On Sat, Feb 18, 2017 at 10:24 PM, Olly Betts via Therion <[email protected]> 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 _______________________________________________ Therion mailing list [email protected] https://mailman.speleo.sk/listinfo/therion
