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

[Tarquin Wilton-Jones]

On 15/08/2021 18:32, Martin Green wrote:
> Do you have two legs between the benchmark and the circle?  Thus making
> it hard to determine the vertical offset?

In my real case, I have 3. Because trigpoints are made from reinforced
concrete, and the iron causes a Disto (or compass) to give wildly
erroneous readings, so I needed to get quite some distance away, and
didn't have a useful place to put a tripod where I could see everything.
Had to do it in 3 hops.

(For anyone planning to do this in future, a plumb is a whole lot
easier, as long as the top of the trigpoint is level.)

Like I said, that's OK in this specific case, I could add the vertical
components together, get the top height, and then fix just the top. And
then leave a comment saying why the top has the "wrong" height (not the
one you see in OS records), and why the benchmark doesn't have one at all.

> 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.

Yes, I thought of this as a possibility. It made me feel like I needed
to take a shower ;)

It works, but it has subtle effects when other fixes are involved
elsewhere in the survey. It's certainly not something that is intuitive
for anyone else maintaining the data.

> 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.

Exactly that :)

You know how clino readings can be left blank with a - character. That
would be lovely for the *fix command.

Would that actually be possible to add?