It should be possible to fix x,y separately from z

[Tarquin Wilton-Jones]

With some surveys, it might only be possible to fix the horizontal x,y coordinates accurately at a different location from the z height. An example would be a trig point for x,y, and a benchmark for the z (this is my actual use-case), which could be in completely different locations in the surface survey. Another would be for a marine cave where the z can be fixed at the water surface, while the x,y might be taken from local mapping at an obvious landmark. In the UK, it is common for benchmarks to have very imprecise locations but highly accurate heights, and trigpoints often do not have a benchmark.

Currently, the *fix command insists on having x,y and z coordinates for every fix. It would be useful for the *fix command to support - as a measure like this:

*fix trig 1234 5678 - *fix benchmark - - 317

The present workaround is quite laborious:

1. Fix the location of the benchmark with random x,y and actual z.
2. Process the survey and read the z of the trigpoint.
3. Remove the benchmark fix.
4. Add a fix for the trigpoint, using the calculated z, and the actual x,y.
5. Hope that loop closure never changes the height difference between the points.
6. Add a load of comments to explain why the benchmark has no height fix, but the non-benchmark does.

Or: Specify approximate heights or x,y for the unknown quantity at each of the two points, using standard deviations to enable the bad coordinates to be ignored.