Backsights and default accuracy estimates

[Olly Betts]

On Sun, May 12, 2002 at 12:17:16AM +0100, Brian Clipstone wrote:
> 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.

You need to take care with this approach - the risk is that all the readings
in the cave could be made more wrong by a bad reading on the surface.  A good
precaution is to take 3 readings before the trip (and recheck if they disagree
noticeably) and repeat this after the trip.

I imagine Brian's aware of the issues, but I thought it best to point it out as
his description could be interpreted as suggesting you just take a reading and
blindly stick it in as the *calibrate value.

> 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.

You can set the calibrations, units, and standard deviations for compass and
backcompass (and clino and backclino) separately:

*calibrate compass 0.5 degrees
*calibrate backcompass 0.0 degrees

Or if they're the same, you can set both at once:

*calibrate compass backcompass 0.5 degrees

Cheers,
Olly