Are there any surveying issues regarding volcanic systems, ie magnetic anomalies, which needs be taken into account by budding surveyors?

I believe there are issues but that a fairly accurate magnetic survey can be obtained by taking fore and back sights and by staying in the middle of the passage (e.g. not near the walls). Ed W will tell you exactly how accurate the surveying can be as he has done quite accurate magnetic surveys in Iceland.

Multi entrance tubes or braided tubes will give opportunities to check the accuracy by a surface survey between entrances or around loops to check misclosure

I would guess that for real critical surveys then a theodolite survey is your best bet.

I know where there is a theodolite, Chris.  8)

Chris, I have sent you a little something which may be of help.

Thank you, Ed; very comprehensive - haven't read it all yet but shall do over the Chrimbley break when I'm not underground!

Did my first lava tube surveying today; we used a Disto and SAP combination and logged about 400m, yet to compile, in a maze-system.

cap 'n chris said:

Did my first lava tube surveying today; we used a Disto and SAP combination and logged about 400m, yet to compile, in a maze-system.

Click to expand...

Be interested to see the results 

er,...not that interesting, sadly; the side passages are omitted on this route to our pre-planned objective, but a result nonetheless.