'Find' command to find index of lat and long from an array

I am trying to extract the index of latitude and longitude from an array using 'find' command. My lat/long array contains latitude and longitude for the whole globe. I am using a code where I require only couple of lat/longs to be matched from the global array but its showing me that there is no match. Below is the code if anybody can help:

LG=[52.25 53.25 54.25]; % desired longitude values
LT=[21.65 22.65 23.65];  % desired latitude values
% Step 2 : Display all varaibles in nc file using command: ncdisplay
filename='3B-DAY.MS.MRG.3IMERG.20180203-S000000-E235959.V06.nc4'
ncdisp(filename,'/','min')
% Step 3 : Read varaibles from nc file by command : ncread
%Variable=input('write your main variable=');
precip=ncread(filename,'precipitationCal');
long=ncread(filename,'lon');
lat=ncread(filename,'lat');
time=ncread(filename,'time');
% Step 4 : Find index of longitude and latitude in the dimension of array 
% using command : find
for i=1:3
    LGG=find(long == LG(i));
    LTT=find(lat  == LT(i));
    if isempty(LGG)||isempty(LTT)
        disp('NaN')
        break
    end

Here I get 'NAN' display. Can somebody tell where is the error? I have attached the nc file as well.

Answer 1

[KSSV]

You should not use find. As lat, lon are floating point numbers using == will not work. You need to define a small tolerance value and check the difference.

tol = 10^-3 ; 
for i=1:3
    LGG=abs(long-LG(i))<tol;
    LTT=abs(lat-LT(i))<tol;
    if LGG || LTT  % check this logic accordingly 
        disp('NaN')
        break
    end
end

You need not to use a loop. You can do this in one line.

Also have a look on knnsearch

[Faisal Baig]

Thanks for the answer. I checked this, first the LGG and LTT difference arrays are becoming all zeros. And its giving me following error:

Operands to the logical and (&&) and or (||) operators must be convertible to logical scalar values.

Error in Extract_Imerge_Coordinates (line 23)

if LGG || LTT % check this logic accordingly

[KSSV]

Try

LGG | LTT

[Walter Roberson]

You want the difference to be < tol, so your NaN test should be

LGG=abs(long-LG(i))<tol;
LTT=abs(lat-LT(i))<tol;
LLGp = find(LGG);
LTTp = find(LTT);
if isempty(LGGp) | isempty(LTTp)
    disp('NaN')
else
    precip(LLGp, LTTp)   %might have to be precip(LTTp, LLGp)
end

[KSSV]

Ohh yes...@Walter Roberson. I missed the output is an logical array.

[Faisal Baig]

Thanx alot, It solved my issue.

Answer 2

[Walter Roberson]

Your code assumes that exactly those latitudes and longitudes appear in the file. You are testing for bit-for-bit identical representations of numbers that binary floating point inherently cannot store the exact representation of. Binary floating point cannot represent 1/10 or 1/100 exactly so for example,

fprintf('%.999g\n', 21.65)

That would have to be the exact value stored in the file, not just something that rounded to the 21.65 to the nearest 1/100.

I suggest that you use interp2() instead of looking for exact equality.

[Faisal Baig]

Yes, I understand that point. I've also given the exact matched values as my desired numbers after looking in the array for entire globe, still its not working.