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<p>Hi,</p>
<p>VDATUM will change the GEIOD anywhere on earth so you start with
a vertical transformation in gdal and then with the horizontal
transformation in VDATUM.<br>
</p>
<p>Nicolas<br>
</p>
<div class="moz-cite-prefix">On 2021-04-19 11:04 a.m., Raife Crookes
wrote:<br>
</div>
<blockquote type="cite"
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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Good
afternoon Patrick and Nicolas,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Many
thanks for your input,
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Nicolas
unfortunately the VDatum doesn’t yet have transformation for
UK co-ordinate systems.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Patrick,
your code below
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">“</span>gdalwarp
-s_srs "+proj=longlat +datum=WGS84 +no_defs
+geoidgrids=egm96_15.gtx" -t_srs "EPSG:4326" dem-geoid.tif
dem-hae.tif”<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Has done the job if I do it in two steps:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">1<sup>st</sup> import the EPSG:27700 data
set and translate this to EPSG:4326.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">2<sup>nd</sup> Run the code you suggested
to correct for elevation.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It would be ace to have found a way to do
this in one step but frankly the two step seems workable.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Kind regards,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Raife<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span
lang="EN-US"> Patrick Young
<a class="moz-txt-link-rfc2396E" href="mailto:patrick.mckendree.young@gmail.com"><patrick.mckendree.young@gmail.com></a>
<br>
<b>Sent:</b> 19 April 2021 03:31<br>
<b>To:</b> Raife Crookes <a class="moz-txt-link-rfc2396E" href="mailto:raife@tp-regen.co.uk"><raife@tp-regen.co.uk></a><br>
<b>Cc:</b> <a class="moz-txt-link-abbreviated" href="mailto:proj@lists.osgeo.org">proj@lists.osgeo.org</a><br>
<b>Subject:</b> Re: [PROJ] Assistance with GDALWarp -
ESPG:27700 to ESPG:4326 - Elipsod level not sea level<o:p></o:p></span></p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal">Hi Raife, <o:p></o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">I think what is missing is the explicit
vertical datum associated with that code. I've come
across this myself when converting a DTM to ellipsoidal
heights. Here's an example from my bash history that
converts the heights with gdalwarp:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">gdalwarp -s_srs "+proj=longlat
+datum=WGS84 +no_defs +geoidgrids=egm96_15.gtx" -t_srs
"EPSG:4326" dem-geoid.tif dem-hae.tif<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">I think the trick is just figuring out
the right proj strings/finding the right EPSG compound
code (there's an example at the bottom of this page <a
href="https://gdal.org/programs/gdalwarp.html#gdalwarp"
moz-do-not-send="true">https://gdal.org/programs/gdalwarp.html#gdalwarp</a>).
I think under the hood it's using this fellow, so at the
end of the day you just have to find the right grid:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"><a
href="https://gdal.org/api/gdal_alg.html?highlight=applyvert#_CPPv426GDALApplyVerticalShiftGrid12GDALDatasetH12GDALDatasetHiddPPCKc"
moz-do-not-send="true">https://gdal.org/api/gdal_alg.html?highlight=applyvert#_CPPv426GDALApplyVerticalShiftGrid12GDALDatasetH12GDALDatasetHiddPPCKc</a><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">P<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal">On Sun, Apr 18, 2021 at 9:17 AM Raife
Crookes <<a href="mailto:raife@tp-regen.co.uk"
moz-do-not-send="true">raife@tp-regen.co.uk</a>>
wrote:<o:p></o:p></p>
</div>
<blockquote style="border:none;border-left:solid #CCCCCC
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<div>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Good
evening,<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I
am hoping that someone could assist me with the
completion of a reprojection of a geotiff lidar data
set to WGS84.
<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">The
transformation should me making an elevation change of
roughly 50m for known points however after running
GDAL warp the elevations are staying the same when
viewed through QGIS.
<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">For
some reason it seems to be defaulting to the WGS84
geoid height not the ellipsoid height I require for
import onto our drone. (My knowledge is that EPSG:4326
should equate to the wgs lat/long and ellipsoid
height.)<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">GDALWARP
instructions:<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">gdalwarp
-s_srs EPSG:27700 -t_srs EPSG:4326<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">gdalwarp
-s_srs -t_srs EPSG:4326<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Proj
info return:<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">C:\>projinfo
-s EPSG:27700 -t EPSG:4326 -o PROJ<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Candidate
operations found: 1<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Note:
using '--spatial-test intersects' would bring more
results (7)<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">-------------------------------------<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Operation
No. 1:<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">unknown
id, Inverse of British National Grid + OSGB 1936 to
WGS 84 (9), 1 m, UK - Britain and UKCS 49<span
style="font-family:"Arial",sans-serif">┬░</span>46'N
to 61<span
style="font-family:"Arial",sans-serif">┬░</span>01'N,
7<span
style="font-family:"Arial",sans-serif">┬░</span>33'W
to 3<span
style="font-family:"Arial",sans-serif">┬░</span>33'E<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">PROJ
string:<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">+proj=pipeline
+step +inv +proj=tmerc +lat_0=49 +lon_0=-2
+k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy
+step +proj=hgridshift
+grids=OSTN15_NTv2_OSGBtoETRS.gsb +step
+proj=unitconvert +xy_in=rad +xy_out=deg +step
+proj=axisswap +order=2,1<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I
can send copies of the original file if requested.
<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hoping
that its something stupid like im using the wrong EPSG
code but have a terrible feeling that a catch all to
prevent users accidentally using the ellipsoid height
is catching me out. <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">(Ground
height OSGB – 30m in WGS84 = 82m)<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Really
look forward to your help/<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Kind
regards<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Raife<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
</div>
</div>
<p class="MsoNormal">_______________________________________________<br>
PROJ mailing list<br>
<a href="mailto:PROJ@lists.osgeo.org" target="_blank"
moz-do-not-send="true">PROJ@lists.osgeo.org</a><br>
<a href="https://lists.osgeo.org/mailman/listinfo/proj"
target="_blank" moz-do-not-send="true">https://lists.osgeo.org/mailman/listinfo/proj</a><o:p></o:p></p>
</blockquote>
</div>
</div>
</blockquote>
<pre class="moz-signature" cols="72">--
Nicolas Cadieux
<a class="moz-txt-link-freetext" href="https://gitlab.com/njacadieux">https://gitlab.com/njacadieux</a></pre>
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