<div dir="ltr">Hi<div><br></div><div>Something I learned this year is that those transformations are not like a "conservative field". Many times there are "frictions", approximations, that produce the effect you noticed. In the EPSG you can find many of those transformations. But there is not always a direct transformation from A to C (or different options). Definitely there are different precisions. PROJ does its best to use the proper transformation(s), but it is not easy. Including concatenating some of them.</div><div><br></div><div>AFAIK, PROJ4 always used WGS84 as the pivot CRS. Now it is more complex.</div><div><br></div><div>Maybe you can find more info in <a href="https://proj.org/operations/operations_computation.html" target="_blank">https://proj.org/operations/operations_computation.html</a></div><div><br></div><div>Cheers.<br clear="all"><div><div dir="ltr" data-smartmail="gmail_signature">.___ ._ ..._ .. . ._. .___ .. __ . _. . __.. ... .... ._ .__<br>Entre dos pensamientos racionales <br>hay infinitos pensamientos irracionales.<br><br></div></div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, 5 Sep 2020 at 01:24, <<a href="mailto:henning@kaarta.com" target="_blank">henning@kaarta.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-US"><div><p class="MsoNormal">Hi all,<u></u><u></u></p><p class="MsoNormal">New to PROJ, using 7.1.1 on linux, still trying to learn the basics.<u></u><u></u></p><p class="MsoNormal"><u></u> <u></u></p><p class="MsoNormal">So it seems to me that one should get the same results from<u></u><u></u></p><p class="MsoNormal">cs2cs +init=<crs1> +to +init=<crs2> | cs2cs +init=<crs2> +to +init=<crs3><u></u><u></u></p><p class="MsoNormal">as<u></u><u></u></p><p class="MsoNormal">cs2cs +init=<crs1> +to +init=<crs3><u></u><u></u></p><p class="MsoNormal"><u></u> <u></u></p><p class="MsoNormal">i.e. a transform to an intermediate CRS shouldn’t affect the final result, but it seems in my case at least it does. <u></u><u></u></p><p class="MsoNormal">E.g.<u></u><u></u></p><p class="MsoNormal"><u></u> <u></u></p><p class="MsoNormal">cs2cs -d 6 +init=epsg:4326 +to +init=epsg:7856+5711<u></u><u></u></p><p class="MsoNormal">151 -33 5<u></u><u></u></p><p class="MsoNormal">313152.777214 6346936.495810 -20.742001<u></u><u></u></p><p class="MsoNormal"><u></u> <u></u></p><p class="MsoNormal">vs.<u></u><u></u></p><p class="MsoNormal"><u></u> <u></u></p><p class="MsoNormal">cs2cs -d 6 +init=epsg:4326 +to +init=epsg:32756 | cs2cs -d 6 +init=epsg:32756 +to +init=epsg:7856+5711<u></u><u></u></p><p class="MsoNormal">151 -33 5<u></u><u></u></p><p class="MsoNormal">313153.271255 6346937.914960 -20.853001<u></u><u></u></p><p class="MsoNormal"><u></u> <u></u></p><p class="MsoNormal">Close, but off by about 1.7m. How would I know which one is correct, and why are they different?<u></u><u></u></p><p class="MsoNormal">I must be missing something…thanks!<u></u><u></u></p><p class="MsoNormal"><u></u> <u></u></p><p class="MsoNormal"><u></u> <u></u></p><p class="MsoNormal"><u></u> <u></u></p></div><div id="gmail-m_5335995481913226437gmail-m_7380985001490801748gmail-m_5932857843784429216DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2"><br>
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