[GRASSLIST:4892] conditional growing of a region

GRASS GRASS user
1

Dear all,

I have been trying to regionalise a DEM into particular landforms. In
this instance, I am after identifying terraces. My original attempt was
with a classification type method whereby I identified threshold in
variables such as slope gradients and flatness. While this is one
way of dealing with this question, I am a little disappointed by the
results because they don't respect the visual geomorphological
interpretation of the landscape. It is striking that terraces are chunks
of land with a particular aspect and slope value (shallow slope
obviously), yet thresholding the slope and aspect ignore the
topological adjacency of pixels. Therefore, I'd like to build an
algorithm that would work like the "magic wand" in photoshop. That
is a method that looks at the characteristics of a seed point and test
the membership of adjacent pixels according to some decision rule.

In practical terms, how does one go about comparing each pixel of
a buffer zone to a seed point. I found the r.grow module which will
do to expand the seed point by one pixel in all direction, but once
this ring of neighbours is determined, how does one test them one
by one? Or is there more nifty trick to deal with this?

Thanks for your help,

Thomas

Thomas Dewez
PhD Student
Dept Geography & Earth Sciences
Brunel University (West London)
Uxbridge UB8 3PH, Mddx
United Kingdom

Phone: +44-(0)1895-203215
Fax: +44-(0)1895-203217

e-mail: thomas.dewez@brunel.ac.uk

2

Dear all,

I have been trying to regionalise a DEM into particular landforms. In
this instance, I am after identifying terraces. My original attempt was
with a classification type method whereby I identified threshold in
variables such as slope gradients and flatness. While this is one way
of dealing with this question, I am a little disappointed by the
results because they don't respect the visual geomorphological
interpretation of the landscape. It is striking that terraces are chunks
of land with a particular aspect and slope value (shallow slope
obviously), yet thresholding the slope and aspect ignore the
topological adjacency of pixels. Therefore, I'd like to build an
algorithm that would work like the "magic wand" in photoshop. That is a
method that looks at the characteristics of a seed point and test the
membership of adjacent pixels according to some decision rule.

In practical terms, how does one go about comparing each pixel of a
buffer zone to a seed point. I found the r.grow module which will do to
expand the seed point by one pixel in all direction, but once this ring
of neighbours is determined, how does one test them one by one? Or is
there more nifty trick to deal with this?

You can get quite a long way with the indices used by r.mapcalc:

layer[-1,-1] is offset left and down (check the manual - directions are
not always intuitive!!) by one cell from the [0,0] cell.

You could also look at r.mfilter - by designing a filter matrix, you can
"package" the comparison style you would like without having to repeat
r.mapcalc scripts. I would try with r.mapcalc first to get a feel for
things, then convert your best filter to an r.mfilter matrix. For
instance, Chrisman's textbook has a filter example on p. 166 (1st edition)
of a slope filter:

TITLE 3x3 Sobel x filter
MATRIX 3
-1 0 1
-2 0 2
-1 0 1
DIVISOR 8
TYPE P

TITLE 3x3 Sobel y filter
MATRIX 3
-1 -2 -1
0 0 0
1 2 1
DIVISOR 8
TYPE P

with pseudo-code to filter:

slope.in.x = Filter elevation
      using "Sobel x filter";
slope.in.y = Filter elevation
      using "Sobel y filter";
slope = sqrt((slope.in.x ^ 2)
      + (slope.in.y ^ 2));

An r.mfilter matrix for taking the average of N, S. E, and W cells - one
step - is:

TITLE 3x3 rook's move
MATRIX 3
0 1 0
1 0 1
0 1 0
DIVISOR 0
TYPE P

r.mfilter gives lots of flexibility, matrices can be bigger, and you can
fine-tune the filtering to pick out the kinds of things you need -
provided they don't change too much across the region (aspect might if
valleys curve).

Roger

Thanks for your help,

Thomas

Thomas Dewez
PhD Student
Dept Geography & Earth Sciences
Brunel University (West London)
Uxbridge UB8 3PH, Mddx
United Kingdom

Phone: +44-(0)1895-203215
Fax: +44-(0)1895-203217

e-mail: thomas.dewez@brunel.ac.uk

--
Roger Bivand
NHH, Breiviksveien 40, N-5045 Bergen, Norway
(travelling but still accessible)