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

[Martin Green]

Do you have two legs between the benchmark and the circle?  Thus making it
hard to determine the vertical offset?

You could try x, y, z fixes with rough estimates for the unknown axes. Then
specify zero standard error for the known axes and a large standard error
for the unknown axes.

*fix 2 1234.5 6789.0 123.4 10 10 0 ; benchmark height (x,y roughly
estimated)
*fix 4 1234.5 6789.0 123.4 0 0 10 ; Circle on top of trig point (height
roughly estimated)

This is not something I have done, and it is a shame that you need very
roughly estimate the parts of the fix you would rather leave blank.

You probably want decent comments if you do this.

Regards,
Martin

On Sun, 15 Aug 2021, 18:03 Tarquin Wilton-Jones, <
tarquin.wilton-jones at ntlworld.com> wrote:

> 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.
>
> --
> Cave-Surveying https://lists.survex.com/mailman/listinfo/cave-surveying
>
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