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<DIV><FONT size=2>I've been using Survex as a means of generating a reasonably
accurate architectural drawing. The technique is to assume the builder got it
right and that the walls of a rectangular room are aligned at 0, 90, 180 or 270
degrees. You then just go round each room with a tape measure, producing a loop
per room, connected at the doorways. This works really well but seems to
demonstrate a possible error in the Survex loop-closure algorithm.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>To demonstrate this, take a simple survey of two perfect
loops, joined by a single leg:-</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>*begin
test<BR>1 2 10 90 0<BR>2 3 10 180 0<BR>3 4 10 270 0<BR>4 1 10 0 0<BR>4 5 10 270 0<BR>5 6 10 0 0<BR>6 7 10 270 0<BR>7 8 10 180 0<BR>8 5 10 90 0<BR>*end
test</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Now introduce an error into one of the loops:-</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>*begin
test<BR>1 2 10 90 0<BR>2 3 10 180 0<BR>3 4 10 270 0<BR>4 1 10 0 0<BR>4 5 10 270 0<BR>5 6 10 0 0<BR>6 7 10 270 0<BR>7 8 7 180 0
; Gross error in this leg only<BR>8 5 10 90 0<BR>*end
test</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Survex moves not only the survey legs in the "bad" loop but
also adjusts the connecting leg (i.e. test.4 - test.5) and moves the station in
the "good" loop (i.e. test.4) where the connecting leg joins. The "error" is
only 1cm which wouldn't show in most cave surveys (with me using the
instruments!) . However, the error is still 1cm if you scale the loops down to,
say, 1m per leg - which becomes quite noticeable when the loops are small and
generally well-closed. </FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Is this an error in the algorithm or just the result of me
using Survex in a way that wasn't intended?</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Mike</FONT></DIV></BODY></HTML>