Assign new dimension to xarray.Dataset using existing coordinates

Question

I have an xarray.Dataset that looks like this, which is available here: https://psl.noaa.gov/thredds/dodsC/Datasets/NARR/Dailies/monolevel/acpcp.1979.nc:

   <xarray.Dataset>
Dimensions:            (time: 365, y: 277, x: 349, nbnds: 2)
Coordinates:
  * time               (time) datetime64[ns] 1979-01-01 ... 1979-12-31
  * y                  (y) float32 0.0 3.246e+04 ... 8.927e+06 8.96e+06
  * x                  (x) float32 0.0 3.246e+04 ... 1.126e+07 1.13e+07
    lat                (y, x) float32 1.0 1.104 1.208 ... 46.93 46.64 46.35
    lon                (y, x) float32 -145.5 -145.3 -145.1 ... -2.644 -2.57
Dimensions without coordinates: nbnds
Data variables:
    Lambert_Conformal  int32 ...
    time_bnds          (time, nbnds) float64 ...
    acpcp              (time, y, x) float32 ...
Attributes: (12/17)
    Conventions:                     CF-1.2
    centerlat:                       50.0
    centerlon:                       -107.0
    comments:                        
    institution:                     National Centers for Environmental Predi...
    latcorners:                      [ 1.000001  0.897945 46.3544   46.63433 ]
    ...                              ...
    history:                         created Mon Mar 21 17:56:47 MDT 2016 by ...
    dataset_title:                   NCEP North American Regional Reanalysis ...
    references:                      https://www.esrl.noaa.gov/psd/data/gridd...
    source:                          http://www.emc.ncep.noaa.gov/mmb/rreanl/...
    _NCProperties:                   version=2,netcdf=4.6.3,hdf5=1.10.5
    DODS_EXTRA.Unlimited_Dimension:  time

And I want to be able to select a given point using .sel based on the lat and lon coordinates like:

ds.sel(lat=point_lat, lon=point_lon, method='nearest')

But I get KeyError: 'no index found for coordinate lat'. I think this makes sense because lat is a coordinate but not a dimension.

So, what would be the best way to be able to use .sel in this case - is there a way to simply add lat and lon to the set of dimensions?

I am pretty sure there must be a global attribute by the name of "featureType" with a value of "timeSeries". Can you please confirm? — Patrick
– Patrick, Commented May 30 at 17:42
@Patrick Updated example for a case that isn't a single point with a link to the file — pbreach
– pbreach, Commented Jun 2 at 12:40

Patrick · Accepted Answer · 2025-05-31 09:02:17Z

0

This netCDF file has only a single latitude-longitude point. lat and lon coordinates use the station dimension which has a length of 1. lat and lon are thus also not coordinate variables (in the sense of the CF Metadata Conventions) and I am a bit surprised that xarray would list them as coordinates but then not allow you to select on them (which indeed does not make sense). I see a lat value of 45.16, which would place this in Lake Michigan or Lake Huron, most likely.

What you have here is most likely a netCDF file with discrete sampling geometries (more CF-speak), in this case a timeSeries of 1-minute interval (time, 1 million+ length over nearly 2 years) of temperature measured at 18 depths (z). You can select on either of these two dimensions as usual.

In short, this is data for a single location. If you want to select data for another location, you should get a gridded data product.

answered May 31 at 9:02

Patrick

33k7 gold badges73 silver badges102 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

pbreach Jun 2 at 12:41

You're right, this isn't the best example with only a single lat-lon point. I've updated for a case where the question still applies and included a link to the file.

Collectives™ on Stack Overflow

Assign new dimension to xarray.Dataset using existing coordinates

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related