Grid convergence

Martin Sluka martinsluka at
Mon Feb 22 16:34:59 GMT 2016

Therion has the identical feature, each survey may have it’s manually defined declination.


> 22. 2. 2016 v 11:44, Olly Betts <olly at>:
> On Tue, Feb 16, 2016 at 02:20:47PM +0100, Mateusz Golicz wrote:
>> Let us consider this test. Suppose an unsuspecting user surveyed a
>> very straight Chinese borehole in Austria. It starts with a huge
>> entrance at station "0" and ends in a choke at station "1".
>> [...]
>> The way UTM works is that if you go 1000 m towards the
>> north pole from 328000 5260000, you in fact are roughly at 328029
>> 5260999, you like it or not. This is some 29 m away from what Survex
>> indicates.
> Shouldn't be a problem - a proper Chinese borehole passage would be
> at least 58m in diameter.
>> Of course this "entrance hunting" example is a bit strange, but I
>> guess we all realize this has big consequences in closing loops in
>> multiple-entrance, multiple-fix situations. How do "we" deal with
>> this? Any plans to fix that? If not, shouldn't we at least warn the
>> unsuspecting user in our manual?
> It's something I'm aware really ought to be handled, but I haven't
> looked much at how best to do so.  I know therion has code to deal with
> grid convergence, so doing something similar might make sense.
> For the magnetic declination, the main difference in the approach I
> took for Survex is that you have to explicitly specify a location
> for the declination calculation - if I follow therion's code, it uses
> the average of the fixed points as an implicit location, but that means
> the declination is affected by adding a fixed point, which seems
> surprising and unhelpful.
> Survex's approach also allows specifying declinations for different
> parts of the survey, which is good if you are putting in data for caves
> over a wide enough area that the declination varies significantly.
> It seems therion calculates convergence at the same point, so I think
> it probably makes sense to handle this in the same way as the
> declination for Survex.
> Cheers,
>    Olly
> -- 
> Survex

More information about the Survex mailing list