r.slope.aspect

[Michael Shapiro]

I, too, would like to know if there is something wrong with the alogorithm.
It is based on Horn's method:

    Horn, B. K. P., 1981, Hill Shading and the Reflectance Map,
    Proceedings of the I.E.E.E., 69, 14.

You could create 3x3 maps (using r.in.ascii) and run r.slope.aspect
on these artifically created maps to see what the center cell aspect
for each map is.

There is a new version of r.slope.aspect ftp from moon.cecer.army.mil.
It uses the same alogorithm for slope and aspect, but rather than
creating classes of aspect it creates apsects from 0-360 degrees.
Get the new one and see if it has the same bias you found in the current
one.


|
|
|Recently I wanted to find out what the relationship between archaeological
|sites and aspect was in an area of the southern Netherlands. So I ran a
|report on site occurrence. I noticed that r.slope.aspect seems to prefer
|the main aspects (E, NE, N, etc) to those aspect cats lying in between:
|in fact they are just about twice as popular. In my area I have 33% of 
|cells with no aspect, 34% with a main aspect and 33% with a minor aspect.
|Note that there are 8 major aspects and 16 minor aspects.
|
|So, if there ain't something wrong with my area, what's wrong with the aspect
|calculation? Did anyone else have comparable results?
|Solving this one would be a great help in my research, because I can't run
|a correlation test involving aspect as long as that map layer can't be 
|trusted.
|
|Martijn van Leusen
|
|


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Michael Shapiro                        U.S. Army CERL                  
email:   shapiro at zorro.cecer.army.mil  Environmental Division          
phone:   (217) 373-7277                P.O. Box 9005                   
fax:     (217) 373-7222                Champaign, Ill. 61826-9005
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