[Qgis-user] Does QGIS require units "m" for the projection coordinate variables in a netCDF/CF file?

[Richard McDonnell]

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.




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Richard McDonnell MSc GIS, FME Certified Professional
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From: QGIS-User <qgis-user-bounces at lists.osgeo.org> On Behalf Of Thomas Lavergne via QGIS-User
Sent: 15 September 2023 13:34
To: qgis-user at 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<http://infile.nc/> outfile.nc<http://outfile.nc/>
ncatted -O -a units,xc,m,c,"m" outfile.nc<http://outfile.nc/>
ncatted -O -a units,yc,m,c,"m" outfile.nc<http://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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