How to extract values from a 3-d .mat file?

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Keegan Carvalho 2020-3-20

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编辑： Keegan Carvalho 2020-3-22

summean.mat

Hi,

I loaded a netcdf file and derived a .mat file which is a 3-D matrix of lon x lat x value (360 x 40 x 111). I wanted to extract the third dimension as entire column.

lon=ncread('sstarc.nc','lon');
lat=ncread('sstarc.nc','lat');
summean=ncread('sstarc.nc','sst');

I used the above code and got the mat file. The third dimension has 111 values which I wanted to extract the "111" values for the entire 360 x 40 grid as a column (eg. time series). Attached .mat file!

Looing forward to your help. Thanks!

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Answer 1

Cris LaPierre 2020-3-21

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Your mat file contains variables. Loading the mat file loads the variables back into the workspace.

load summean.mat

If there are multiple variables stored in the mat file, you can tell it to load specific variables instead of all of them.

load summean.mat filteredA

As for the values to extract, you can do that using indexing on the variable containing the values.

The third dimension does not have 111 values. Instead, you have 111 matrices of dimension 360 x 40 stacked on top of each other. To extract the last one, you could do the following

vals = filteredA(:,:,111);

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Cris LaPierre 2020-3-22

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The variable filteredA contains the data for Jun-Aug for all 37 years (512 x 50 x 111). Now you want this data as a time series. What does that mean to you? Knowing there are now 2,841,600 values, how do you envision this data being organized in a vector? Do you want to preserve location information? Month? Year?

I would be inclined to create a timetable, thus preseving the month and year info. I'd add variables (columns) for sst value, lon and lat.

Following your example above, this is what the code might look like.

% extract data from nc file
lon=ncread('sst.nc','longitude');
lat=ncread('sst.nc','latitude')';
sst=ncread('sst.nc','variable');
% identify summer months
startDate = datetime(1982,1,1,"Format","MMM-uuuu");
endDate = datetime(2018,12,31,"Format","MMM-uuuu");
dates = startDate:calmonths(1):endDate;
summerMonth = month(dates) >= 6 & month(dates) <= 8;
% Set up variables for creating timetable
datesTT = permute(repmat(dates,512,1,50),[1,3,2]); % create a date for each value
datesTT = datesTT(:,:,summerMonth);
[LAT, LON] = meshgrid(lat,lon);
latTT = repmat(LAT,1,1,sum(summerMonth)); % create matrix same size as datesTT
lonTT = repmat(LON,1,1,sum(summerMonth)); % create matrix same size as datesTT
sstSummer = sst(:,:,summerMonth);   % Extract data for summer months
% Create timeTable
summerTT = timetable(datesTT(:),sstSummer(:),lonTT(:),latTT(:));
summerTT.Properties.VariableNames = {'sst','lon','lat'};

Here's a preview of what the table looks like (first 8 rows).

      Time       sst        lon       lat  
    ________    ______    _______    ______
    Jun-1982    271.46          0    89.463
    Jun-1982    271.46    0.70313    89.463
    Jun-1982    271.46     1.4063    89.463
    Jun-1982    271.46     2.1094    89.463
    Jun-1982    271.46     2.8125    89.463
    Jun-1982    271.46     3.5156    89.463
    Jun-1982    271.46     4.2188    89.463
    Jun-1982    271.46     4.9219    89.463

Cris LaPierre 2020-3-22

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As for taking the mean when there are NaN entries, you can use the following syntax

mean(A,'omitnan')

You can see an example here.

Keegan Carvalho 2020-3-22

编辑：Keegan Carvalho 2020-3-22

在 MATLAB Online 中打开

Thank you Cris. Very grateful for the explanations. I actually have another query on the same. I am trying to create a map of seasonal means and thereby want to have 37 years of data (360 x 40 x37). I used the following code:

lon=ncread('sstarc.nc','lon');
lat=ncread('sstarc.nc','lat');
sst=ncread('sstarc.nc','sst');
DJF = sort([[1:12:444],[2:12:444],[12:12:444]]);
Wacc = cumsum(sst(:,:,DJF));
Wavg = Wacc(:,:,3:3:end)/3;
meanDJF=mean(Wavg,3); 
h=pcolor(lon,lat,meanDJF);

However I seem to get odd temperature values of 100 degrees and above at certain places. I asked this question: (https://se.mathworks.com/matlabcentral/answers/512283-how-to-extract-seasonal-means)

I know this may seem to be going too far, but I would gladly appreciate your help as I am working on a project.

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How to extract values from a 3-d .mat file?

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