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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Hi Nathan,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><span style="mso-list:Ignore">1)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">cs2cs, and the underlying pj_transform(), need this to know that it should do a datum-shift. You need either
a +datum, +towgs84 or a +nadgrids parameter set on both source and target. <o:p>
</o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><span style="mso-list:Ignore">2)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Optimization – The alternative is to perform a 7-parameter shift for every possible coordinate transformation.
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><span style="mso-list:Ignore">3)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Yes.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">This all happen in parts of PROJ that can be considered legacy code. As you may know we are in the process of giving PROJ
a big overhaul that, amongst other things, are meant to make things like this more transparent. From PROJ v. 6 and on your cs2cs call will internally be translated to something like<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:65.2pt"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:65.2pt"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">+proj=pipeline
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:65.2pt"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">+step +inv +proj=utm +zone=55 +south +ellps=GRS80<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:65.2pt"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">+step +proj=hgridshift +grids=C:\dev\proj-datumgrid\oceania\GDA94_GDA2020_conformal.gsb<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:65.2pt"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">+step +proj=utm +zone=55 +south +ellps=GRS80
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">which in my opinion is a lot simpler to understand.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">/Kristian<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">Fra:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> PROJ <proj-bounces@lists.osgeo.org>
<b>På vegne af </b>Nathan Woodrow<br>
<b>Sendt:</b> 22. januar 2019 01:00<br>
<b>Til:</b> proj@lists.osgeo.org<br>
<b>Emne:</b> [PROJ] Help understanding grid files and +towgs84<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hi,<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 I'm mainly just trying to understand how and why this works just for my own knowledge.<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 currently working on a GDA94 > GDA2020 transform project and noticed that the proj4 string that ships with everything (GDAL, QGIS, PostGIS) only has <span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#263238">+proj=utm
+zone=55 +south +ellps=GRS80 +units=m +no_defs but in order to get the transform to work correctly I need to add the grid to the source and the +towgs84=0,0,0,0,0,0,0 to the target (GDA2020) proj4 sring.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">ogr2ogr command was:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#263238">ogr2ogr -s_srs "+proj=utm +zone=55 +south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs +wktext +nadgrids=C:\temp\GDA94_GDA2020_conformal.gsb" <br>
-t_srs "+proj=utm +zone=55 +south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs +wktext" out.shp in.shp</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#263238">Now I'm just trying to understand mainly:</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#263238">1) Why is </span><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#263238">+towgs84=0,0,0,0,0,0,0 needed on the target? As if it's not there it will just return
the same values as the input</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#263238">2) Why is that not included as default? </span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#263238">3) Is my understanding of the following correct? Proj is really doing: GDA94 -> WGS84 -> Shifted WGS84 via grid -> GDA2020?</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#263238">Thanks,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#263238">Nathan</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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