Backsights and default accuracy estimates

[Martin Green]

Hi,
The grade system was made up by the BCRA, I do not why they made there
decisions, but it is common practice here to use them.  I can not find a
link to a BCRA set of definitions, but here is a similar one.
http://malaysiancave.tripod.com/cave_maps_survey_grading.htm
My comments on the accuracy was a fairly hand wavy order of magnitude
approach, to demonstrate that the errors can get big quick, ie if your
errors manage to swing the cave around.  Therefore it is best to avoid that
approach unless it will give more accurate results.   After all in a long
enough cave, with an inaccurate enough measuring system, with 15 degree
magnetic shifts, it is still more accurate to assume that magnetic north is
absolute north than it is to measure angles. To properly solve the problem
would require a Baysian logic approach, with prior probabilities for
magnetic variations determined from a suitable geologist, and well
determined probability distributions of the surveyors when they read
instruments, then solved using a suitable optimisation alogrithem.
Any way we seem to be talking about the same things, and disagreeing.
But I would like to say that I think that a measurement of a position is
useless with out an estimation of the associated error.  Otherwise how can
you tell blunders from errors?  Wheather you are 10m from another cave or
within a hundred meters in some direction?  So trying to determine the error
in the general case scenario for people adhering to BCRA grade 5 surveying
is not at all moot, as they are unlikely to do it themselves, nor is the
volume of blunderless data available to make generalisation for all cavers.
Martin
(I am just about aware that n^1=n, and that the problem is a system of
probabalistic non-linear 3d equations, but aren't they ever so hard to solve
with out the odd approximation).