fixed points in survex

[Erin M. Lynch]

On Tue, 5 Nov 2002, Olly Betts wrote:

> On Tue, Nov 05, 2002 at 02:54:32AM -0800, Erin Lynch wrote:
> > In the long run, it'd be dead handy if survex incorporated the maths so 
> > that users can just enter the coordinates they get out of their GPS.  But 
> > it's worth having a think about whether it would be better to 
> > change the coordinate system of the fixed points to an arbitrary local 
> > grid where north is True North, or to leave the fixed points in the 
> > UTM/UPS grid and rotate/stretch the cave survey so it is in UTM/UPS
> > coordinates (which would be useful for map overlays and surface 
> > prospecting).
> 
> I think this is the hardest part actually.  The maths required isn't
> hard to find - there's probably even GPL-ed C code suitable for
> assimilation.  But you need to decide what grids users will want to be
> able to transform it into, and how to specify this clearly and
> succinctly.

For the time being, I'd settle for having a way to record the zone, datum 
(and maybe EPE?) of a fixed point, so when the mathy bits are implemented 
I don't have to go through and change all of my old data files. 

> For your purposes, simply converting coordinates in one zone into the
> extension of coordinates for the neighbouring zone is probably good
> enough.

No, unfortunately it's not :(

> Grid north and true north for a UTM zone are presumably only exactly
> aligned at the centre of each zone.  What's the divergence of the two
> at the edge of a zone?  If it's major, then we need to think about
> compass corrections for grid north...

The compass correction varies slightly depending on the direction of your 
bearing, but to give you an idea of the size of the error, I've calculated 
the correction for a northwards bearings in a few caving areas in China.  
Houping (on the edge of the 48R Zone) is the worst at 1.5 degrees.  Nandan 
is 1.1, Tianxing -1, Youyang -1, Leye 0.6 etc.  The problems this can 
cause are best illustrated in Houping, where a 700m surface survey 
produced a 26m "loop closure error" between two GPS points if we 
assumed the GPS points and survey were in the same grid.  Once the 
GPS points were converted to the grid of the survey the error was only 7m.

Cheers,
Erin