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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 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. – whuber Sep 11 '12 at 21:56
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! – giohappy Sep 12 '12 at 15:50
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. – whuber Sep 12 '12 at 18:41

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