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I've started to read a bit more into coordinate systems, projections and the problems around them and so far I understand that in the WGS 1984 Web Mercator (auxilliary sphere) coordinate system a scale bar on a map layout cannot be correct (unless the map extent is at the equator).

https://support.esri.com/en/technical-article/000011356

Now the part I need help understanding: Web mercator is the coordinate system used by Google Maps and most other online map services, so how is it possible they are showing a correct scale bar in their maps (or are they not)? I'm probably misunderstanding something here, so I would be very greatful if someone can explain this to me!

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  • Google Maps no longer uses Web Mercator see gis.stackexchange.com/questions/302795/…
    – nmtoken
    Commented Mar 17, 2023 at 9:57
  • From the tweet this post is linking to it seems like this is the case for the globe view, but for the "flat" version it looks the same to me or at least like a very similar projection (also not an equal area one so it should come with the same measurement problem)
    – danielaandrea
    Commented Mar 17, 2023 at 10:07

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If you compare https://www.google.com/maps/@0,20,12z and https://www.google.com/maps/@60,20,12z, the scale bar is much longer in the latter. Google maps know the distortions of the coordinate system and compensate it by the latitude when drawing the scale bar.

Scalebar at latitude 0

enter image description here

Scalebar at latitude 60

enter image description here

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  • So the problem in a map layout in ArcGIS ist that the scale bar ist static and not automatically adjusting like in Google Maps?
    – danielaandrea
    Commented Mar 17, 2023 at 11:30
  • Right. But even adjusting the length of the scalebar is a useful workaround for web maps it would not remove the problems with measuring lengths and areas in EPSG:3857.
    – user30184
    Commented Mar 17, 2023 at 11:54
  • Okay thank you, I understand things better now! I also just found out that in the layout options of ArcGIS Pro there is an option for the scale bar called "Compute at center" which calculates the map scale using the center of the map frame and when I check it, the scale bar is correct for my extent (very large scale map but quite far away from the equator). So that seems like a solution for giving a correct sized scale bar for the map layouts my organization is producing, no?
    – danielaandrea
    Commented Mar 17, 2023 at 13:44

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