Thank you Richard, 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.
>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). 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. 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. All the best, Thomas fre. 15. sep. 2023 kl. 15:16 skrev Richard McDonnell < richard.mcdonn...@opw.ie>: > Thomas, > > I’m open to correction on this, but meters would generally be the default > unit used in GIS, when dealing with metric datasets. > > What units you use to measure on your Map Canvas or store as attribution > is then a different thing. > > > > I had a look for the EPSG codes for EASE2 but all I could get was *EPSG:6931 > <https://epsg.io/6931>* > > If you look at the units there you can see meters as the UoM. > > I hope that helps, > > > > Richard. > > > > > > > > —— > Richard McDonnell MSc GIS, FME Certified Professional > *FRM Data Management* > > —— > Oifig na nOibreacha Poiblí > Office of Public Works > > Sráid Jonathan Swift, Baile Átha Troim, Co na Mí, C15 NX36 > Jonathan Swift Street, Trim, Co Meath, C15 NX36 > —— > M +353 87 688 5964 T +353 46 942 2409 > https://gov.ie/opw > > —— > To send me files larger than 30MB, please use the link below > https://filetransfer.opw.ie/filedrop/richard.mcdonn...@opw.ie > > Email Disclaimer: > https://www.gov.ie/en/organisation-information/439daf-email-disclaimer/ > > —— > MSc GIS, FME Certified Professional > > —— > Oifig na nOibreacha Poiblí > Office of Public Works > > Sráid Jonathan Swift, Baile Átha Troim, Co na Mí, C15 NX36 > Jonathan Swift Street, Trim, Co Meath, C15 NX36 > —— > M +353 87 688 5964 T +353 46 942 2409 > https://https://gov.ie/opw <https://www.opw.ie> > > —— > Email Disclaimer: > https://www.gov.ie/en/organisation-information/439daf-email-disclaimer/ > <https://www.opw.ie/en/disclaimer/> > > *From:* QGIS-User <qgis-user-boun...@lists.osgeo.org> *On Behalf Of *Thomas > Lavergne via QGIS-User > *Sent:* 15 September 2023 13:34 > *To:* qgis-user@lists.osgeo.org > *Subject:* [Qgis-user] Does QGIS require units "m" for the projection > coordinate variables in a netCDF/CF file? > > > > Dear QGIS community, > > > > We recently released a number of sea-ice climate data records stored in > netCDF/CF files. > > An example file is here > <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> > : > > > > Our files use a polar equal area projection (EASE2) and the x / y axis > values are given with units "km" (kilometers): > > > > double xc(xc) ; > xc:axis = "X" ; > xc:units = "km" ; > xc:long_name = "x coordinate of projection (eastings)" ; > xc:standard_name = "projection_x_coordinate" ; > double yc(yc) ; > yc:axis = "Y" ; > yc:units = "km" ; > yc:long_name = "y coordinate of projection (northings)" ; > yc:standard_name = "projection_y_coordinate" > > > > When opening these in QGIS, they are placed correctly on the map, but the > message "Unkown CRS" appears in the lower-right corner. > > > > By manipulating our files, we could get QGIS to recognize the correct CRS. > > > > ncap2 -s 'xc=xc*1000f;yc=yc*1000f' infile.nc outfile.nc > ncatted -O -a units,xc,m,c,"m" outfile.nc > ncatted -O -a units,yc,m,c,"m" outfile.nc > > > > The NCO commands above change the x / y axis variables to units "m" > (meters). > > > > The CF convention > <http://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html> > 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. > > > > 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? > > > > All the best, > > Thomas >
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