Interpolating point data

[CGenie]

Hello,

I have point data of form

latitude longitude obs_value

(I generated .nc files from .ascii ones using met's ascii2nc).

These are all indexed by one common index parameter, and when I load that in iris, I get a CubeList with separate cubes for latitude, longitude, obs_value, etc. Is there a way to combine these into 1 cube? Specifically, I'd like to interpolate the sample (latitude, longitude, obs_value) values onto a regular grid so that these 3 parameters fit into a cube (I want contour plots etc for that).

Best

[ESadek-MO]

@SciTools/peloton On the obs_value variable add an attribute obs_value:coordinates='latitude longitude', and this might fix it for you! If it doesn't fix it, an ncdump -h would give us a bit more insight.

[CGenie]

Thank you for your answer. I'm not sure however where to add this, iris doesn't allow to set cube.attributes['coordinates'] = 'latitude longitude' because, apparently, it's not a permitted attribute.

ncdump -h gives me this:

netcdf \2023-06-01_00-observations {
dimensions:
	mxstr = 16 ;
	mxstr2 = 40 ;
	mxstr3 = 80 ;
	nhdr = 502 ;
	nobs = 502 ;
	nhdr_typ = 1 ;
	nhdr_sid = 502 ;
	nhdr_vld = 1 ;
	nobs_qty = 1 ;
	obs_var_num = 1 ;
variables:
	int hdr_typ(nhdr) ;
		hdr_typ:long_name = "index of message type" ;
		hdr_typ:_FillValue = -9999 ;
	int hdr_sid(nhdr) ;
		hdr_sid:long_name = "index of station identification" ;
		hdr_sid:_FillValue = -9999 ;
	int hdr_vld(nhdr) ;
		hdr_vld:long_name = "index of valid time" ;
		hdr_vld:_FillValue = -9999 ;
	float hdr_lat(nhdr) ;
		hdr_lat:long_name = "latitude" ;
		hdr_lat:_FillValue = -9999.f ;
		hdr_lat:units = "degrees_north" ;
	float hdr_lon(nhdr) ;
		hdr_lon:long_name = "longitude" ;
		hdr_lon:_FillValue = -9999.f ;
		hdr_lon:units = "degrees_east" ;
	float hdr_elv(nhdr) ;
		hdr_elv:long_name = "elevation" ;
		hdr_elv:_FillValue = -9999.f ;
		hdr_elv:units = "meters above sea level (msl)" ;
	int obs_qty(nobs) ;
		obs_qty:long_name = "index of quality flag" ;
	int obs_hid(nobs) ;
		obs_hid:long_name = "index of matching header data" ;
		obs_hid:_FillValue = -9999 ;
	int obs_vid(nobs) ;
		obs_vid:long_name = "index of variable names at var_name" ;
		obs_vid:_FillValue = -9999 ;
	float obs_lvl(nobs) ;
		obs_lvl:long_name = "pressure level (hPa) or accumulation interval (sec)" ;
		obs_lvl:_FillValue = -9999.f ;
	float obs_hgt(nobs) ;
		obs_hgt:long_name = "height in meters above sea level or ground level (msl or agl)" ;
		obs_hgt:_FillValue = -9999.f ;
	float obs_val(nobs) ;
		obs_val:long_name = "observation value" ;
		obs_val:_FillValue = -9999.f ;
	char hdr_typ_table(nhdr_typ, mxstr2) ;
		hdr_typ_table:long_name = "message type" ;
	char hdr_sid_table(nhdr_sid, mxstr2) ;
		hdr_sid_table:long_name = "station identification" ;
	char hdr_vld_table(nhdr_vld, mxstr) ;
		hdr_vld_table:long_name = "valid time" ;
		hdr_vld_table:units = "YYYYMMDD_HHMMSS UTC" ;
	char obs_qty_table(nobs_qty, mxstr) ;
		obs_qty_table:long_name = "quality flag" ;
	char obs_var(obs_var_num, mxstr2) ;
		obs_var:long_name = "variable names" ;

// global attributes:
		:MET_Obs_version = "1.02" ;
		:use_var_id = "true" ;
		:FileOrigins = "File / generated 20230807_173720 UTC on host fb9b4a62cd29 by the MET ascii2nc tool" ;
		:MET_version = "V11.0.2" ;
		:MET_tool = "ascii2nc" ;
}

When I load the file, I get these cubes:

[<iris 'Cube' of index of variable names at var_name / (unknown) (-- : 502)>,
<iris 'Cube' of index of matching header data / (unknown) (-- : 502)>,
<iris 'Cube' of pressure level (hPa) or accumulation interval (sec) / (unknown) (-- : 502)>,
<iris 'Cube' of valid time / (unknown) (-- : 1; -- : 16)>,
<iris 'Cube' of index of quality flag / (unknown) (-- : 502)>,
<iris 'Cube' of longitude / (degrees) (-- : 502)>,
<iris 'Cube' of index of message type / (unknown) (-- : 502)>,
<iris 'Cube' of variable names / (unknown) (-- : 1; -- : 40)>,
<iris 'Cube' of station identification / (unknown) (-- : 502; -- : 40)>,
<iris 'Cube' of index of station identification / (unknown) (-- : 502)>,
<iris 'Cube' of observation value / (unknown) (-- : 502)>,
<iris 'Cube' of height in meters above sea level or ground level (msl or agl) / (unknown) (-- : 502)>,
<iris 'Cube' of message type / (unknown) (-- : 1; -- : 40)>,
<iris 'Cube' of latitude / (degrees) (-- : 502)>,
<iris 'Cube' of quality flag / (unknown) (-- : 1; -- : 16)>,
<iris 'Cube' of index of valid time / (unknown) (-- : 502)>,
<iris 'Cube' of elevation / (unknown) (-- : 502)>]

As you see, there are 502 observations. latitude, longitude and observation value are in separate cubes, they are all indexed by the index of the observation number.

[trexfeathers]

From @SciTools/peloton: sorry we weren't clear, you would need to make this modification BEFORE loading with Iris - as you have noticed, you can't edit coordinates within Iris because it's an interpreted attribute.

You either want to use the NetCDF4 Python library, or use ncatted on the command line.