Not sure if it's noted in the areadocs, but this is a worthwhile change for anyone making changes to the maps.
In magic.c, find the teleport spell.
map = ( number_range( 1,3 ) -1 );
map = number_range( 0, MAP_MAX-1 );
This means you'll never have to worry about changing it again
Course, this also means that for every map you add or remove, you MUST make sure you modify overland.h