[GRASS5] Re: [GRASSLIST:7896] Slope and Aspect Algorithms in GRASS

Helena Mitasova 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)


   Beaudette wrote:
> Greetings,
> After reading a recent publication on the pros/cons of various algorithms for 
> calculating slope and aspect from gridded data [1], 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:
> r.slope.aspect
> r.param.scale
> v.surf.rst
> r.resamp.rst
> According to the publication [1] , 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 
> r.slope.aspect)
> thanks!
> References:
> 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, .
> http://www.sciencedirect.com/science?_ob=GatewayURL&_method=citationSearch&_uoikey=B6VHC-4G1WYF2-1&_origin=SDEMFRASCII&_version=1&md5=ed8513abe73e9f178164c3bb35ca4ba3

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