Backsights and default accuracy estimates

[John Halleck]

On Fri, 10 May 2002, Olly Betts wrote:

Forgive my slow reply, I've been out of the country for about 5 weeks...

> On Thu, May 09, 2002 at 07:05:04PM +0100, M.J. Green wrote:
> > Strictly with the grade 5 BCRA limits, errors should be called for greater
> > than 2 degrees, for fore sight to back sight discrepancies.

  Fore and back site discrepancies of much larger than that can be
  *CORRECT* for places with large magnetic problems.  Such data can
  be processed properly to distinguish errors from magnetic
  anomolies.  (As I've written up before.)

  However, if the program doesn't know how to handle such issues
  correctly (and it is a difficult issue to retrofit in) then
  I agree that descrepancies greater than (say) 2SD should be
  flagged.

> [...]

> > Perhaps this could be defined so that 2 degrees is when it shouts.

  If you are going to shout, then a figure based on the SD's that you
  expect seems cleaner.    For extreme example, if the instrument I'm using
  is my naked eyeball without an instrument, and I have (for example)
  10 Degree SD, then a 2 degree discrepancy is not remarkable.
  If I'm using a Total station then a few minutes of arc descrepancy
  might possibly be appropriate.

> [...]