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Occasionally experimenting in arcmap with openstreetmaps basemap and in google maps i noticed something strange. I measure the distance of 2 locations in google maps (screenshot 1) scrennshot1 and on another website which uses osm service http://www.pifpafpuf.de/cycleroute.html (screenshot 2) enter image description hereand the distance appears to be correct nearly the same. When i measure the same distance in arcmap 10.3 (screenshot 3) enter image description here with the measure tool Line measurement (Planar) it changes a lot , exactly there is a difference of nearly 50 km. I know this maps use the same projection method WGS_1984_Web_Mercator_Auxiliary_Sphere EPSG 3857. I do not understand why does this big difference occur.

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    That is a big difference. Are you sure you're measuring in the same spatial reference and using the same method (ArcMap is using planar, perhaps the others are geodetic). What are you hoping to use the measurement for? – Michael Stimson May 23 '16 at 23:38
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    You are so right (facepalm) i just noticed and wanted to answer my own question i was using planar instead of geodetic. – Marian Pepa May 23 '16 at 23:39
  • That's it. Now answer your own question to guide future users. – Michael Stimson May 24 '16 at 0:05
  • I was hoping you would do it , you were faster hahah. – Marian Pepa May 24 '16 at 0:15
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A lot of online services display in WGS84 Web Mercator, which calculates distances terribly, so they calculate distance along the ellipsoid (geodetic distance) in the background, rather than the direct planar distance. (Whether they are using the correct ellipsoid formula or not is for another question.)

ArcMap is more of a professional tool and makes no assumption, so distance calculations are done as-is; it does not check whether the projection is a good fit for the data or not.

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