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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi Raven,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The reason why there’s only one transformation is only because when the decoupling occurred, the dictionaries where implemented in order to provide the same
level of functionality as before the decoupling (i.e. before the creation the transformation dictionary).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The goal was that the new engine would not impact any existing results. It would also not provide any new system definition. For the end user, changes were
seamless.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">It doesn’t mean that we cannot have more than one transformation for a given datum.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">It’s actually one of the goals of the decoupling.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">If we have more than one transformation, the engine needs to make a choice between the different options.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">How could this happen?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">A possible solution to this issue could be that each time the system encounters this situation, ask the user to choose which one to use.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">This solution, however, presents big problems:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l2 level1 lfo3"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">1><span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The dialog asking the user for import would have to be inserted in tons of places, and the engine could not run smoothly without user interaction.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l2 level1 lfo3"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">2><span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The user probably wouldn’t know which one to pick, and just make a guess.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l2 level1 lfo3"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">3><span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">If all users in a shop don’t pick the same one, the data will start to creep.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">For that reason the concept of transformation path was introduced.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">For instance, if we have DATUM1 and want to have a Geodetic Transformation to WGS84 for instance, and have multiple Geodetic Transformations for this, called
GT1, GT2, GT3, a Geodetic Transformation Path has to be created, GTP_DATUM1_TO_WGS84, inside which, you specify which geodetic transformation has to be used.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">If you don’t do that, the engine will have to pick one transformation and that might not be the one you’d like to use. I think it would be the first one the
engine finds in the dictionary (not sure, to be dbl checked).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">You cannot have more than one Geodetic Transformation Path for a given source => target datum.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">In case of complex transformations involving multiple datums here is how it works.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The building of a complex transformation from one datum to another is a four phase process:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">1><span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Check the Path Dictionary. If a path from Source to Target is defined, it is used and the process is complete.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">2><span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Check the Transformation Dictionary for a transformation which converts directly from the source to the target. If such exists, it is used and the
process is complete.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">3><span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Using a specific pivot datum, examine the transformation dictionary for two complementary transformations: source to the pivot and, pivot to target.
If this is successful, a path of two transformations is generated and this path is used and the process stops. (The code will work through a prioritized list of pivot datums. Currently, this list consists of a single datum: “WGS84”.)<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">4><span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">If we still don’t have a transformation path after all of this, we examine the transformation dictionary for transformations from the source datum,
and transformations to the target datum. These definitions are considered if, and only if, there is a single transformation in the dictionary which references that datum. These unique transformations replace the current source and target datums and the whole
process starts over at phase one. That is, if there is only one transformation in the entire transformation dictionary that converts from Datum1 (the source datum), then clearly, any valid path will need to start with this transformation. Similarly, with
the target datum. If there is only one transformation in the entire dictionary that produces target datum coordinates, than that datum must indeed be the last transformation in the path.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">This process fails if:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">A><span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">A path from the source to target is not found, or<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">B><span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">More than one path definition from the source to the target is found.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Back to your question, we can therefore “</span>start seeing multiple transformations attached to the same datum<span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The only important thing this makes me think about though, is that if we do that, we cannot really add a path to choose this new transformation and risk to
impact an existing user out there. This would change users’ result and we always work CS-Map changes around that so that this doesn’t occur.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Such path would have to be customized locally after install to choose this new transformation.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hugues<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> metacrs-bounces@lists.osgeo.org [mailto:metacrs-bounces@lists.osgeo.org]
<b>On Behalf Of </b>Raven Kopelman<br>
<b>Sent:</b> Friday, January 17, 2014 9:02 PM<br>
<b>To:</b> metacrs@lists.osgeo.org<br>
<b>Subject:</b> [MetaCRS] CS-Map policy on multiple transformations per datum<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hi all,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I'm noticing that in spite of the effort to decouple datums from transformations, most (all?) datums still only have a single transformation defined. This includes the new JGD2011 datum (<a href="http://trac.osgeo.org/csmap/ticket/153">http://trac.osgeo.org/csmap/ticket/153</a>),
of which the related systems seem to have been very carefully tailored to use a combination of two datums to avoid adding multiple transformations for the datum.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">My suspicion is that this all ties into the transformation picking algorithm that fails if there are multiple valid choices.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Going forward, are we likely to start seeing multiple transformations attached to the same datum? This is interesting to me as it will impact how compatible the FME specific definition changes I am making are going to be with the mainline
development.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">--<br>
Raven Kopelman | Software Developer<br>
<br>
Safe Software Inc.<br>
T 604.501.9985 x 331<br>
<a href="mailto:raven.kopelman@safe.com" target="_blank">raven.kopelman@safe.com</a> |
<a href="http://www.safe.com" target="_blank">www.safe.com</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><a href="http://www.safe.com/learning/events/"><span style="text-decoration:none"><img border="0" id="_x0000_i1025" src="http://i943.photobucket.com/albums/ad276/Safe_Software/FME-UCampWT-email-signature-2_zpsc0db77c5.png"></span></a><o:p></o:p></p>
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