[Therion] North True Grid Magnetic Astronomical?

[Bruce Mutton]

The Therion book states

.The declination is positive when the magnetic north is east of true north.

and

 north <true/grid> . specify default orientation of the map. By default,
true (astronomical) north is used. It is ignored when used with local
coordinate system.

 

I'm confused.  

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, 

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

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)

 

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

 

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.

 

Is there a mistake or am I not understanding something?

Regards

Bruce

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