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<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
11.0pt;font-family:Arial;color:blue'>Gotcha. Frank answered your question yesterday:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
11.0pt;font-family:Arial;color:blue'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
11.0pt;font-family:Arial;color:blue'>The parameters are always expressed in
terms of what it takes to transform to WGS84. If PROJ is transforming
from WGS84 it will internally reverse the sense of the transformation. The
parameters are always expressed in terms of what it takes to transform to
WGS84. If PROJ is transforming from WGS84 it will internally reverse the
sense of the transformation.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
11.0pt;font-family:Arial;color:blue'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
11.0pt;font-family:Arial;color:blue'>And you can test this as you propose.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
11.0pt;font-family:Arial;color:blue'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
11.0pt;font-family:Arial;color:blue'>But this test tells you nothing about the
correct rotation sense to use. For that you need an external test (truth)
point with coordinates in both datums.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
11.0pt;font-family:Arial;color:blue'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
11.0pt;font-family:Arial;color:blue'>Noel<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<div>
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face="Times New Roman"><span style='font-size:12.0pt'>
<hr size=2 width="100%" align=center tabindex=-1>
</span></font></div>
<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
proj-bounces@lists.maptools.org [mailto:proj-bounces@lists.maptools.org] <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
7:52 AM<br>
<b><span style='font-weight:bold'>To:</span></b> <st1:PersonName w:st="on">PROJ.4
and general Projections Discussions</st1:PersonName><br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [Proj] cs2cs's
+towgs84 usage</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>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><span style='font-weight:bold'>+towgs84
(-1)dx,(-1)dy,(-1)dz</span></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>
<o:p></o:p></span></font></p>
<div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>On Sat, Apr 24, 2010 at 3:06 PM, Noel Zinn <<a
href="mailto:ndzinn@comcast.net">ndzinn@comcast.net</a>> wrote:<o:p></o:p></span></font></p>
<div link=blue vlink=blue>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=2 color=navy face=Arial><span style='font-size:10.0pt;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><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=2 color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'> </span></font><o:p></o:p></p>
<div>
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face="Times New Roman"><span style='font-size:12.0pt'>
<hr size=2 width="100%" align=center>
</span></font></div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><b><font
size=2 face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma;font-weight:
bold'>From:</span></font></b><font size=2 face=Tahoma><span style='font-size:
10.0pt;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<o:p></o:p></span></font></p>
<div>
<p class=MsoNormal><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma'><br>
<b><span style='font-weight:bold'>To:</span></b> <st1:PersonName w:st="on">PROJ.4
and general Projections Discussions</st1:PersonName><br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [Proj] cs2cs's
+towgs84 usage<o:p></o:p></span></font></p>
</div>
</div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'>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?<o:p></o:p></span></font></p>
<div>
<div>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'> <o:p></o:p></span></font></p>
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<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;margin-bottom:12.0pt'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'>ilumas<o:p></o:p></span></font></p>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'>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:<o:p></o:p></span></font></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'>Le 23/04/2010
14:24, Ilumas a écrit :<o:p></o:p></span></font></p>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'>><br>
> Anyway, I believe that the behaviour of a certain towgs84 string is easy<o:p></o:p></span></font></p>
</div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'>> to test with
cs2cs itself: just do the transformation to one direction<o:p></o:p></span></font></p>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;margin-bottom:12.0pt'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'>> (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<o:p></o:p></span></font></p>
</div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'>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:#888888'><br>
Jean-Claude</span></font><o:p></o:p></span></font></p>
<div>
<div>
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<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><font
size=3 face="Times New Roman"><span style='font-size:12.0pt'> <o:p></o:p></span></font></p>
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