UTM/UPS fixed points in survex

[Erin M. Lynch]

Guys,
  I've been pondering how to deal with grid/true north convergence and
large systems that span two UTM/UPS grids.  I know that for the moment 
survex doesn't deal with grids, but I think it'd be good if *fix were 
fettled to accommodate fields for Zone and Map Datum.  Or even better would 
be to introduce a *gpsfix which for the moment would just act as *fix, 
but in the future would deal with the convergence and multiple zones 
problems.

I've come up with a quick and dirty solution for the time being. I've 
swiped some code from various sites and cobbled together a javascript 
which you can see at www.survex.com/~erin/mag/conv.html
The useful bit is the part that calculates a distance and bearing relative 
to true north for UTM/UPS coordinates in the WGS84 map datum.  I'm 
using this to create fictitious surface survey legs between my gps points, 
and it seems to work well.  

In the Houping data set www.survex.com/~hmg/survex/houping/nov02002/ I've
fixed the entrance to ErWang Dong (surface.1) and displayed 3 different 
candidates for the entrance of SanWang Dong:  

sanwang-gps : the GPS point entered as a *fix, ignoring the difference 
  between grid north and true north
sanwang-convgps : the GPS point with it's location entered as a leg from 
  surface.1 based on the javascript output
sanwang.main.1 : the GPS point as measured by surface survey.

Over a 700m surface survey, the difference between sanwang-gps and 
sanwang.main.1 is 26m, which is pretty appalling, but it's only 7m between 
sanwang-convgps and sanwang.main.1

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).

Cheers,
Erin