[Therion] North True Grid Magnetic Astronomical?

[Bruce Mutton]

Eric

You are strictly correct of course. 

However there are cases where all we have is a paper map, no computer,
usually no date (well we do have a date within a decade or so), certainly no
survey data, and where survey practices result in gross errors that swamp
any probabilistic or systematic ones.

It's good to air the rigorous approach as well as the casual.
(Usually I take the rigorous side)
Thanks
Bruce

-----Original Message-----
From: therion-bounces at speleo.sk [mailto:therion-bounces at speleo.sk] On Behalf
Of Eric Madelaine
Sent: Wednesday, 30 July 2008 12:09 a.m.
To: List for Therion users
Subject: Re: [Therion] North True Grid Magnetic Astronomical?

Hi Bruce,

    Well, I don't agree with you...
    There is a difference between the accuracy of your measures (that 
correspond to probabilistic errors within a given estimation), and a 
systematic error, as the one you would introduce if you don't take into 
account the variation of declination between two surveys that would have 
been done 10 years apart... So this error is not included in your 
measure uncertainty but it adds to it.
    Moreover, in most survey programs, it is more or less easy to take 
into account the declination, or even compute it from the date... It is 
even easy to add this information a posteriori, if you get old survey 
data, as soon as you have the date.

Eric.


Bruce Mutton wrote:
> Eric wrote
>
>   
>> I would add to Martin explanation that while the meridian convergence is 
>> fixed (for a given location), the declination varies significantly with 
>> time. So if you are managing a cave survey project for say, 10 or 20 
>> years... then the difference in declination will be significant (can be 
>> several degrees)
>>
>> Eric.
>>     
>
> Eric
> I guess we are lucky in NZ, the declination varies by about a degree a
> decade, and we've only been producing maps for about four decades. The
> overall accuracy today of most of our surveys is probably no better than
> this (although we would kid ourselves otherwise).
> Bruce
>