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

[Glynn Clements]

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.

The nonlinear nature of the median can exacerbate discontinuities. The
median chooses a single value from the set of inputs; it doesn't
average them.

This can make it a poor choice for a smoothing filter, particularly if
you are going to calculate derivatives (e.g. r.slope.aspect) on the
result.

If you want to eliminate outliers, a better option might be an average
over some quantile range (e.g. 20%-80%), or a weighted average where
values near the median get a larger weight.

If you're trying to remove spatial noise, you normally want a filter
which gives greater weight to the values nearer the centre. To avoid
"contamination" by noise, one method is to use a variety of linear
filters and take the median of the results.

-- 
Glynn Clements <glynn at gclements.plus.com>