i sometimes help a fellow dept. (the civil dept.) and they suggested a routine that they say doesn't exist. So i did create a limited lisp routine to help. what they want is to place multiple raindrops (say 100 to 500) and track each to it's destination, THEN (this is the unavailable part) group them by basins of drainage (i loop a spline around the various areas). does anybody know of commercial third party (that is more robust and flexible) that can do this? i have checked the internet, a few seminars and vendors, to no avail.