Backsights and default accuracy estimates

[Olly Betts]

On Thu, May 16, 2002 at 10:23:17AM +0100, Andy Waddington on Survey stuff wrote:
> > 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
> 
> And how does Survex deduce which of the two readings is which ?

Same way it can tell which is the compass reading and which the clino - the
*data command tells it.  It's really no different to the case without the
backcompass reading.

> It would seem to depend on the ordering of the legs to deduce which
> "direction" the survey is going

No.  You don't need to deduce the general direction of the survey to know which
way the compass reading is going - it goes from "from" to "to", unless there's
a direction reading of "B", in which case it goes the other way.

Or if you have interleaved data, compass goes from the "station" in the line
before to the "station" in the line after, unless reversed by a direction
reading of "B".

The backcompass reading goes the other way to the compass reading (hence the
prefix "back").

> Additionally, if both foresight reader and backsight reader are
> leapfrogging, you need to have some sort of tag to identify which reading is
> which.

You have - see the "direction" reading in the *data command.

Cheers,
Olly