On Thu, Aug 15, 2019 at 10:06:05PM +0100, Olly Betts wrote:
> The colour scale is fixed such that blue is "good" and anything else
> is suspect. You probably want to look at the worst first.
>
> Once you have a suspect traverse identified, you can look it up in the
> .err file which is produced alongside the .3d file when you process the
> data. Here's an example entry:
>
> 76.brave.18 - 76.brave.17 - 76.brave.16 - 76.brave.15 - 76.brave.14 -
> 76.brave.13 - 76.brave.12 - 76.brave.11 - 76.brave.10
> Original length 32.35m ( 8 legs), moved 1.26m ( 0.16m/leg). Error 3.90%
> 4.920289
> H: 5.888476 V: 0.864600
>
> Here 4.920289 is the number of sds (which is what aven colours by). But
> below that the H and V are the equivalent numbers looking only at the
> horizontal and vertical errors.
Survex 1.2.42 (released yesterday) adds the ability to colour by
horizontal or vertical error (H and V above), so you can zoom in on
a suspect traverse based on the overall error, and then check how
its H and V values compare based on the colouring.
> So here the vertical error is very good,
> and the horizontal error lousy. That suggests the error is probably either
> in a compass reading or the tape reading in a low-inclination leg. (If
> H was fine and V bad, you'd be looking for a clino error or a tape error
> in a steep leg or plumb.)
>
> So you can now look at each of the legs in the list on the first line and
> look at their compass readings and the tape readings for any low-inclination
> legs. For manually entered data, compare with the original notes.
Another tip is that a common error type is a "mis-tie" - connecting a
new survey to the wrong existing station(s).
This is probably especially prevalent in long running survey projects
with a turnover of the people involved. For example, it's all to easy
to tie to the "first bolt of the traverse" but fail to realise
somebody's extended the traverse since the original survey. This sort
of problem affects both electronic and traditional instruments.
My experience is that the largest errors tend to turn out to be
mis-ties (but not all mis-ties result in large errors). Mis-ties will
tend to have bad H and V, though maybe H will tend to be a bit less bad
(because it's easier to mis-tie to something in the same passage or
chamber, and harder to mix up top vs bottom of a pitch).
If you suspect a mis-tie, I would try simply disconnecting each end of
the problematic survey in turn. If that makes the remaining network
look good then you can see what stations the disconnected end is near
to give you some candidates to consider for where it perhaps should be
tied to.
Cheers,
Olly
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