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

[Helena Mitasova]

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" <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
>
>