<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<blockquote type="cite"
cite="mid:DU2P192MB2235E2289ED1F870C41C4D1EEF76A@DU2P192MB2235.EURP192.PROD.OUTLOOK.COM">
<div class="WordSection1">
<p class="MsoPlainText"><span lang="EN-GB">[GT]: So was the bug
confusing orthometric and ellipsoidal?<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">No, PROJ used an inappropriate geoid. The 2D
Amersfoort datum of the 2D target CRS RD has an ellipsoidal
height within 1 m of MSL in the Netherlands. PROJ
nevertheless thought it was a good idea to take the geoid
for the vertical target CRS NAP that is to be used on 3D
ETRS89 and apply it on the Amersfoort ellipsoidal height.</span></p>
</div>
</blockquote>
<p>A bit more subtle than that. It did:</p>
<p>- WGS 84 + EGM 2008 -> WGS 84 3D geographic using EGM2008
height: OK</p>
<p>- use of "<code class="notranslate">Amersfoort to WGS 84 (4)"
Helmert transformation in reverse direction, promoting the
Amersfoort CRS to a 3D geographic one and do the Helmert
transformation in 3D. That's the part that was questionnable
(yeah, wrong) in this context since that Helmert
transformatioon is likely only intended for 2D. Unfortunately
the EPSG database doesn't distinguish cases where this is valid
and those where it is not. The only hint is that if there's no
official record for a 3D geographic CRS, doing the Helmert
transformation in 3D is likely to not be correct</code></p>
<p><code class="notranslate">- apply the "Amersfoort to ETRS89-NLD
[AGRS2010]" transformation using the 2D grid</code></p>
<p><code class="notranslate">- apply the "</code><code
class="notranslate">ETRS89-NLD [AGRS2010] to NAP height (2)"
transformation using the geoid grid</code></p>
<p><code class="notranslate">- apply the </code><code
class="notranslate">"Amersfoort to ETRS89-NLD [AGRS2010]" in the
reverse direction, to finally get Amersfoort + NAP height
coordinates</code></p>
<pre class="moz-signature" cols="72">
--
<a class="moz-txt-link-freetext" href="http://www.spatialys.com">http://www.spatialys.com</a>
My software is free, but my time generally not.</pre>
</body>
</html>