Geographic boundaries in GRASS raster/vector files

[Eric G . Miller]

On Fri, Apr 07, 2000 at 07:33:32PM -0700, Rich Shepard wrote:
> 
>   The layer boundary is defined by four values: "north", "east", "south" and
> "west". The values (represented in the documents and the spearfish data set)
> are in meters and represent the geographic position of the outside edges of
> the cells (not the center). That I understand. However, GRASS originally
> developed in a simple world where all data sets were projected in UTM. This
> results in nice, rectangular map layers with perfectly horizontal and
> vertical edges.
> 
>   However, almost all of our work is in State Plane Coordinates. As a matter
> of fact, several regulatory agencies specify this projection. Well, SPC
> results in a layer which is not orthoginal to the sides of the monitor in
> which it is displayed. For example, the upper left corner has a northing
> value less than that of the upper right corner (same for the lower corners).
> At the same time, the upper left corner has an easting with a lesser value
> than the lower left corner.
> 
>   Given this lack of uniformity in values, how to I enter values when
> asked by the program? In other GIS software, the bounds are represented by
> the (easting, northing) values of the corners, not the edges. Wouldn't it be
> nice if GRASS understood corners, too?

There's no difference between corners and edges. Other software just
takes the least/greatest values for min(x,y),max(x,y) to form the
bounding box -- grass refers to edges.  So you just take the extrema and
use them as your edges.  You might add a little to get an even cell
size.  Maybe make a MASK layer to block out those areas outside your
area of interest -- you could use an SPC boundary map to create one.

I've yet to get the State of California (as a whole) to fit in a UTM
projection ;-).  So, we use an Albers Equal Area Projection.


-- 
+----------------------------------------------------+
| Eric G. Miller                        egm2 at jps.net |
| GnuPG public key: http://www.jps.net/egm2/gpg.asc  |
+----------------------------------------------------+