[_petronius]

Dumb question, maybe, but how long before that data becomes inaccurate? Or rather, are there any areas where the change in elevation for the purpose of this system could be big enough in a 30ish year timescale that it would cause problems?

I assume no geological process alters the land drastically enough, quickly enough, that you'd notice, but what about water-level changes (dammed rivers?), melting glaciers, etc? Is "hard" ground consistent enough that no human processes are going to cause the data to diverge from the database drastically without the chance to update the database with new topographical surveys?