[Therion] Therion uses wrong declination for surveys 'in the future'

[Tarquin Wilton-Jones]

> We've had to use data back to about 1910 in Ireland but I'm not
> aware of anyone needing to go back before 1890. I'm not sure if
> there is/was any conventional cave survey data that old.

Pen Park Hole
"It also has the honour of having the oldest published survey, dating to
1683"
But sorry, I do not know the equivalent for a survey with centreline data.

</trivia>

>> You can't always avoid mixing dated and undated surveys
>> (sometimes the older survey data might be undated), so using the
>> min(survey_dates) as a proxy for undated surveys (and producing a
>> warning as well) should be a reasonable approach.
> 
> Very good point. We have a lot of data imported from other
> programs which ends up with no date. And fake data generated from
> existing survey plots also ends up dateless. This is definitely a
> thing.

Often such data has declination specified, at least in the cases I have
handled. The declination is not just magnetic, but also the calibration
of the device (or its user) at the same time. In the case where there is
"calibrate compass" but no date, and the compass is calibrated to 4
degrees or so, that's usually the magnetic in there, and a warning is
valid but unhelpful (warnings are not errors, etc.).

I don't know if it currently assumes that "calibrate compass" and
"calibrate declination" are both reasons not to complain, but I would
suggest that it could, likely without harm.

Tarquin