Tom Charnock writes:
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 etcAt 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 hurtsHas anyone addressed this problem before?
You can probably tell I'm no geologist or database engineer, comments,
discussion and references welcomemuch cheer Tom.
I haven't worked with models quite like yours, Tom, but I have given this
quite a bit of thought. Think of your collection of raster layers as being
a conventional tabular database. Each spatial location, represented by a cell,
would represent a database record. Each raster layer holds a piece of
information about that record - a field, if you will. The name of that raster
layer would be the heading for the field. You add additional fields by
creating new raster layers.
You might use one raster layer to define the upper limit of a geologic layer,
and a second raster layer to define the bottom, both in absolute elevations.
Another raster layer would hold one of the constants, such as hydraulic
conductivity. As additional pieces of information are needed for each spatial
location, new raster layers are created. r.mapcalc will be your primary
analysis tool, enabling you to conditionally mix and match the different
raster layers. As long as you are ultimately interested in outputting
*numbers* from your model, this approach should work well. If you want
to display data in three dimensions, you will have a tough time with it (unless
you have a Silicon Graphics machine, enabling you to use SG3d). You'll just
have to be creative.
I believe that your concerns about "slivers" are unfounded, since you are
really only dealing with values in a fixed matrix.
Hope that this gives you some ideas!
--
Malcolm D. Williamson - Research Assistant E-mail: malcolm@cast.uark.edu
Center for Advanced Spatial Technologies Telephone: (501) 575-6159
Ozark Rm. 12 Fax: (501) 575-3846
University of Arkansas
Fayetteville, AR 72701