# How to subtract two different netcdf files after reading them

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Karthik M 2021 年 9 月 13 日
Hi Freinds
I have objective to find chlor conc. hotspot of particular month during successive year
For example I consider 2 year(2015 and 2016) nc data (Feb)month
I have extracted the chlor_a data and could plot with respective x and y coordinates
I used the below code for 1st nc file to read, the same code can be used for 2nd file
ncfile_1 = 'A20150322015059.L3m_MO_CHL_chlor_a_4km.nc';
[X_1,Y_1] = meshgrid(lon_1,lat_1) ;
nn = 1000;
xi_1 = linspace(30,100,nn) ;
yi_1 = linspace(0,30,nn) ;
[Xi_1,Yi_1] = meshgrid(xi_1,yi_1);
iwant_1 = interp2(X_1,Y_1,chlor_a_1',Xi_1,Yi_1)' ;
ind_1 = find(iwant_1>10 | iwant_1<0);
%ind = find(iwant_2 > 0 & iwant_1<10);
iwant_1(ind_1) = NaN;
%[min_val_1,min_ind_1] = min(iwant_1',[],'all','linear');
[min_val_1,min_ind_1] = min(reshape(iwant_1',[],1));
[r1_1,c1_1]=ind2sub(nn,min_ind_1);
x_min_1 = xi_1(c1_1);
y_min_1 = yi_1(r1_1);
%[max_val_1,max_ind_1] = max(iwant_1',[],'all','linear');
[max_val_1,max_ind_1] = max(reshape(iwant_1',[],1));
[r2_1,c2_1]=ind2sub(nn,max_ind_1);
x_max_1 = xi_1(c2_1);
y_max_1 = yi_1(r2_1);
figure(1)
c = colorbar;
cmap = jet(255);
cmap(:,2) = 0;
colormap(cmap)
caxis([0, 5])
hold on
plot(x_min_1,y_min_1,'k +', 'MarkerSize', 10);
plot(x_max_1,y_max_1,'m +', 'MarkerSize', 10);
hold off
legend('','min','max');
I find error while trying to execute below steps
while try to save both files in nc format
while subtraction both files to spot seasonal trends
This comparison will identify max persistence of chlor_a
[+ve] value means max chlor_a conc. [-ve] value means less
I need to get the plot as such after subtraction, please find the attached image
similary need to work on to compare weekly data
Kindly help, waitng for your valuable inputs
regards

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