[GRASS5] raster inaccuracy

[Hamish]

> I use Grass in part to support finite difference ground water models.  There
> is a substantial inaccuracy in Grass' raster representation that I've known of
> for some time.    The problem surfaced again recently, caused errors in my
> reported results and required me to repeat several hours of work at
> substantial costs.  I think this problem has been present from early betas of
> 5.0 through 5.3 (our current installation).
> 
> The png file at
> 
> http://members.spinn.net/~roger/mismatch.png


is your region something like this:

e=100025
w=200075
n=300025
s=400075
res=50


or more to the point is your input data like the above and your region like:

e=100000
w=200000
n=300000
s=400000
res=50

?

as a test, do

g.region res=25  # or res="some common denominator"

and see if things start to line up in the XDRIVER.
(the PNG driver is much less tested than XDRIVER, maybe other bugs..)


I've had the same trouble with importing model data; setting the region 
resolution to half, or setting the bounds as the first example above (25-75)
fixed it.. but you have to be careful it doesn't inadvertently switch back,
also when you do overlays with maps which use a 0-50 bounds settings the 
computer is doing what you thing it is doing...


maybe it?


Hamish