Multiple readings from one but not all instruments

[Andrew Atkinson]

On 15/11/2022 20:58, Olly Betts wrote:
> I don't think this can easily be extended in this way.  This feature
> averages the vectors for each of the legs, but with an omitted reading
> we don't have a full vector for that leg.  Filling in the blank from the
> first leg doesn't seem satisfactory as it effectively attributes
> additional accuracy to that reading (and it doesn't easily extend to
> having 3 repeat legs, two of which have a tape reading).

I was sort of hoping that you had bodged it and just averaged the three 
readings, but in my heart I knew you would not have done that
> 
> I'd suggest just handling the extra compass and/or clino here as
> backcompass/backclino suitably calibrated so they actually work
> in the same direction as the forward instruments:
> 
> *calibrate backclino 0 -1
> *calibrate backcompass 180
> 
> Survex also supports backtape (since 1.2.25) so you can take your pick
> of which instruments you have a secondary for (backtape doesn't need a
> special calibration for this use, since distance is the same whichever
> way round you measure it).

I like that, works for this situation, but wouldn't get round the SAP 
taking 3 readings with only one tape. I agree adding in the missing 
readings would not be correct
> 
> You can specify a separate accuracy for backcompass and compass, so this
> is supported via the backsight approach.  Averaging will take into
> account the accuracies specified via *sd so you'll get an average heavily
> weighted towards the better compass here.
> 
> The main thing that isn't possible is more than two instruments of the
> same sort.  If people really need that we could perhaps extend the
> backsight handling to support this (and to more explicitly support
> additional forward instruments).
> 
> There is actually some unfinished support for repeat readings - I think
> the sketched out syntax for multiple readings was:
> 
> *data normal from to tape compass clino
> 
> 1	2	3.22	{ 178 179 }    { -3 -4 }
> 
> I don't remember why this didn't get finished, but maybe lack of anyone
> actually asking for it back then.  The code there is for this assumes
> the same SD for each reading inside the {...} though.

Phil had asked about multiple readings for forward and back readings, I 
guess he is pre-processing to get the data in. As far as I remember he 
was using 2 sets of instruments and taking forward and back with both 
sets. it's somewhere in the mail list archive. There was a solution but 
it was very messy and difficult to implement. I wonder how much demand 
is missed as people get round it and lots are not using this list?
> 
> If I follow, we have a tape/compass/depth survey with an additional set
> of depth readings.  I'm not sure being able to "vertically equate"
> stations would help here as we can't simply represent this as two
> traverses as we lack full data for the return traverse (reusing the
> tape and compass readings would effectively treat them as having extra
> accuracy).
> 
> We don't have "BACKFROMDEPTH", etc so this isn't currently possible by
> creative use of the backsight support, but perhaps that's the natural
> way to handle this.

That might work, I'm not actually sure how the person is doing it, all 
the information I have is on the thread I quoted

Thanks Olly for your continued hard work

Andrew