<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>