On Sat, Feb 18, 2017 at 10:24 PM, Olly Betts via Therion <therion@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