Do more sophisticated grid convergence.

[Eric C. Landgraf]

Currently, we compute grid convergence for the point specified in *declination auto. Typically, this is fine: if you're not at polar regions or at the very edge of a UTM zone, it's likely less than 1°.

However, for very large cave systems or cave regions, this method is insufficient: it requires you set *declination auto at several different points. This is ususally a sensible idea: if your cave system is large, declination is likely to change across linear space, sometimes quite agressively, and it is hard to effectively predict this behaviour as it shifts temporally and spacially.

Grid convergence (difference between grid north and true north) is "simple" in comparison, and changes less dramatically over a region. But for multi-entrance systems could still result in major shifts between entrances

We should compute it at more than a central point, especially for "large" systems. This likely will have to be a configurable knob.

Some caveats that Olly and I have discussed:

• grid convergence for any given leg both depends on and controls station location
• station location cannot be ascertained without closing loops and solving the system.

The only way to *perfectly* account for grid convergence requires iteratively re-solving the entire system (an operation which can be quite costly) with new convergence corrections based on final location. This is going to be most costly for the cave systems where it is most important. So what we want is an "approximate" solution that can be done quickly, and generally, without users having an in-depth understanding of grid convergence themselves.

Therion uses the centroid of all fixed points. Walls appears to semi-iteratively solve for grid convergence (although I may be wrong).