Backsights (and lava caves)

[John Halleck]

On Mon, 20 Jan 2003, Bill Frantz wrote:

> [...]

> I wonder how many of the failures to agree in
> Lilburn (a marble cave), were the result of the many granite boulders which
> have washed into the cave, rather than errors on the part of the instrument
> people.

  If there are UNCORRECTED fore and back sights, and there are loops, it is
  a simple matter to identify the differences between blunders and magnetic
  anomolies.

  If there are uncorrected fore and back sights, and there are no loops,
  then it MIGHT be possible to tell the difference between blunders and
  magnetic anomolies.  But it takes a lot of statistics.

  If you don't have fore and back sights, or if they have been "corrected"
  or averaged, you are probably screwed.

> [...]

  And Bill is very right that there is an assumption here that the compass
  is in the same place for both the fore and back sight.  (Not always possible)