Need assistance properly plotting a 3D variable from a NetCDF (.nc) file.

Question

Austin M. Weber 2021-12-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1615620-need-assistance-properly-plotting-a-3d-variable-from-a-netcdf-nc-file

回答： Cris LaPierre 2021-12-21

I want to spatially show the evapotranspiration data contained within a NetCDF file, but almost all of the values appear the same color when plotted, when in reality there should be a wide range of colors. I think I may need to employ the Attributes of the variable in some way, but I am not sure how. Or, perhaps it is an issue with some outlier value that is making the majority of data points look the same. Here is what I have:

%% LOAD THE FILE INTO THE WORKSPACE
%    Save the NetCDF file name as a character variable:
        filename = 'Evapotranspiration-Data.nc';
    
%    Display contents of NetCDF file (output pasted at the bottom of the question)
        ncdisp(filename);
    
%    Get the structure of the file
        S = ncinfo(filename);
%    Extract the variables
        lon = abs(ncread(filename, 'longitude')-360); % longitudes in degress W
        lat = ncread(filename, 'latitude'); % latitudes in degrees N
        t = ncread(filename, 'time'); % time in 'hours since 1901-01-01 00:00:00'
        ET = ncread(filename, 'evavt'); % Evapotranspiration in 'meters H20 equiv.'
    
%    Standardize ET data
%      1.  Extract missing value from structure
                mv = S.Variables(4).Attributes(4).Value; % missing value
%      2.  Standardize
                ET = standardizeMissing(ET,mv); % Set all "missing values" to NaN
%% PLOT THE DATA
    figure
    imagescn(lat,lon,ET(:,:,1)) % plot first month of ET data
%      I am using the function imagescn() from
%      https://www.mathworks.com/matlabcentral/fileexchange/61293-imagescn,
%      but I am also happy to work with:
%          contourf(lat,lon,ET(:,:,1),'EdgeColor','none')
%      Although the results are basically the same.
    
%    Adjust orientation of figure and set color scheme
        view([-90 90])
        colormap winter
    
%    Make a colorbar and give it a label
        cb = colorbar;
        cb.Label.String = 'Evapotranspiration (m w.e.)';

Here is the figure:

Here is the output from ncdisp(filename):

Dimensions:
           longitude = 451
           latitude  = 301
           time      = 732
Variables:
    longitude
           Size:       451x1
           Dimensions: longitude
           Datatype:   single
           Attributes:
                       units     = 'degrees_east'
                       long_name = 'longitude'
    latitude 
           Size:       301x1
           Dimensions: latitude
           Datatype:   single
           Attributes:
                       units     = 'degrees_north'
                       long_name = 'latitude'
    time     
           Size:       732x1
           Dimensions: time
           Datatype:   int32
           Attributes:
                       units     = 'hours since 1900-01-01 00:00:00.0'
                       long_name = 'time'
                       calendar  = 'gregorian'
    evavt    
           Size:       451x301x732
           Dimensions: longitude,latitude,time
           Datatype:   int16
           Attributes:
                       scale_factor  = 1.1369e-07
                       add_offset    = -0.0037253
                       _FillValue    = -32767
                       missing_value = -32767
                       units         = 'm of water equivalent'
                       long_name     = 'Evaporation from vegetation transpiration'

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Cris LaPierre 2021-12-21

Can you share you nc file? You will have to change the extenstion to .txt to attach it here (using the paperclip icon)

Austin M. Weber 2021-12-21

Hi @Cris LaPierre, I tried changing the file extension to .txt, but the file is still too large to upload, even after compression into a .zip folder.

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Answer 1

Cris LaPierre 2021-12-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1615620-need-assistance-properly-plotting-a-3d-variable-from-a-netcdf-nc-file#answer_859780

在 MATLAB Online 中打开

See my posts here and perhaps here.

The difference is that your color variable is 3D. Each sheet corresponds to a different time. For visualizing, I think you want to pick a specific time.

filename = 'Evapotranspiration-Data.nc';
lon = ncread(filename, 'longitude'); % longitudes in degrees W
lat = ncread(filename, 'latitude'); % latitudes in degrees N
t = ncread(filename, 'time'); % time in 'hours since 1901-01-01 00:00:00'
ET = ncread(filename, 'evavt'); % Evapotranspiration in 'meters H20 equiv.'
    
%    Standardize ET data
mv = S.Variables(4).Attributes(4).Value; % missing value
ET = standardizeMissing(ET,mv); % Set all "missing values" to NaN
[Lat Lon] = meshgrid(lat_range,lon_range);
%% PLOT THE DATA
figure
axesm('eqdcylin',"MapLatLimit",lat([1,end]),"MapLonLimit",lon([1,end]))
pcolorm(Lat,Lon,ET(:,:,1));
demcmap(ET(:,:,1))
tightmap
colorbar