[Therion] fixed point std error causes extreme survey network distortion

Bruce bruce at tomo.co.nz
Sun Dec 22 01:35:25 CET 2013


I got some unexpected results recently when I added some very rough gps
coordinates to my survey for a cave that has three entrances.  Because they
are approximate I added some estimates of standard errors as below, so that
I could get Therion to factor the relative accuracy into the loop closure.

 

   fix 12 at 01    1562372 5439558 992    20 20 20  # entrance

   fix 0 at 10      1562117 5440239 1080  5  5   20  # another entrance

   fix 120 at 05  1562124 5439500 1020  10 10 10  #entrance three

 

With any ONE of the entrances fixed as above, the cave plots accurately - I
can verify this in Google Earth.

As soon as I have any two or more entrances fixed, then the cave distorts
significantly, and the entrances are stretched some hundreds of metres away
from the centroid of the cave.  Also quite often getting 'scrap exceeding
maximal scale' errors if I output to pdf.

 

I checked this was not just a feature of that dataset by mocking up a
similar situation in two other datasets, and got similar results.

 

The work around seems to be to avoid using the standard errors;

 

   fix 12 at 01    1562372 5439558 992    #20 20 20   entrance

   fix 0 at 10      1562117 5440239 1080  #5  5   20   another entrance

   fix 120 at 05  1562124 5439500 1020  #10 10 10  entrance three

 

but this then results in each gps location receiving equal weighting.

I think I might have noticed this problem a few years ago, and gave up on
trying to sort it out because I wasn't able to identify the characteristics
of the problem well enough.

 

Anyone else having problems like this?

Or identify a reason why what I am trying to do is not possible?

Or is it a bug?

 

Bruce

 

 

 

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