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<p class="MsoNormal">Thanks, Kristian, for the thoughtful and extensive response,
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">And the more I think about it, the more it makes sense what you say, that stepping through WGS84 as a hub is often the only practical choice.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It is also helpful to receive the affirmation that I've got it right.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-Don<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman",serif">--<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman",serif">Don MacQueen<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman",serif">Lawrence Livermore National Laboratory<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman",serif">7000 East Ave., L-627<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman",serif">Livermore, CA 94550<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman",serif">925-423-1062<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman",serif">Lab cell 925-724-7509<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman",serif"> <o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black"><proj-bounces@lists.maptools.org> on behalf of Kristian Evers <kreve@sdfe.dk><br>
<b>Reply-To: </b>"PROJ.4 and general Projections Discussions" <proj@lists.maptools.org><br>
<b>Date: </b>Monday, September 24, 2018 at 9:08 PM<br>
<b>To: </b>"PROJ.4 and general Projections Discussions" <proj@lists.maptools.org><br>
<b>Subject: </b>Re: [Proj] Question about migrating to Proj 5.x.x<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Don, <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 see that you are looking at an outdated development version of the docs. I should have removed that a long time ago. Please go to
<a href="https://proj4.org">https://proj4.org</a> instead. The migration guide is the same but the rest of the docs are significantly improved.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Regarding the pipeline you’ve described below, yes it is exactly same as what cs2cs is doing. While stepping through WGS84 as a hub is generally a bad idea, quite often it is the only practical choice. Either because you haven’t got access
to a better way to do the transformation or because that is simply how they are defined. Often times there will be a better way to do the transformation but we still haven’t got a mechanism for helping you set that up. We are working on that though (or Even
is, I should say).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">So in summary, you’ve got it all right. I acknowledge that we haven’t made it particularly easy to do transformations as simple as they are done with pj_transform(). The function proj_create_crs_to_crs() is meant to replace it but so far
it has been restricted to +init setups which unfortunately has led to most people doing similar pipelines as below invoked with proj_create(). I intend to make this easier in PROJ 6.0.0.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">For examples of how the pipeline framework can be used to do transformations without stepping through WGS84 have a look at <a href="https://github.com/NordicGeodesy/NordicTransformations">https://github.com/NordicGeodesy/NordicTransformations</a>.
These are a collection of transformations used in the Nordic countries and none of them use WGS84 a hub. Some are quite complicated (have a look at the resources/NKG file) and some are rather similar to what you are used to from cs2cs.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">/Kristian<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<o:p></o:p></p>
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<p class="MsoNormal">On 25 Sep 2018, at 01:01, MacQueen, Don <<a href="mailto:macqueen1@llnl.gov">macqueen1@llnl.gov</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">In effect, this is a question about whether or how well I understand the new pipeline approach.<br>
<br>
Here's an example transformation I have been using at my site, using cs2cs. It is considered to be correct.<br>
<br>
echo 1501839.77549 456388.976491 | \<br>
cs2cs \<br>
+from \<br>
+init=epsg:26743 \<br>
+nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat \<br>
+to \<br>
+init=epsg:2227 \<br>
+nadgrids=null<br>
<br>
The online Version 4 to Version 5 API migration guide<br>
( <a href="https://kbevers.github.io/development/migration.html">https://kbevers.github.io/development/migration.html</a> )<br>
describes that cs2cs does a transformation like this one by passing "through the ill-defined WGS84 reference frame, using it as a hub" and points out shortcomings of that approach.
<br>
Various descriptions I've seen of the new pipeline approach have seemed to imply that it's better at least in part because it doesn't use WGS84 as a "hub".<br>
<br>
Following the example in the migration guide, here's what I've found for performing the same using cct:<br>
<br>
echo 1501839.77549 456388.976491 | \<br>
cct -z 0 -t 0\<br>
+proj=pipeline \<br>
+step \<br>
+inv \<br>
+init=epsg:26743 \<br>
+nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat \<br>
+step \<br>
+init=epsg:2227 \<br>
+nadgrids=null<br>
<br>
The respective outputs match -- great!<br>
<br>
But it also eems to me that by specifying +inv, it is still passing through WGS84. It's now explicit, which is good, but is it really fundamentally different? Is there some other way to get from one projected system to another?<br>
<br>
Or am I just completely confused?<br>
<br>
Thanks<br>
-Don<br>
<br>
--<br>
Don MacQueen<br>
Lawrence Livermore National Laboratory<br>
7000 East Ave., L-627<br>
Livermore, CA 94550<br>
925-423-1062<br>
Lab cell 925-724-7509<br>
<br>
<br>
<br>
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<a href="mailto:Proj@lists.maptools.org">Proj@lists.maptools.org</a><br>
http://lists.maptools.org/mailman/listinfo/proj<o:p></o:p></p>
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