Exporting an entire terrain can be tedious, especially if you require data for what is outside the camera view.
An interesting workflow I have only discovered is to use a spherical camera to render your geometry export.
1. Position a camera at a point of interest, this could be at 0,0,0 but be mindfull to raise the altitude above the terrain.
2. Set it to Spherical.
3. Set your render quality, it will work with a square or lat-long ratio 1:1, 2:1 or whatever you want.
The obj that is exported will be in the round. The only caveat is detail will diminish in resolution the further away from the point of the camera.
Hope you find that useful :-)