Fixing a station's height only, or x,y only

[Tarquin Wilton-Jones]

Thanks for the comments, folks.

> As Les says surveying from the bench mark to the circle appears to be
> the obvious fix.  This can be done with a plumb leg, such that any clino
> corrections are not applied eg. If the bracket to the top was 80cm.
> 
> *data normal from to tape compass clino
> benchmark circle 0.8 - up

Yes, I can indeed do that, and that is what I have done (but I used a
Disto rather than a plumb). The question is how to represent that
correctly in Survex (or Therion). There are hacks and workarounds, but
is there a way to represent the actual situation correctly, so that you
can show exactly which positions got taken from exactly which station?

I can of course do the maths (a few sin(clino)*length added together),
and add the height from the benchmark to the trig top, and then *fix the
trig top with the x,y,calculated_height.

But this works only if the trig and benchmark are in nearly the same place.

Imagine your surface survey includes a trigpoint that does not have a
benchmark bracket, and separately also a flush bracket benchmark from
the side of a building elsewhere on the surface. The two are not close
enough to each other to measure between without a large number of legs.

Again, you can ask Survex to do the maths by adding fake x,y coordinates
to the benchmark, see what you get as the height for the trigpoint, then
use that in a fix statement for the trigpoint, and remove the original
fix for the benchmark, but this is a really messy solution.

Does Survex (or Therion) have a way to represent this, so that you can
set x,y for one station, and z for a different station? If not, it
really should, since in traditional levelling and tiangulation, the two
are completely unrelated and calculated independently. We therefore have
a lot of places in the UK where you get one but not the other. Many (but
definitely not all) trigpoints have a benchmark bracket. Most benchmarks
do not have an accurate location.