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

[Tarquin Wilton-Jones]

Hi,

I have a survey that starts at a trigpoint. The trigpoint has a height
specified for the benchmark bracket on its side, and x,y coordinates for
the circle on its top. The survey has stations on both the benchmark and
the centre of the top.

I need to be able to specify the x,y for the top, and the z for the
bracket on the side.

Can survex allow me to do this, or do I need to do all the maths myself
to work out the height of the top so I can fix all three coordinate
dimensions for a single point?

What I want is this:
*fix 2 - - 123.4
*fix 4 1234.5 6789.0 -

>From what I can see in the docs, none of these parameters are optional,
so how can you deal with situation in survex?

Cheers,

Tarquin