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<p class="MsoPlainText" style="margin-left:70.8pt"><span lang="EN-GB">I wrote:<o:p></o:p></span></p>
<p class="MsoPlainText" style="margin-left:70.8pt"><span lang="EN-GB">> To prevent users to make different choices in similar use cases, the documentation should indicate what to do for Lowest Astronomical Tide (LAT) or other Chart Datum grids.<o:p></o:p></span></p>
<p class="MsoPlainText"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoPlainText" style="margin-left:35.4pt"><span lang="EN-GB">ER wrote:<o:p></o:p></span></p>
<p class="MsoPlainText" style="margin-left:35.4pt"><span lang="EN-GB">> Cf https://github.com/OSGeo/PROJ/pull/3668 + https://github.com/OSGeo/PROJ-data/pull/97<o:p></o:p></span></p>
<p class="MsoPlainText"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span lang="EN-GB">Thanks! This means that the Geodetic TIFF Grid for Chart Datum of Norway (and possibly also the one of Denmark that Kristian Evers is working on) wrongly use type VERTICAL_OFFSET_VERTICAL_TO_VERTICAL instead of VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL.
But as that works too, it’s not a big deal.<o:p></o:p></span></p>
<p class="MsoPlainText"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span lang="EN-GB">I have two questions:<o:p></o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoPlainText" style="margin-left:-18.0pt;mso-list:l0 level1 lfo1"><span lang="EN-GB">Are you sure about the tag “hydroid_height”? A Google search (<a href="https://www.google.com/search?q=%22hydroid+height%22">https://www.google.com/search?q=%22hydroid+height%22</a>)
gives only 1 relevant hit. I have no opinion on this, as it is not my expertise. I’m just helping the people at our hydrographic service. <o:p></o:p></span></li><li class="MsoPlainText" style="margin-left:-18.0pt;mso-list:l0 level1 lfo1"><span lang="EN-GB">Why limit the description to only predefined options? I can imagine that one could want to use a reference surface that is not a geoid nor a hydroid, for example
an ellipsoidal DTM grid to transform to/from Above Ground Level heights. So it might be easier to allow any description to prevent future problems.<o:p></o:p></span></li></ul>
<p class="MsoPlainText"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span lang="EN-GB">I will probably use type VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL with a new description (e.g. hydroid_height) for the tif grid. But I’ll also make a gtx grid for users of PROJ versions that don’t accept the new description.
I might use the gtx grid for application in EPSG for that reason. That might be easier anyway, since there are not yet tif grids in EPSG as far as I know.<o:p></o:p></span></p>
<p class="MsoPlainText"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoPlainText"><span lang="EN-GB">Jochem<o:p></o:p></span></p>
<p class="MsoPlainText"><span lang="EN-GB"><o:p> </o:p></span></p>
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