[Therion] Fix station point using only 2D

[kevin dixon]

Adam,

Radio-location can give you depth but accuracy is not too good.
How have your surface positions at the entrance(s) and the radio location
been determined ?
Professional GNSS (~3cm), DGPS(~1m) or hand held (~3m) ?

I have radio-location fixes at the far end of a system to control any
accumulated azimuth bias.
For these I have assigned an elevation to the radio-location equivalent to
the cave survey at the radio location point and used an appropriate SD
value for the radio location data. You may have to iterate a few times and
experiment with SD values to see how it changes your survey with and
without the radio location fix(s).

Also better if you have radio location fixes at different points because
geology effects the position.
Use several in the survey - you should see how they fit (or not) - reject
the wildly out ones, adjusting to the rest will help reduce any bias.

150m is a lot of difference so I would be looking at the positional method
used, datum and grid, survey instrument calibrations, declination values
and then get additional radio locations to confirm. If you are dealing with
older survey data then you may want to consider a compass bias - does a
survey rotation significantly reduce the difference ?

Kevin Dixon

On Tue, Aug 21, 2018 at 10:41 AM, Adam Pyka via Therion <therion at speleo.sk>
wrote:

> Hi all,
>
> How to 'fix' the survey station point (to morph the cave plan) having
> only 2D coordinates?
>
> Using the radiolocation we get (with quite good accuracy) the station
> point position which is about 150m beyond the estimated location we
> get from surveying. So now I wanted to use this position to make a
> correction to the main polygon. But the 'fix' command need a 3D coords
> so how should I do that (no, there is no point to measure the depth by
> radiolocation by some reasons).
>
> Thanks in an advice,
> Adam
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