[R-sig-Geo] Best way to plot cross sections of discrete-valued grids

[Waichler, Scott R]

I have a 3-D grid where the values of the cells are a small number of
discrete values.  (The grid represents subsurface soil and geology
types.)  Contiguous regions of the same value are relatively few in
number and large in size in comparison to the whole domain.  I am
looking for advice on two things:

1)  What is the best way to sample the grid along any transect defined
by a line between (x1, y1) and (x2, y2), and plot the 2-D vertical cross
section?  I would like to sample not only in the orthogonal directions
defined by the grid but along any other compass bearing as well.
Lattice plotting capability is desired.

2)  Is it better to plot the same-valued regions as polygons instead of
discrete cells?  EPS plots using levelplot() tend to show the grid as
faint lines, and have large file sizes because data on each cell is
retained instead of data for single-valued regions that tend to be
relatively large.

Thanks for any help,

Scott Waichler, Senior Research Scientist
Pacific Northwest National Laboratory
scott.waichler _at_ pnl.gov
http://hydrology.pnl.gov