Sorry for the late response, busy weekend.

a) Identify a point on the exterior of the line set.
b) Walk around the exterior of the line set till you get back to start.
This identifies all the lines required to define the exterior; and then ones that you can remove are all those not belonging to the exterior.

I thought about that while in bed but wasn't sure what the best way of doing that would be but I think I can work with what you've given me and have it working tonight.

Yeah, I am assuming he has a triangulation of the lines rather than arbitrary polygons, or just a list of lines.

Correct, I should have mentioned this.

The data is coming to me triangulated and is for multiple purposes, "identifying the exterior lines" for my purposes would be a better way of putting what I wanted to do moreso than "removing these interior lines".

Sorry for the late response, busy weekend.

a) Identify a point on the exterior of the line set.
b) Walk around the exterior of the line set till you get back to start.
This identifies all the lines required to define the exterior; and then ones that you can remove are all those not belonging to the exterior.

I thought about that while in bed but wasn't sure what the best way of doing that would be but I think I can work with what you've given me and have it working tonight.

Yeah, I am assuming he has a triangulation of the lines rather than arbitrary polygons, or just a list of lines. Triangulation would probably be a good first step though.

Correct, I should have mentioned this.

The data is coming to me triangulated and is for multiple purposes, "identifying the exterior lines" for my purposes would be a better way of putting what I wanted to do moreso than "removing these interior lines".