[Therion] Declination handling imprecise?

[Benedikt Hallinger]

Thank you everybode for those insights!
It is good to hear therion is this accurate, however i need to learn what i 
understand wrong in the test data.

Is the centerline shown in aven correct and only the displayed angles 
rounded (i.e. the calculated coordinates are valid)?
How about loch?



Am 2017-02-18 23:24, schrieb 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