On Mon, Mar 15, 2021 at 05:50:18PM +0000, alastair gott wrote:
> I'm afraid I there is no answer as to whether I mean calibration,
> declination or calibrate declination.
>
> As the survex model has the *calibrate compass -2.65 already in it,
> and I don't know whether this is because they knew the compass was
> out, or whether they corrected for the declination in the survex model
> when the data was put in.
It would help a lot to determine what -2.65 includes.
If it wasn't recorded and you can't usefully ask the people who recorded
the data, you could try adjusting the data assuming this includes the
declination and again assuming it doesn't and see which version gives
better loop errors. Aven can colour by error which provides a way to
visualise this.
> When I refer to the calibration in Therion, I've just applied the
> same function calibrate compass 357.3
>
> I was hoping that running the comparison between therion and survex on
> the first leg of the survey, I could work out by trial and error what
> I should do to make the data the same. I'm happy to apply a fix to all
> the .th files, to ensure that the models align, but this doesn't seem
> like the therion way given that the underlying data is the same.
A key difference here is that Therion automatically corrects for
declination and for grid convergence (the difference between true north
and grid north) if you specify a coordinate system and put a date on a
survey. It does this by calculating the corrections at the location
which is the average of all the specified fixed points (this doesn't
seem to be documented, but that's my understanding from reading the
source code.)
Survex takes a different approach and you have to turn on the equivalent
feature and specify the location to calculate the corrections at, e.g.:
*declination auto 0410600 5282000 1234
I didn't follow therion's automatic approach because it has the downside
that adding a new fixed point slightly changes the location of *ALL*
survey stations (apart from those which are also fixed points).
> PS. I know there will be parts of the datasets where there are
> differences, but till I can make the first leg the same, I think it
> will be difficult for me to spot even the biggest difference.
Rather than trying to maintain two parallel version of the data set,
my suggestion would be to save yourself a lot of work and just tell
Therion to use a .3d file produced from the Survex data set for the
station coordinates (see the "import" command).
Cheers,
Olly
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