[GRASS5] Re: [GRASSLIST:7896] Slope and Aspect Algorithms in GRASS
hmitaso at unity.ncsu.edu
Mon Aug 15 11:12:08 EDT 2005
the algorithms used in these programs are very well documented but it
would be great if we had a volunteer who would include the equations
into the code for the doxygen documentation. I will be happy to provide
the equations in latex format.
This is where you can find the equations and algorithms:
Neteler, Mitasova GRASSbook (see appendix B.3) http://mpa.itc.it/grassbook2/
M. Neteler, H. Mitasova, 2004. Open Source GIS: A GRASS GIS Approach.
Second Edition. 424 pages, Kluwer Academic Publishers, Boston,
Dordrecht, ISBN 1-4020-8064-6 (Also published as eBook, ISBN 1-4020-8065-4).
or get the papers listed in the manual pages (but the book has it all on
4 pages as close to the code as possible)
Also, there is a developer on this list (Brad, can you give us an
update?) who has added additional algorithm for computation of
slope/aspect into r.slope.aspect but it has not been submitted yet
(curvatures may be holding it)
> After reading a recent publication on the pros/cons of various algorithms for
> calculating slope and aspect from gridded data , I became very curious
> about which algorithms are implemented in some of the r.* and v.* commands .
> A link to the science direct webpage and article text is included at the
> bottom: note the long URL.
> For example, the following commands can generate slope and aspect maps:
> According to the publication  , there are a number of ways in which slope
> and aspect can be calculated (pages 4 - 6) : i.e. "three-point plane" , "four
> closest neighbors" , "eight neighbors unweighted" ...
> According to the paper, the "eight neighbors unweighted" algorithm, was the
> most robust (at least when tested with a Monte Carlo style error analysis)
> for slope, aspect, and topographic index calculations.
> I looked through the source for r.slope.aspect, and could not determine which
> method was being used.
> Do any of the developers know which algorithms are used for the various
> commands mentioned above to calculate slope and aspect values?
> I think that it would be a good idea to:
> 1. document the slope/aspect algorithms being used in the various r.* commands
> 2. add some code to allow for different algorithms to be used (maybe just in
> 1. Lynn D. Raaflaub and Michael J. Collins, The effect of error in gridded
> digital elevation models on the estimation of topographic parameters,
> Environmental Modelling & Software, In Press, Corrected Proof, Available
> online 27 April 2005, .
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