suggested "fix":
  r.mapcalc "new_map = if(old_map < min_value, min_value, old_map)"


Hamish