Sometimes when I'm building a sim, the owner wants things kept hidden from the world map until they're ready to reveal. The map pics the highest objects that are under 400M to display. Taking advantage of the ancient SL art of skywriting, we can protect an island sim from prying eyes.
The easiest covering is a 256x256x1 phantom prim with this script in it.
on_rez( integer p )
vector targetposition = < 128, 128, 390 >;
while (llVecDist(llGetPos(), targetposition) > 0.001)
When the prim is rezzed, it walks its way into position. And after the next mapper pass, curious eyes will only see that big boring prim.
Unfortunately, 400M is low enough to shadow the whole sim at ground level. A prim has to be 40% or more transparent to not cast shadows. After another mapper pass, I should know whether or not skywriting will still work at that transparency.