[Therion] Declination handling imprecise?

[Martin Budaj]

On Sat, Feb 18, 2017 at 10:24 PM, Olly Betts via Therion
<therion at speleo.sk> wrote:
> For a few km the variation is probably fairly small for most of the world
> compared to the errors from the instrument readings, though the instrument
> errors are random and the declination discrepenacy is systematic
> - systematic errors are worse because they don't lessen when you combine a
> lot of readings (for random errors the error of the sum increases as the
> square root of the number of readings - e.g. for 100 readings, the random
> error only increases 10 times).
>
> The grid convergence may well be larger (IIRC therion calculates that at
> the same average fixed point location; I know that Survex calculates it at
> the same coordinates you give in "*declination auto").  To quantify these
> errors, for the coordinates in your test file, 5km E means about 0.015
> degrees change in declination and about 0.049 degrees change in grid
> convergence (in opposite directions).

BTW, there is even more significant source of (possibly systematic)
error: daily variation of magnetic declination.

A real world example of the daily variation of declination from an
average value in the central Europe is:
0 AM: 0'
5 AM: -6'
10 AM: 0'
1 PM: +7'
6 PM: +1'
10 PM: -1'

In this case you get a systematic error of about 4'-7' (0.07-0.12
degrees) if you regularly survey between 11 AM and 3 PM in the summer
(the daily variation is lower in the winter).

For other example end an explanation check
http://www.geomag.bgs.ac.uk/education/earthmag.html#_Toc2075560

Martin