[Therion] North True Grid Magnetic Astronomical?

[Martin Budaj]

On Sun, Jul 27, 2008 at 2:00 AM, Bruce Mutton
<bruce.mutton at paradise.net.nz> wrote:
> Presumably almost all cave surveys start out aligned to magnetic north
> because most of us use compasses that adopt the local magnetic north as
> their datum.
>
> Then, often, we either align the survey to true (astronomical) north
> manually by specifying a declination,

Exactly

>                            Or align the survey to grid north by specifying a
> co-ordinate system (that calculates declination automatically)

No. You usually align the survey to grid north if you are using
theodolite to measure angles and your measurements are based on points
with known coordinates in the coordinate system used ("grid").

The reason why the grid north is not parallel with true north is the
projection of the spherical Earth surface into plane. There is usually
one meridian where grid north is exactly the true north, but if you
move east or west from this meridian there will be increasing
difference between both (called meridian convergence).

The compass readings are converted to grid north by applying both the
declination (local) and the meriadian convergence (local) corrections.

> The grid north is usually not the same as the true (astronomical) north, but
> for the purposes of cave maps and Therion I would assume it is usually
> considered to be so.  (Not strictly correct, but I imagine the error would
> not be detectable on any printed cave map in most co-ordinate systems)

The difference is up to 3 degrees for UTM projection or up to 10
degrees for Czech Krovak projection.

> So what is intended by Therion?  Presumably some distinction between
> magnetic and grid north?

As described above, the inclusion of meridian convergence effect.

> I have experimented and found that 'north grid' 'north true' appear to
> produce identical outputs regardless of whether a 'co-ordinate system' or a
> 'declination' or a 'rotation' is specified.

It could be the case if you accidentally happened to live near the
central meridian of your coordinate system (grid); e.g. longitudes 3,
9, 15 etc. degrees east for UTM.

Martin