[GRASSLIST:7620] Re: surface interpolation

[Maciek Sieczka]

From: "Ksenia Konwicki, RPF" <kes at timberline.ca>

> Good morning,
>
> I am a new GRASS user, and I am trying to create a DEM to do surface
> analysis on. We have elevation data that was collected / designed for
> use as a TIN with breaklines. I have checked out the various surface
> interpolation modules in GRASS 6.0, but I can't find anything that would
> incorporate point and line data to create a surface.

E.g. v.surf.rst can do it, but first you have to connect your points and
line layers into one layer, where the elevation value would be stored in the
same column. The problem is that the Grass command for patching vectors,
v.patch, is not completely ported in Grass 6 yet and currently patches only
the geometry and preserves categories of both input layers, so you will end
up with a layer where objects have no attributes and some of them have the
same cat, so it is quite impossible to attach elevation to such a datatable
manually afterwards in case of bigger datasets. The other module, v.append,
available on Grass wiki, should do fine if both input layers have exactly
the same structure of datatables, but is really slow. Eg. I tried to
patch two 3" SRTM tiles converted to vector points. On Athlon-XP 3000,
1GB RAM after the whole night the progress was 2%. Anyway, at least it
works - thanks to Michael Barton.

Yet there is an option to import your layers into Grass 5.4, use v.patch
there and port back into Grass 6 (v.convert) for further processing.
Although the vector engine of Grass 5 is much worse than in 6, v.patch plays
the role fine there, being able to populate attributes from input to output
properly.

Maciek