[R-sig-Geo] How to assess orientation of a topographic feature?

[Michael Treglia]

Roland,

Do you have these features already defined and such? One thing that might
help in some of this is calculating the Multiresolution Index of Valley
Bottom Flatness (MRVBF) - it identify valleys and ridges in a quantitative
way.  I'm not aware of an implementation of this directly in R, but it is
implemented in SAGA GIS (
http://www.saga-gis.org/saga_module_doc/2.1.3/ta_morphometry_8.html) and
can be scripted through the RSAGA package (
https://cran.r-project.org/web/packages/RSAGA/index.html).

Potentially you could define your ridges and valleys using MRVBF or other
preferred method, then vectorize those areas, and then calculate the
average aspect for the area.  This average would represent the direction
that the feature runs, assuming it is a fairly linear feature.  (if you
have a valley or ridge that runs north/south, most slopes will be facing
west (270 degrees) or east (90 degrees); the average would be approximately
180 degrees (south), indicating that the feature runs North/South. If
features aren't linear, you'll have to think about the implications of
that, and consider breaking features up into multiple components or
something... depends on what you're ultimately interested in.

This is just an idea of potential workflow... can obviously be done in
various ways with various tools, but is definitely scriptable in R using
combination of RSAGA, sp, and raster packages.

hope that helps,
mike

On Fri, Nov 13, 2015 at 11:37 AM, Roland Pape <roland.pape at uni-oldenburg.de>
wrote:

> Dear list,
>
> is there any means to (automatically) assess the orientation of
> topographic features like a ridge or a valley based on a digital elevation
> model? To calculate the aspect (as direction of largest change) yields
> obviously not the wanted result, but what about using the direction of
> smallest change? Any help is greatly appreciated!
>
> Kind regards,
>
> Roland
>
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