Hi Grasshoppers
thanks to everyone who replied to my question about boreholes a few
weeks ago.
I am trying to link GRASS to a ground-water flow model. At the
moment am a considering the problem of representing geology within
the GRASS database. The model I am using divides the geology up into
3d cells with constant properties (eg hydraulic conductivity) within a
cell. Rasters are two dimensional, so the only two ways I can see of
representing geology are as a series of planar slices through the
geology, or as a series of "DEMS" following strategraphic surfaces.
Either way I can see a lot of problems looming, particularly with
maintaining consistency between raster layers as GRASS has no way
(that I know) of linking rasters except by giving them sensible
names; eg layer_1 layer_2 etc
At the moment I favour using strategraphic surfaces as this fits the
model I am using most closely, and GRASS is quite rich with surface
interpolation routines. I am concern however that when I
change region, resampling will lead to layers intersecting, a kind of
3d analogy to the problem of slivers in vector data.
my head hurts 
Has anyone addressed this problem before?
You can probably tell I'm no geologist or database engineer, comments,
discussion and references welcome 
much cheer Tom.
Tom Charnock
Dept Civil Engineering
Aston University
Birmingham B7 5NJ UK
charnotw@aston.ac.uk