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

GRASS GRASS user
1

heloo

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 ?

regards

--
Ahmet Temiz

--
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.

2

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 ?

Temiz

Can you put a sample + region settings + v.surf.rst syntax so somebody
can try to reproduce?

Maciek

3

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