The Viscount had some great info!
Depending on how serious you are about mapping...
Read some articles about cave mapping. That's as close as you will get to tunnel mapping and the techniques will be the same.
Take measurements and sketches as you go, then create your map when you get home. Maybe use 3D modeling software like 'sketch up' to create computer models. Google "cave mapping software". Some interesting results.
A couple of mapping tools if you are a bit OCD.
- A sighting compass to accurately gather bearings
- 100' tape for distances (laser measure if you have the $$$ since they can measure larger distances)
- Inclinometer if you want to record drop / rise
I've gone mine exploring with a guy who was AMAZING at mapping mines. His diagrams were works of art in their own right. It takes time and practice to develop the skills.
Abby Normal