Ruth:
Cool! I visited Scotland in 2000 and spent most of my time in the West of Scotland. Isle of Skye, Isle of Harris & Lewis, etc. Beautiful country!
Anyway, I created an ArcMap document, set the coordinate system to British National Grid and then I created a square polygon on top of the town of Ullapool that is 500m by 500m and exported it to a shapefile. I zippped the shapefile and uploaded it to AGOL. I zoomed to the polygon and measured it corner to corner in the east/west direction and north/south direction. I get 500m plus or minus one meter (mouse clicking accuracy) every time. See attached shapefile and give it a try yourself.
I'm fairly certain that the display coordinate system in AGOL cannot be changed from Web Merc. Do you think that it is purdent to be using Google Maps as your gold standard for accuracy of measurements? Do you have ArcGIS for Desktop (ArcMap) available at your desktop?
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