Writing lat and lon attributes in a NetCDF file

Hello!

I'm here to ask help for a matlab scrip. I have a matrix of data that covers a part of europe (it's a tile of a MODIS product) and I want to write it in a netcdf file in order to represent the map with QGIS. I write the information of latitude and longitude with the function ncwriteatt with the following script. However, when I use the command ncinfo the attributes are empty and when I represent it of QGIS the file is not georeferenced.

Can anyone help me? Thanks in advance,

Margherita

xmin=0.003241+(0.004464/2);
xmax=xmin+2399*0.004464;
ymin=40.002083+(0.004464/2);
ymax=ymin+2399*0.004464;

cx=[xmin:0.004464:xmax]';
cy=flip([ymin:0.004464:ymax]');

coord_x=zeros(length(cy(:,1)),length(cx(:,1)));
coord_y=zeros(length(cy(:,1)),length(cx(:,1)));

for i=1:length(cy)
    coord_y(i,:)=cy(i);
end
for i=1:length(cx)
    coord_x(:,i)=cx(i);
end

dim_lon=length(cx);
dim_lat=length(cy);

nccreate('MYD16A2_2021.nc','ET_500m', 'Dimensions',{'lon',dim_lon,'lat',dim_lat}, 'FillValue','disable');
nccreate('MYD16A2_2021.nc','lon','Dimensions', {'lon',dim_lon})
nccreate('MYD16A2_2021.nc','lat','Dimensions', {'lat',dim_lat})
ncwrite('MYD16A2_2021.nc','ET_500m', averageData)
ncwrite('MYD16A2_2021.nc','lat',cy)
ncwrite('MYD16A2_2021.nc','lon',cx)

ncwriteatt('MYD16A2_2021.nc', 'lon', 'standard_name', 'longitude');
ncwriteatt('MYD16A2_2021.nc', 'lon', 'units', 'degrees_east');
ncwriteatt('MYD16A2_2021.nc', 'lon', 'axis', 'X');
ncwriteatt('MYD16A2_2021.nc', 'lat', 'standard_name', 'latitude');
ncwriteatt('MYD16A2_2021.nc', 'lat', 'units', 'degrees_north');
ncwriteatt('MYD16A2_2021.nc', 'lat', 'axis', 'Y');
ncwriteatt('MYD16A2_2021.nc', 'ET_500m', 'coordinate_system', 'WGS84')
ncwriteatt('MYD16A2_2021.nc', 'ET_500m','long_name','Evapotranspiration from MODIS data')
ncwriteatt('MYD16A2_2021.nc', 'ET_500m','units','mm/8days')
ncwriteatt('MYD16A2_2021.nc', 'ET_500m','missing_value',-9999)

Answer 1

[Balaji]

Hi Margherita

I understand you are facing issue in the ‘netcdf’ file you have created.

Based on the provided script, it seems like you are correctly creating the ‘netcdf’ file and writing the data and attributes. However, there are a few modifications you can make to ensure proper georeferencing and attribute assignment. Here are the suggested changes:

Specify the coordinate variables ‘lon’ and ‘lat’ as 1D arrays instead of 2D arrays. This will make it easier to assign the coordinate values using ‘ncwrite’.

nccreate('MYD16A2_2021.nc','lon','Dimensions', {'lon',dim_lon}) 
nccreate('MYD16A2_2021.nc','lat','Dimensions', {'lat',dim_lat}) 
ncwrite('MYD16A2_2021.nc','lon',cx) 
ncwrite('MYD16A2_2021.nc','lat',cy) 
ncwriteatt('MYD16A2_2021.nc', 'lon', 'standard_name', 'longitude'); 
ncwriteatt('MYD16A2_2021.nc', 'lon', 'units', 'degrees_east'); 
ncwriteatt('MYD16A2_2021.nc', 'lon', 'axis', 'X'); 
ncwriteatt('MYD16A2_2021.nc', 'lat', 'standard_name', 'latitude'); 
ncwriteatt('MYD16A2_2021.nc', 'lat', 'units', 'degrees_north'); 
ncwriteatt('MYD16A2_2021.nc', 'lat', 'axis', 'Y'); 
averageData = ... % Define or load the data matrix here 
    ncwrite('MYD16A2_2021.nc','ET_500m', averageData) 

Hope this helps!

Thanks

Balaji