Here the green point is on the midline between Beginning and End and is over land. So I know that the coast between Beginning and End forms a peninsula. If there is water, I have a bay:
(In practice, it's better to check a number of points along the line between B and E and make sure they're all land, because in some situations where B and E are straddling a long narrow bay the midpoint might be land with water to one side.) Once a peninsula is detected, the next step is to determine where to anchor an island chain and which direction it should point. This starts by placing a new point on the Middle point of the coastline between Beginning and End:
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