That is what I call a gridding effect -
check it out here - and this was done only with simwe and r.mapcalc
- no r.flow, r.neighbors, etc.
(some of the problem actually may be hidden in r.mapcalc computations)
http://skagit.meas.ncsu.edu/~helena/gmslab/oxfordms/oxf.html
you can see how it evolves here (you start seeing it after several
iterations - it is there from the start
but it is small at the beginning and just grows over time until you
start seeing it)
http://skagit.meas.ncsu.edu/~helena/gmslab/oxfordms/eledhedge.gif
I thought that smoothing will take care of it but apparently just the
opposite
We need to find out what to do about it - I think that
it is more related to the modeling approach than GRASS itself,
BTW as I understand it median is computed by dividing the values into
a finite number of classes
so the median from a continuous field would not change continuously
from cell to cell and
the effect you are getting could be expected especially for larger
number of cells.
Mean is computed directly from the floating point values - is that
right?
Helena
Helena Mitasova
Dept. of Marine, Earth and Atm. Sciences
1125 Jordan Hall, NCSU Box 8208,
Raleigh NC 27695
http://skagit.meas.ncsu.edu/~helena/
On Aug 1, 2007, at 11:39 PM, Michael Barton wrote:
Tests over the last couple days suggest that r.neighbors may be
the, or one
of, the causes. We lose most of the artifacts if we turn off
smoothing using
r.neighbors, and the artifacts are much worse with a neighborhood
of 7x7
than 3x3.
We're probably wrong about the date, however. This seems to only
show up
clearly in very long runs (a simulation of 50 recursive models) and
is most
pronounced with larger smoothing neighborhoods. Previously we'd
done a small
neighborhood of 3x3 and done most of our tests for no more than 10
iterations. We only did a couple of long ones and were looking more
at stats
from the output than the maps themselves. Now we are doing a number
of 50+
iteration runs (the most recent one ran for nearly 600 years simulated
time).
using a median smoother gives much worse results than a mean smoother,
though a median ought to be better the larger the neighborhood is,
since it
should not be affected as much by extreme values.
Our next test it to find out if there is some kind of issue with
region
setting that is interacting with the smoothing. Probably not, but
we need to
make sure.
I'll report more later after. If anyone is interested in seeing
what I've
tried to describe, I've posted images of the effects of different
smoothing
parameters at:
<http://www.public.asu.edu/~cmbarton/files/LandDyn>
Michael
On 8/1/07 7:34 PM, "Glynn Clements" <[EMAIL PROTECTED]> wrote:
Michael Barton wrote:
Helena also mentioned r.neighbors as a possible culprit, as I'd
forgotten to
mention that we also use this to smooth results.
Neither r.neighbors or lib/stats changed during the first half of
July.
__________________________________________
Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University
phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton
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