This a followup to my previous question: How to implement physics and AoE spells in an MMO game?.

There, we concluded that all physics have to be done on the server, and that I should use cylinders for calculations.

Now, how can I check for collision detection on a ground-to-player basis on the server? It's fairly easy if the ground is a flat space, I just check if the player's z coordinate is lower than some value and voila, but, what if the map/ground itself is a model? How do I know where hills are on the server-side? How do I know when object collisions happen?

I'm using node.js and socket.io.