Backsights (and lava caves)

[Ken Grimes]

At 07:47 PM 19-01-2003 -0700, P A Hill & E V Goodall wrote:
>
>I was wondering:
>
>Garry, have you ever seen the foresite and backsite not agreeing because of 
>a magnetic bias do to a instrument too close to either the 
>wall/floor/ceiling of the cave or too close to a peice of breakdown?
>
>Anyone else ever actually observed this in the field?
>

In the Lava caves of Victoria (Australia) I have had differences of up to 29 degrees between fore- and back-sights along a single leg.  Differences of up to 10 degrees are quite common.  Moving away from the rock may reduce the problem, but does not always remove it completly.

In one cave at Mt Eccles, called (surprisingly!) "North Pole Cave", there is a nice pointy rock at a bend in the tube that is ideally placed for a survey station.  All bearings taken from that point read due north.  As you move the compass around the rock the needle swings to always point to the top of the rock.

Fortunately my main objective has always been to produce a picture of what the cave looks like rather than an accuracte location of its parts for engineering/survey purposes.  So the survey line is just for control of the sketching.


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Ken Grimes                               Phone +61 +3 5573 4503
Regolith Mapping,  PO Box 362,  Hamilton,  Vic 3300,  Australia
Cainozoic Geology, Geomorphology, Karst & Speleology.
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