ArcMap lets you put a scalebar inside a layout referncing a WGS84 data frame. How is the scale calculated? Is it a "mean" scale? And, in case it's right, what "mean" scale could mean?
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It is the scale at the equator.
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1+1 I was able to confirm this by creating a long horizontal window and panning vertically within it, going from south pole to north pole. The scalebar never changed and gave correct distances at the equator.– whuberCommented Sep 11, 2012 at 21:56
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Thanks a lot. QuantumGIS uses the Harvesine formula calculated at the middle latitude of the mapcanvas extent. It's a much better aproximation. I see many map printings in geographical CRS (strait lon/lat grids) with metrical scalebars. It's nonesense, besides ArcMap caluclates that scalebar as if it was on the Equator!– giohappyCommented Sep 12, 2012 at 15:50
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Not quite nonsense: the scale varies with orientation. In the north-south direction, ArcMap's scale is correct at any latitude. The QGIS solution is a compromise but arguably is not any more correct.– whuberCommented Sep 12, 2012 at 18:41