[GRASS-dev] question about any July changes in hydrology functions

[Michael Barton]

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" <glynn at gclements.plus.com> 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