Magnetic declination in Survex

[Olly Betts]

On Fri, Feb 21, 2020 at 07:49:34AM -0500, Luc Le Blanc wrote:
> From what I read and understood in 2014, in Survex, a negative
> declination is East in the *calibrate declination command. Yet, when I
> check the more recent *declination command, I read:
> 
> "Compass bearings are measured relative to Magnetic North - adding the
> magnetic declination gives bearings relative to True North."
> 
> This suggests a negative West declination.
> 
> What's the truth?

What you just wrote is all true, as documented under the *declination
command at the end of the section you quote above:

| Note that the value specified uses the conventional sign for magnetic
| declination, unlike the old *calibrate declination which needed a value
| with the opposite sign (because *calibrate specifies a zero error), so
| take care when updating old data, or if you're used to the semantics of
| *calibrate declination.

Now we have "*declination auto", it's best to just specify the survey
date and take advantage of that unless you really have a better source
of declinations than the current IGRF model.  If you specify explicit
declination values then they're fixed from then on, whereas "auto"
will update when a newer IGRF model is issued.  Also "auto" corrects
automatically for grid convergence too.

Cheers,
    Olly