[Qgis-user] How slope is calculated in this 3*3 pixel example?

[Pawel Stankiewicz]

Hi

I've cropped GTOPO30 W020N90 to 3*3 pixels geo tiff:
https://docs.google.com/file/d/0B9DMholuObszUWVhU0xiQlBzVTQ/edit?usp=sharing
It contains the steepest slope in Europe, in Switzerland, at 711 m
resolution.

I reprojected W020N90 to Lambert azimuthal equal area projection with
central meridian set on 9 E.
So the unit is meter

Elevations:
1709 1819 1972
2061 2273 2645
2967 3499 3362

Differences with regard to the central pixel:
564 454   301
212 _0_   372
694 1226 1089

In the metdata tab pixel size is 711*711 what is a geometric mean from what
one can masure 765 N-S * 662 E-W.
So using official pixel size slopes in % with regard to the central pixel
are:
     64
30 _    52
    172
Right?
Well, according to QGIS the central pixel slope is 110.
Why? How?

Kind regards



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