s.surf.tps

[Precision Farming Project]

In article <199603252141.PAA04862 at rogue.gis.uiuc.edu>, 
helena at gis.uiuc.edu says...

>> [...]
>>  If I somehow stumble across a
>>set of parameters (dmin, tension, smooth, segmax, npmin) that
>>will surface through 6 points, and immediately re-run s.surf.tps
>>with EXACTLY the same parameters, it only surfaces through 3 points.
>>This seems likely to be a problem of using some value somewhere
>>in s.surf.tps BEFORE it is set.
>
>Has anybody used s.surf.tps with this version of GRASS?
>Do you get any warnings or error messages from
>the program, have you checked your region and resolution properly
>(run d.sites with the same region you are using when running
>s.surf.tps)

Using d.sites and d.rast, the 1, 2, 3 or 6 data points it
manages to fit are all within the displayed region, as are
the 34x sites it says are not in the region.


>The selection of points which are taken for the interpolation is 
 completely
>controled by g.region. The program would remove the data points
>within the given region if they are closer to each other than dmin,
>but that would mean that your region has only couple of rows and
>columns when you get only 6 points, so I guess that the problem
>is in the region.

I can set d.min to ridiculously small or large values, based on
the point density, with NO effect on the number of points
chosen (well, if I set d.min to more than about half that maximum
distance in the region, it only finds one or two points, instead
of 3 or 6).

The fact that re-running the program with the SAME DATA and the
SAME command line parameters, and yet the ``supposed'' number of
valid data points changes between the two runs leads me to
believe that the problem is not with the data or the command
line parameters.  The most likely suspect is (IMHO) the use
of some variable in s.surf.tps before it is set.  Was s.surf.tps
run through lint at some time (this would catch this kind of problem).


>
>Helena Mitasova
>GMS Lab
>University of Illinois
>

Gordon Haverland, P.Eng.
Precision Farming Project
Alberta Agriculture
ghaverla at freenet.edmonton.ab.ca
goddard at gpu.srv.ualberta.ca