Surveys without centrelines

[Olaf Kahler]

I haven't thought it all the way through, but one other problem is that
you'd only get a very flat survey. The distances alone won't allow you
to get any elevations, or at least I don't see how. It's fine if you are
working on a plane like at sea level, but might prove impractical as
soon as your measurements deviate and are taken outside the plane, which
would result in distortions in the survey. You could of course measure
distances from every point to three distinct other points, as GPS does,
but two distances will only suffice for 2-D problems...

A very unusual cave surveying technique anyway.

Cheers,
    Olaf

On Fri, 6 Sep 2013 05:39:09 -0700 (PDT)
kevin dixon <kdxn at yahoo.com> wrote:

> The term for this type of survey is Trilateration.
> 
> One problem you will have with this is that where there are only two
> distances to a point, there will be two valid position solutions,
> equidistant either side of the baseline that has been used to measure
> from. In this situation, you will need to have some way of telling
> the algorithm which side of the baseline is correct.
> 
> Should this unusual survey method be added to Survex ? 
> Would it be better to do the trilateration calculations in excel and
> then output data suitable for use in Therion/Survex ?
> 
> Kevin Dixon 
> 
> 
> ________________________________
>  From: Wookey <wookey at wookware.org>
> To: Survex User Group <survex at survex.com> 
> Cc: Stelios Zacharias <stelios.zacharias at selas.org> 
> Sent: Friday, 6 September 2013, 13:20
> Subject: Fw: Surveys without centrelines
>  
> 
> This was posted to the therion list, but is really a survex question. 
> 
> > Stelios Zacharias <stelios.zacharias at selas.org> wrote:
> 
> > Can Therion put together scraps on the basis of the polygonal system
> > of survey? (I think this is what it is called - it uses as set of
> > triangles and only distance measurement rather than the distance,
> > azimuth, slope measurements we are used to).
> > 
> > A friend has a survey constructed with only distances taken from two
> > points (and successive points) so as to create a flat area of
> > triangles (it is flat because the "floor" is the sea and the water
> > level is constant). There is therefore no centreline, but a series
> > of points, each joined by two measurements to other points and all
> > coming back to a known baseline. In the days of paper survey, the
> > plan would be drawn with just a compass from one point to the next,
> > beginning off the baseline.
> 
> 
> Survex has a 
> *data cylpolar from to tape compass fromdepth todepth
> entry form, which is along the right lines, but that
> still expects bearings and depths.
> 
> As Bruce suggests, guessing the bearnings (or putting in arbitrary
> values) and turning the SD up with 
> *sd bearing 180 degrees
> might reuslt in the right answer.
> 
> Better would be a new input format for this form of data
> *data triangulation from to length fromdepth todepth
> and probably more usefully
> *data triangulation station depth newline tape
> 
> I'm not sure how hard it is to add this. (Is it just a new data
> format, or do the solver algorithms have to change?) If the former it
> should be quite straightforward.
> 
> 
> Wookey