<div dir="ltr">Thank you Richard,<div><br></div><div>I am not from the GIS world and not even a QGIS user, so bear with me if my answers do not use the expected vocabulary.</div><div><br></div><div>From my point of view, my netCDF files describe their geolocation in a CF-compliant manner, with a grid_mapping CRS variable, and the x and y projection coordinate variables. But QGIS does not recognize the CRS, seemingly because I have my x and y as km (and I do specify :units="km", I am not hiding this).</div><div><br></div><div>Since I am also a programmer, I tried to find where in the QGIS code the netCDF/CF geolocation is read and decoded, to see if there was a strict test on :units="m". But I failed to find this in the code.</div><div><br></div><div>Before I fill out a bug report, I wanted to hear from a QGIS developer (or someone who knows how to navigate the software code) if we can identify where the strict test on :units="m" is in the code, and see if alternative solutions (accepting other units, all multiples of meters) was feasible.</div><div><br></div><div>All the best,</div><div>Thomas</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">fre. 15. sep. 2023 kl. 15:16 skrev Richard McDonnell <<a href="mailto:richard.mcdonnell@opw.ie">richard.mcdonnell@opw.ie</a>>:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div class="msg-1032967726710090140">
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<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">Thomas,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">I’m open to correction on this, but meters would generally be the default unit used in GIS, when dealing with metric datasets.
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">What units you use to measure on your Map Canvas or store as attribution is then a different thing.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">I had a look for the EPSG codes for EASE2 but all I could get was
<b><a href="https://epsg.io/6931" target="_blank">EPSG:6931</a></b><u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">If you look at the units there you can see meters as the UoM.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">I hope that helps,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">Richard.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11pt;font-family:Calibri,sans-serif">From:</span></b><span lang="EN-US" style="font-size:11pt;font-family:Calibri,sans-serif"> QGIS-User <<a href="mailto:qgis-user-bounces@lists.osgeo.org" target="_blank">qgis-user-bounces@lists.osgeo.org</a>>
<b>On Behalf Of </b>Thomas Lavergne via QGIS-User<br>
<b>Sent:</b> 15 September 2023 13:34<br>
<b>To:</b> <a href="mailto:qgis-user@lists.osgeo.org" target="_blank">qgis-user@lists.osgeo.org</a><br>
<b>Subject:</b> [Qgis-user] Does QGIS require units "m" for the projection coordinate variables in a netCDF/CF file?<u></u><u></u></span></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">Dear QGIS community,<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">We recently released a number of sea-ice climate data records stored in netCDF/CF files.<u></u><u></u></p>
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<p class="MsoNormal">An example file is <a href="https://thredds.met.no/thredds/catalog/osisaf/met.no/reprocessed/ice/drift_455m_files/merged/2020/12/catalog.html?dataset=osisaf/met.no/reprocessed/ice/drift_455m_files/merged/2020/12/ice_drift_nh_ease2-750_cdr-v1p0_24h-202012211200.nc" target="_blank">here</a>:<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">Our files use a polar equal area projection (EASE2) and the x / y axis values are given with units "km" (kilometers):<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">double xc(xc) ;<br>
xc:axis = "X" ;<br>
xc:units = "km" ;<br>
xc:long_name = "x coordinate of projection (eastings)" ;<br>
xc:standard_name = "projection_x_coordinate" ;<br>
double yc(yc) ;<br>
yc:axis = "Y" ;<br>
yc:units = "km" ;<br>
yc:long_name = "y coordinate of projection (northings)" ;<br>
yc:standard_name = "projection_y_coordinate" <u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">When opening these in QGIS, they are placed correctly on the map, but the message "Unkown CRS" appears in the lower-right corner.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">By manipulating our files, we could get QGIS to recognize the correct CRS.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">ncap2 -s 'xc=xc*1000f;yc=yc*1000f' <a href="http://infile.nc/" target="_blank">infile.nc</a> <a href="http://outfile.nc/" target="_blank">outfile.nc</a><br>
ncatted -O -a units,xc,m,c,"m" <a href="http://outfile.nc/" target="_blank">outfile.nc</a><br>
ncatted -O -a units,yc,m,c,"m" <a href="http://outfile.nc/" target="_blank">outfile.nc</a><br clear="all">
<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">The NCO commands above change the x / y axis variables to units "m" (meters).<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">The <a href="http://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html" target="_blank">CF convention</a> does not impose that x / y axis variables are given with unit meters, and shows several examples using
units "km". But this is apparently an issue for QGIS.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">Can someone please confirm that QGIS requires "meters" for these variables, and comment if this is a desired feature or if I should open a bug report at github?<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">All the best,<u></u><u></u></p>
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<p class="MsoNormal">Thomas<u></u><u></u></p>
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