[Therion] Declination handling imprecise?

[Olly Betts]

On Sat, Feb 18, 2017 at 10:20:54AM +0200, Benedikt Hallinger via Therion wrote:
> If therion calculates the average position of all fixed points this is fine.
> the cave spans about 5km w/e and about 3km n/s.

Ah, the 100km is total passage length not extent then.

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).

> For the date-observation, indeed my conclusion came from the summary in the
> log. Thank you for claryfying this. Good to hear, therion uses fractions
> here, and 1/12 is perfectly good.

To be clear, therion calculates the fractional year taking into account the day
of the month - it just doesn't assign quite equal lengths to every day of the
year, but instead puts the start of each month 1/12 of a year apart, and then
splits up each 1/12 into 28, 29, 30 or 31 equal pieces.  Really it should split
the year into 365 or 366 equal pieces.

> However i have seen that it looks like the calculated declination is maybe
> rounded to full degrees? Or is this just an display thing in aven viewer?

Aven currently only shows a whole number of degrees, which is unhelpful for
this sort of thing.  It probably ought to show one decimal place.

> Based on what you have said, i would assume that the problem source lies in
> therion itself summarizing all centreline dates into one survey average
> date, and not just by centerline as i would assume, when each centerline has
> a date specification.

I think you need Stacho or Martin to comment on whether that's happening.

Cheers,
    Olly