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Hi Bruce,<br>
<br>
I have hundreds of fixed points with different accuarcy and never
noticed the effect you are describing. However, I am not fixing
directly the coordinates of stations of my survey. I always create
independant fixed points and then equate them to the survey
stations. In your case, this would give something like:<br>
<br>
fix GPS_12 1562372 5439558 992 20 20 20 # entrance<br>
equate GPS_12 12@01<br>
fix GPS_0 1562117 5440239 1080 5 5 20 # another
entrance<br>
equate GPS_0 0@10<br>
fix GPS_120 1562124 5439500 1020 10 10 10 #entrance three<br>
equate GPS_120 120@05<br>
<br>
An advantage of this strategy is that I can comment all the equates
but one to start with and verify with Aven on the 3d file where are
the GPS points located with respect to the cave survey. That way, I
can spot the outliers in the GPS points (or the errors in their
retranscription) and iteratively validate the equates.<br>
<br>
Xavier<br>
<br>
PS: between us, I would not qualify your gps stddev below as very
rough. A stddev of 5 meters about the smallest I use when I get long
GPS averages (more than 30 minutes) with one of the a differential
satellites. <br>
<br>
<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">Le 22/12/2013 01:35, Bruce a écrit :<br>
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<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">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.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"> fix 12@01
1562372 5439558
992 20 20 20 # entrance<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"> fix 0@10
1562117
5440239 1080 5 5 20 # another entrance<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"> fix 120@05
1562124 5439500 1020 10
10 10 #entrance three<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">With any ONE of the
entrances fixed as above, the
cave plots accurately – I can verify this in Google Earth.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">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.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">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.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">The work around
seems to be to avoid using the
standard errors;<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"> fix 12@01
1562372 5439558
992 #20 20 20 entrance<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"> fix 0@10
1562117
5440239 1080 #5 5 20 another entrance<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"> fix 120@05
1562124 5439500 1020 #10
10 10 entrance three<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">but this then
results in each gps location receiving
equal weighting.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">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.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">Anyone else having
problems like this?<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">Or identify a
reason why what I am trying to do is
not possible?<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">Or is it a bug?<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ">Bruce<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:
10.0pt;font-family:Arial" lang="EN-NZ"><o:p> </o:p></span></font></p>
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