Index in position 1 is invalid. Array indices must be positive integers or logical values.

Hello,

I am trying to find the values of variable Z at the indices of lat_txt and lon_txt.

[m,n] = size(M) ; % m= 2030, n =1354
x0 = min(lon(:)) ; x1 = max(lon(:)) ; 
y0 = min(lat(:)) ; y1 = max(lat(:)) ; 
x = linspace(x0,x1,m) ; 
y = linspace(y0,y1,n) ; 
[X,Y] = meshgrid(x,y) ; 
Z = interp2(X,Y,M',lon,lat)';
Z = Z'; %2030x1354

Out = Z(lat_txt, lon_txt);
Index in position 1 is invalid. Array indices must be positive integers or logical values.

lat_txt = 8119x1 double (containing lat values);

lon_txt = 8119x1 double(containing lon values);

Z contains integers 1,2,3,4. How can I find value of Z? As it gives me error here. However for each lat lon I just want one value of Z.

Answer 1

[Star Strider]

To reference ‘Z’ as a matrix, ‘lat’ and ‘lon’ must be integers greater than 0, or logical values.

The griddata function may be the best option here, speciffically —

Z = griddata(X(:),Y(:),reshape(M',numel(M),1),lon,lat)';

I cannot test this because I have none of the variables. However this is more llikely to produce the desired result than interp2, at least in the context of the posted code. Since I have no idea what the variables are and cannot test this, it may need to be tweaked.

.

[Zhou Ci]

Lat, lon and M here are from hdf file with size: Lat = 406×270; Lon = 406x270; M = 2030x1354; There's a txt file that contains a column vector of Lat and a Column vector of Lon (I've named as lat_txt, lon_txt). All I want is to get the Value of M (from hdf) according to the Lat and Lon values of txt file. This is why I've done interpolation above and passing lat_txt, lon_txt to get the Value of Z.

[Star Strider]

To use griddata, ‘lat’ and ‘lon’ must also be vectors, so that assignment becomes:

Z = griddata(X(:),Y(:),reshape(M',[],1),lon(:),lat(:))';

I cannot test this, so experiment with it with the actual data to get the desired result.

.

[Zhou Ci]

@Star Strider Unfortunately I still get the same error. Using above line of code to find the values of Z at the indices of lat_txt and lon_txt gives me this error again:

Out = Z(lat_txt, lon_txt);
Index in position 1 is invalid. Array indices must be positive integers or logical values.

[Star Strider]

It is not correct now for the same reason it was not correct this morning.

Use the griddata call (or something similar to it, since I cannot test it with the data that I do not have access to) that I posted in my previous Comment, instead of using ‘lat’ and ‘lon’ as indices into ‘Z’, that will never work, for the same reasons I described earlier.

.