Logbook entry: Whoops... magnetic anomalies (2009-12-08) at Hut cave system
About happenings on Dec. 8, 2009, Aaron Curtis wrote:
Nial and Aaron headed over to Hut Cave for a first stab at surveying using their beautifully calibrated DistoX / Pony system. Things seemed to go well for a few legs, but then one leg taken from the rock came out in completely the wrong compass direction. There was much head scratching, and tests commenced.
A series of repeat forward and reversed legs back and forth between two rocky points in the chamber to the right of the entrance from the hut produced a zig-zag pattern on the survey instead of overlapping lines, indicating a difference of 13 degrees in compass reading at the two points (3m apart). A similar experiment in a different part of the cave also gave a zigzag pattern, with a smaller offset. Aaron then tried holding the DistoX to his chest and then moving it towards the rock at arms length, maintaining it parallel, and found a 6 degree change in the compass reading.
Feeling rather annoyed with phonolite and physics in general, the surveyors returned home and started working out an alternate method of surveying. Three ideas came up. 1. Replacing compass readings with a relative angle between legs using a transit or protractor. 2. (Proposed by Clive O) Increasing the number of legs between survey stations to the point where triangulation of the entire survey would be possible. 3. (Proposed by Bill M) Taking both forward and reverse compass readings from each point, subtracting them, and recording the angle between legs. Since the declination due to rock magnetic properties should be consistent given a small enough movement of the DistoX, this should cancel out the magnetic anomalies.