Use the map to set land elevation limits.

I propose that the map making program set the and elevation limits on the mainland.

The map program can tell when it's seeing land or prims, so why not have it set a bit on each 4x4 sq meter block allowing you to lower the land level when the map program can only see prims.

This would allow people to cover there land with prims and then lower the land as needed to make basements.

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A simpler solution is to re-bake the center limit based on the current land level at some slow rate. Land levels move a lot over time in RL.

This would allow land owners to slowly dig a basement.

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I know this is only a stopgap measure as everyone wants privacy zones at negative coordinates, but I figure that will take time and this would reduce the clutter in the sky.