I have a lot of difficulty with this, and I think a few additional features would help:
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when a co-ordinate is selected, its position displayed somewhere, preferably in a small box next to it.
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keyboard nudging of the selected co-ordinate point. Useful when the bounds of a particular zone need to be very precise.
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with a point selected, the Tab-key will select the next one, clockwise. Shift+Tab; anti-clockwise.
mac