<div dir="ltr"><div dir="ltr"><div>Hi Even.</div><div><br></div><div>The example in data/sql/transformations_czechia_extra.sql was very useful.</div><div>First I am trying to reproduce the transformation with a pipeline, and later I will make the proper operations and concatenated operations in the database.</div><div><br></div><div>However this Greek transformation is more convoluted than I expected (initially I misunderstood some steps).</div><div><br></div><div>a) First they have a Helmert from ETRS89 (from their realization, but I do not care) to GGRS87. So far so good (params are not the ones from EPSG, but no problem)</div><div>b) Then they apply the transverse Mercator to "GGRS87 / Greek Grid". All fine.</div><div>c) In parallel they project ETRS89 coordinates into a different transverse Mercator, called TM07 (not in EPSG; they provide the params). The grid with the horizontal corrections are in TM07</div><div>d) the final projected coordinates in "GGRS87 / Greek Grid" are the values from b) plus the corrections from the grid in c)</div><div><br></div><div>See that the easting-northing coordinates to get the correction from the grid are not the same as the coordinates from b). To make it clear, TM07 has a false-northing of -2000 km</div><div><br></div><div>How can it be done in a pipeline? (later to be converted into something in proj.db)</div><div>I do not know how to keep the corrections somewhere to be added later to the projected coordinates.</div><div><br></div><div>Thanks</div><div>Javier</div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, 3 Feb 2025 at 16:54, Even Rouault <<a href="mailto:even.rouault@spatialys.com" target="_blank">even.rouault@spatialys.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"><u></u>
<div>
<p>Javier,</p>
<p>you can probably hack around BOUNDCRS in a totally PROJ-specific
way by using the "METHOD["PROJ-based operation method:
{proj_pipeline_here}" <br>
<br>
</p>
<p>Otherwise having records in the DB will certainly be cleaner.
data/sql/transformations_czechia_extra.sql can probably be used as
an example of quite advanced transformations, including using a
grid referenced in a projected CRS.</p>
<p>Even<br>
</p>
<p><br>
</p>
<div>Le 03/02/2025 à 16:26, Javier Jimenez
Shaw via PROJ a écrit :<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi</div>
<div><br>
</div>
<div>I am trying to make this transformation (that is not in
EPGS). I do not know how far I can go with PROJ.</div>
<div><br>
</div>
<div>(Apparently this transformation is needed to go from the
coordinates used by HEPOS.gr, the Greek positioning system, to
the official -I think- CRS in Greece, <span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">GGRS87</span>.</div>
<div>The best is that the transformation would be in EPSG. Do
you know anybody in Greece with the proper contacts? I am
happy to talk with them about it, and help if needed)</div>
<div><br>
</div>
<div>Until it is in EPSG, I would like to use PROJ to do it.<br>
</div>
<div><br>
</div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">The
accurate transformation from ETRS89 to GGRS87 is something
like:</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">-
apply a Helmert transformation</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">-
Project into a transverse Mercator</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">-
Apply a correction grid. Yes, in the projected system.<br>
</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">-
(optional) go back from the projected to the geographic crs.</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none"><br>
</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">I
thought about several options:<br>
</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none"><br>
</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">-
First option: create a WKT2 with a BOUNDCRS that defines the
transformation. Can it be done in a single WKT2?<br>
</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">-
Second option: add the transformation to proj.db. Is it
doable?<br>
</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">-
Third option: Create "my own grid" file (in the geographic
system) combining all the steps. Of course, it will loose
some accuracy due to the new grid interpolation.<br>
</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none"><br>
</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">Any
clue?</span></div>
<div><span style="color:rgb(34,34,34);font-family:Arial,Helvetica,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;word-spacing:0px;white-space:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;display:inline;float:none">Thanks.<br>
</span></div>
</div>
<br>
<fieldset></fieldset>
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