[GRASS-dev] using slope/aspect functions inside our module

[Helena Mitasova]

On Jul 9, 2007, at 3:47 PM, Dylan Beaudette wrote:

> On Wednesday 04 July 2007 08:41, Glynn Clements wrote:
>> "Carlos \"Guâno\" Grohmann" wrote:
>>> Interesting.
>>> Why r.slope.aspect uses
>>>
>>> dx = ... / (4 * ewres);
>>>
>>> while r.shaded.relief uses
>>>
>>> dx = ... / (8 * ewres);
>>>
>>> ??
>>
>> Oops. It should be 8; the ewres/nsres values are actually the
>> distances between the centres of the top/bottom rows and left/right
>> columns in the 3x3 window, which are two rows/columns apart. IOW, the
>> 4*ewres should have been 4*(2*ewres) = 8*ewres, ditto for nsres.
>
> Yikes, does this mean that previous calculations are now wrong?

no - it is correctly implemented - you would have found that it is  
wrong by now.
It is just that the variable that you divide with in the r.slope.aspect
uses 2*ewres (twice the resolution, not just one as is usual in
the published equations) - I got caught on that many times when checking
the r.slope.aspect code.

Helena

>
> dylan
>
>
>
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