Extracting a Terrain Boundary to a linestring in ORD

Hi,

I have a Terrain Surface which has a boundary.

I'd like to Extract the Boundary to a Linestring then edit the Linestring into 3 boundaries to reduce the size of the DTM.

I have seen cryptic descriptions of using a Mesh and the extract Boundary Tool for a Mesh.

I don't have a Mesh, and my drawing is too large to replicate the DTM as some sort of Mesh (whatever that is).

I'd simply like to extract the boundary as a linestring straight from my DTM surface (Terrain).

Is this possible in ORD without going through some sort of mesh creation, mesh "top" thingy that some have eluded to.

This was possible in InRoads using the Extract 3-D feature (or something like that).

Thanks

Andrew