Sure,<br><br>but that's not the point. The original question was if the values need to be negated when going FROM wgs84 TO something else. If that was correct, one should do the round trip as follows:<br><br>1. cs2cs +proj=utm +zone=a +datum=wgs84 +to +proj=utm +zone=a +datum=datum2 +towgs84 dx,dy,dz<br>
input: x1,y2<br>output: x2,y2 <br>2. +proj=utm +zone=a +datum=datum2 <b>+towgs84 (-1)dx,(-1)dy,(-1)dz</b> +to +proj=utm +zone=a +datum=wgs84 <br>input: x2,y2<br>output:x1,y1<br><br>Or am I completely lost here?<br><br>Thanks for your patience...<br>
<br>I<br><br><br><div class="gmail_quote">On Sat, Apr 24, 2010 at 3:06 PM, Noel Zinn <span dir="ltr"><<a href="mailto:ndzinn@comcast.net">ndzinn@comcast.net</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
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<p class="MsoNormal"><font face="Arial" size="2" color="navy"><span style="font-size: 10pt; font-family: Arial; color: navy;">Not true. A round trip will always close.
You’ll have learned nothing about the correct rotation sense. You can
test this yourself. Do one round trip with the rotations unchanged and do
another round trip with the rotations negated. They’ll both close on the
original point. Correctness is determined by the intermediate coordinates
(right or wrong), not by the round-trip closure. –Noel Zinn</span></font></p>
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<p class="MsoNormal"><b><font face="Tahoma" size="2"><span style="font-size: 10pt; font-family: Tahoma; font-weight: bold;">From:</span></font></b><font face="Tahoma" size="2"><span style="font-size: 10pt; font-family: Tahoma;">
<a href="mailto:proj-bounces@lists.maptools.org" target="_blank">proj-bounces@lists.maptools.org</a> [mailto:<a href="mailto:proj-bounces@lists.maptools.org" target="_blank">proj-bounces@lists.maptools.org</a>] <b><span style="font-weight: bold;">On Behalf Of </span></b>Ilumas<br>
<b><span style="font-weight: bold;">Sent:</span></b> Saturday, April 24, 2010
3:02 AM<div class="im"><br>
<b><span style="font-weight: bold;">To:</span></b> PROJ.4
and general Projections Discussions<br>
<b><span style="font-weight: bold;">Subject:</span></b> Re: [Proj] cs2cs's
+towgs84 usage</div></span></font></p>
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<p class="MsoNormal"><font face="Times New Roman" size="3"><span style="font-size: 12pt;"> </span></font></p>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">Well, I agree that for assessing the accuracy of the re-projection
results one needs a reference, but that should not be necessary for
understanding the behavior of the cs2cs or the towgs84 string.
Therefore I still argue that the test I suggested below is a valid way to test
whether the parameter values need to be negated or not. Anyone?</span></font></p><div><div></div><div class="h5">
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<p class="MsoNormal" style="margin-bottom: 12pt;"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">ilumas</span></font></p>
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<p class="MsoNormal"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">On Fri, Apr 23, 2010 at 3:39 PM, Jean-Claude REPETTO <<a href="mailto:jrepetto@free.fr" target="_blank">jrepetto@free.fr</a>> wrote:</span></font></p>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">Le 23/04/2010 14:24, Ilumas a écrit :</span></font></p>
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<p class="MsoNormal"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">><br>
> Anyway, I believe that the behaviour of a certain towgs84 string is easy</span></font></p>
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<p class="MsoNormal"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">> to test with cs2cs itself: just do the transformation to one
direction</span></font></p>
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<p class="MsoNormal" style="margin-bottom: 12pt;"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">> (e.g. from WGS84)
and convert the resulting coordinates back to the<br>
> original datum using exactly the same parameter string. If the resulting<br>
> coordinates equal to the original ones, no negation is needed. Right?<br>
><br>
> ilumas</span></font></p>
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<p class="MsoNormal"><font face="Times New Roman" size="3"><span style="font-size: 12pt;">Hi,nding the behavious of the cs2cs<br>
<br>
I think you are wrong. To check the parameters, you need to know the<br>
coordinates of at least one point in both datums, and compare them with<br>
the results of cs2cs.<br>
<font color="#888888"><span style="color: rgb(136, 136, 136);"><br>
Jean-Claude</span></font></span></font></p>
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