Welcome to OSM! The footprint you added here represents a building in the imagery, well spotted! I think this building is actually square becasue the shadow it casts is straight. See how I mapped it in https://osmcha.org/changesets/171898153

If you want to map a round feature press o, (or right click and select the relevant function) after tagging it to circularise it.
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            #REVIEWED_BAD #OSMCHA
            Published using OSMCha: https://osmcha.org/changesets/171897448
            