Declination auto vs manual

[Olly Betts]

On Mon, Nov 04, 2019 at 05:23:42PM +0100, Špela Borko wrote:
> I have a problem using *declination auto. It seems that with "auto" command
> survex consistently uses higher declination value as one can find with
> online IGRF model. The error is consistent in time. I tried to use
> different in and out cs. Bellow is an example with real coordinates from
> where I am seeing this behaviour (13.5091 46.37474, Slovenia).

Thanks for the great example.

> Am I missing something? Can anyone explain?

I think the difference you are seeing is the grid convergence - as the
documentation says:

| Survex 1.2.27 and later also automatically correct for grid
| convergence (the difference between Grid North and True North) when
| *declination auto is in use, based on the same specified
| representative location.

The convergence varies across a projection, but seems to be -1.079
degrees for your location, which seems consistent with the 1.1 degree
difference aven shows for your example manual/automatic pairs.  It's
calculated for the output projection, which is why it isn't affected
by changing the input projection.

It's not currently possible to see the convergence correction values
(without patching the code as I did to get the number above).  I've
noted this on the existing request to report the auto declination
values:

https://trac.survex.com/ticket/92

Cheers,
    Olly