You need client geometry or something that can "mimic" them...
Dawn Of Light server currently have no idea of what the world looks like, for the server all Regions are made up of Zones which are Empty Cubes (65k units by side)
Getting Z-axis ground level is not possible without building some kind of module for DOL that would have data to match the client geometry !
One possibility is to try loading client assets inside Regions (read client files at startup and keep terrains/models in memory), but it will be very memory hungry.
An other Possibility is to build a navigation map out of client files and only keep a simple model with areas where npc/player can "walk"
A minimal version of navigation map would be to use terrain height-maps with an interpolation algorithm without building the whole triangle mesh, this will not work for dungeons or cities, this will not detect buildings / trees / bridges...
If your skilled enough in C# and Geometry you can try to look at my Client Lib project :
https://github.com/dol-leodagan/DaocClientLib
It's not ready to use, I'm having a hard time trying to guess what the client files means, and how to import every relevant in-game content to have an accurate navigation map !