Why using interp2 gives NaN

I am trying to collocate two datesets (Satellite data). I have matched dates and time correcctly but my lat lon collocation step gives me NaN. Here is my code:

% Read Satellite 1 data. 
Data = table2array(DCC);
col_names = {'Year','Month', 'Day',	'HH', 'MM',	'Lat', 'Lon', 'Data_C', 'Salt'};
         
Sat1_data = double(Data);
Sat1_date = NaN(size(Sat1_data,1),1);
Sat1_time = Sat1_date;
% 
for kkk =  1:size(Sat1_data,1)
        Sat1_date(kkk,1) = Sat1_data(kkk,1)*1E4+Sat1_data(kkk,2)*1E2+Sat1_data(kkk,3);
        Sat1_time(kkk,1) = Sat1_data(kkk,4)*1E2+Sat1_data(kkk,5);
end

% read data from 2nd Satellite
Sat2_path = 'E:\aer 2017\';
Sat2_list   = dir(fullfile(Sat2_path,'*.nc4'));
Sat2_date = NaN(size(Sat2_list,1),1);
for iii =  1:size(Sat2_list,1)
    Sat2_date(iii,1) = str2double(Sat2_list(iii).name(28:35));
end

% define output table
Sat1_Sat2 =NaN(size(Sat1_data,1),10);

%% start loop
for i = 1 : length(Sat2_list)
    ind_Sat1 = find(Sat2_date(i) == Sat1_date)
    if isempty(ind_Sat1) == 0
        fname_Sat2 = fullfile(Sat2_path,Sat2_list(i).name)
            if isempty (fname_Sat2)==0
            %% read Sat2 DATA
             Sat2_lat2 = ncread(fname_Sat2, 'lat');  %101x1 double
             Sat2_lon2 = ncread(fname_Sat2, 'lon');  %113x1 double
            [X,Y] = meshgrid(Sat2_lat2,Sat2_lon2);    %113x101 double       
            
            Carbon = squeeze(ncread(fname_Sat2, 'CAMASS'));  %113x101x24 double
            S_Salt = squeeze(ncread(fname_Sat2, 'SAMASS'));  %113x101x24 double
            
            Sat1_sub = Sat1_data(ind_Sat1,:);
            Sat1_time_sub = Sat1_time(ind_Sat1);
            Sat1_aer_sub = NaN(length(ind_Sat1),3)
            
            for j = 1:length(ind_Sat1)
                Sat1_lat = Sat1_sub(j,6)
                Sat1_lon = Sat1_sub(j,7)
                Sat1_time1 = Sat1_time_sub(j,1)
                 Sat1_time2 = Sat1_time1/100

                 if Sat1_time2>=4 && Sat1_time2<5
                    Sub_Carbon4 = Carbon(:,:,4); %113x101 double                
                    data_Carbon4 = interp2(X,Y,Sub_Carbon4,Sat1_lon, Sat1_lat);
                    Sub_S_Salt4 = S_Salt(:,:,4); %113x101 double                
                    data_S_Salt4 = interp2(X,Y,Sub_S_Salt4,Sat1_lon, Sat1_lat);
                    
                    
                elseif MODIS_time2>=5 && MODIS_time2<6
                   Sub_Carbon5 = Carbon(:,:,5);   %113x101 double                
                    data_Carbon5 = interp2(X,Y,Sub_Carbon5,Sat1_lon, Sat1_lat);
                    Sub_S_Salt5 = S_Salt(:,:,5);  %113x101 double                 
                    data_S_Salt5 = interp2(X,Y,Sub_S_Salt5,Sat1_lon, Sat1_lat);
                   
                elseif MODIS_time2>=6 && MODIS_time2<7
                    Sub_Carbon6 = Carbon(:,:,6);   %113x101 double                
                    data_Carbon6 = interp2(X,Y,Sub_Carbon6,Sat1_lon, Sat1_lat);
                    Sub_S_Salt6 = S_Salt(:,:,6);   %113x101 double                
                    data_S_Salt6 = interp2(X,Y,Sub_S_Salt6,Sat1_lon, Sat1_lat);
                   
                end           

            end
            end
    end
end

After extracting the data at matched time from second satellite 'Sub_Carbon4 = Carbon(:,:,4);' I did interpolation to extract the data from satellite 2 at matched lat lon. I need to do interpolation becuase lat from satellite 1 has values 40.65, 24.28 and so on. Similarly longitude values from satellite 1 are 89.95 114.63. But satellite 2 lat is like this as it was a column vector so I used meshgrid (same goes for lon):

This line returns NaN.

data_Carbon4 = interp2(X,Y,Sub_Carbon4,Sat1_lon, Sat1_lat);

I can't figure out why it is giving me NaN. Kindly tell me what mistake I've made. I'll be grateful.

Answer 1

[Jan]

interp2() replies NaNs, if the interpolated points are outside the given set of points, so it is a "extrapolation". So stop the omputations in this line and check, if the points to be interpolated at are inside the defined range.

There is no need to use meshgrid, because the first 2 inouts X and Y of interp2 can be vectors. This is even faster.

You can simplify:

Sat1_date = NaN(size(Sat1_data,1),1);
Sat1_time = Sat1_date;
for kkk =  1:size(Sat1_data,1)
        Sat1_date(kkk,1) = Sat1_data(kkk,1)*1E4+Sat1_data(kkk,2)*1E2+Sat1_data(kkk,3);
        Sat1_time(kkk,1) = Sat1_data(kkk,4)*1E2+Sat1_data(kkk,5);
end

to

Sat1_date = Sat1_data(:, 1:3) .* [1E4, 1E2, 1];
Sat1_time = Sat1_data(:, 4) * 1E2 + Sat1_data(:, 5);

[Zhou Ci]

Hi @Jan Thank you for your response.

I cahnged the line from

data_Carbon4 = interp2(X,Y,Sub_Carbon4,Sat1_lon, Sat1_lat);

To

data_Carbon = interp2(Sat2_lat2, Sat2_lon2, Sub_Carbon4, Sat1_lat, Sat1_lon)

It gives me value now.

But I don't understand that if I put my values like syntax below it gives me NaN:

Vq = interp2(X,Y,V,Xq,Yq)

And using this gives me value

Vq = interp2(Y,X,V,Yq,Xq)

Why it's like this?

And thankyou for showing the simple way to extract day and time from sat1.

[Jan]

You have swapped Sat1_lon and Sat1_lat in the interp2 command. This could make the difference.