[GRASS-user] calculation valley floor width in GRASS or some index indicating valley floor width?

[Hamish]

Helmut wrote:

>>  any hint or pointer to some grass gis procedure/literature/web
>>  link/etc how to caculate valley floor width in GRASS or some index
>>  indicating valley floor width.
>> 
>>  the idea is to get some measure how wide or narrow a valley is.

JWDougherty wrote:
> That is a very involved question, starting with how you define the
> valley floor quantitatively to begin with.  You could calculate an index
> of concavity or average slope and plot the point where the measure (e.g.
> concavity of slope, or average slope) drops below a certain value that
> would be a valley floor boundary by definition.  There should be
> several grass tools that could be applied.

e.g. run 'r.param.scale param=feature' to pull out channels, then
r.watershed to make a rivers map and r.cost to calculate distance
from the center-line (rivers) to the edge of the valley. Of course
rivers are not always in the center of the valley, perhaps take the
r.param.scale valleys and run r.cost to find the cost to the middle.
Then extract the 'ridge lines' of the distance-cost map, again with
r.param.scale, and double the peak values along the ridge.

It is very similar to the classic 'river mile' problem, to define well
a line down the center of the channel when nature isn't as simple as
a pipe and valleys merge, islands happen, etc.


Hamish