On Dec 5, 2008, at 1:08 AM, <grass-user-request@lists.osgeo.org> wrote:
Message: 7
Date: Thu, 04 Dec 2008 18:37:56 +0000
From: John Stevenson <john.stevenson@manchester.ac.uk>
Subject: [GRASS-user] r.neighbors, wide filtering
To: GRASS user list <grass-user@lists.osgeo.org>
Message-ID: <49382384.6010206@manchester.ac.uk>
Content-Type: text/plain; charset=ISO-8859-1; format=flowedHi,
For my research, I am testing a mesh denoising algorithm on topographic
data. It smooths the surfaces much like using r.neighbors
method=average or r.neighbors method=median, and, depending on
settings, gives similar results to r.neighbors when size <5. The
advantage of the algorithm is that it has some ability to preserve
features. In cases where more significant smoothing is necessary
(r.neighbors size > 5) it is much better at preserving minimum and
maximum elevations etc as it converges on a stable solution for the
smoothed landscape.My question relates to understanding in which cases is it necessarily to
smooth to such an extent? From what I have seen e.g. taking speckle out
of SRTM DEMs, smoothing by such extremes removes a lot of useful
information and results in unrealistic surfaces.Has anyone come across a situation/dataset or type of analysis where
they need to smooth with r.neighbors (method=average/median, size > 5)?
I would be interested to know, and to see if this algorithm would be useful.
John,
In fact, we were using r.neighbors median smoothing with size=7. We have created a recursive script to model surface erosion and deposition on large landscapes over long time frames (centuries). We have been using r.terraflow or r.flow to model water accumulation as part of the script. However, both of these create rapidly growing spikes and pits due to approximations in the flow calcuations. We needed a 7x7 median smoother to take these out each cycle.
We have just switched to the new r.watershed (up to 80x faster than the old version in some tests). This is now considerably faster than r.terraflow AND much more accurate. It does not produce the same spikes and pits. Over short time frames (decades) no smoothing is needed; over large time frames (centuries) we hope to get by with much smaller smoothing windows (we are testing 3x3 now).
Michael
____________________
C. Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University
Phone: 480-965-6262
Fax: 480-965-7671
www: <www.public.asu.edu/~cmbarton>