Why is cave surveying harder than land surveying?

Answer 1

• Lack of visual site distances is a big challenge.
• A compass often doesn't necessarily work well underground as iron deposits affect things.
• GPS doesn't work well underground.
• Lack of a visual horizon makes it difficult to stay oriented.

Also,

Difficult to "loop" surveys to check results. Caves/tunnels are linear.

Working underground is more hazardous (rock falls etc).

Transporting equipment is harder.

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From experience, the techniques differ so it's not a very fair comparison.

Iron ore bodies and ferruginous intrusions can affect compasses, in a few areas, but it's not a very widespread problem.

GPS does not work underground full-stop, any more than a portable telephone or other portable radio will work, as these use VHF radio signals that do not penetrate the ground to any extent. Cave radio-location of spot points from the land survace above them, uses a VLF induction method. GPS comes into its own for tying the cave survey to the surface topography, by locating the entrance(s) and other associated features.

The lack of a horizon is never a cave-surveying problem, because you sight the compass and clinometer at a lamp placed at the next survey point, not at the cave feature itself. This also negates the first answer about lack of visual site distances, although zig-zagging meanders with very short sight-lines are very hard to measure accurately over their full length thanks to many small but cumulative and random errors.

Caves are linear. Some are. Most contain at least one loop or other closure-giving feature, and there are well-proven methods for distributing the errors in closing loops. However accuracy is indeed harder to obtain in a purely linear passage.

"Hazardous". Well, yes, caves do contain natural, objective hazards, though rock falls are rare. Most caving accidents are personal slips, trips or falls. Anyway the surface landscape can carry its own hazards too!

Difficult transport. No more so than any other caving equipment. Less so if anything. Don't forget a cave can only be surveyed by cavers of sufficient experience to negotiate the place anyway! The basic surveying tools are water-resistant notebooks & pencils, fabric builders'-type tape-measure, compass and clinometer; and the last 2 are compact devices carried on lanyards worn round the neck.

Modern cave surveying increasingly uses small, portable electronic range-finders like the Leica "Disto" (trade-mark), which uses a low-powered laser. These are used by measuring to chosen points on the cave walls, or heights to unreachable ceilings, rather than target-lamps.