Why using interp2 gives NaN

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Zhou Ci
Zhou Ci on 3 Mar 2022
Commented: Jan on 3 Mar 2022
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.

Accepted Answer

Jan
Jan on 3 Mar 2022
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);
  2 Comments
Jan
Jan on 3 Mar 2022
You have swapped Sat1_lon and Sat1_lat in the interp2 command. This could make the difference.

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