Backsights and default accuracy estimates

[Brian Clipstone]

> I only have one relevant experience of taking both fore and backsights but
> it does throw up some possible problems:
>
I have some experience of using the data from surveyers using two set of
instrument for fore and backsights and it does cause errors.
When we have been surveying a large cave like O.F.D. we calibrate our
Compasses by siteing on a fixed point (above ground), which is actually Grid
North as set out by a Theodolite, this is the number we put in the
*CALIBRATE COMPASS in Survex, this serves two functions, one to set the
magnetic variation and the other to eliminate the reading errors of various
surveyers and the Compasses.
I have found that the reading error of surveyers is quite large, more than
different compass error.
The way we use *CALIBRATE COMPASS eliminates this error, but if you use both
fore and backsights with different compasses then you must be able to use a
seperate *CALIBRATE COMPASS on both compasses used or there will be an error
in the readings.

Brian