How to find a value in a matrix

Hi

I have a matrix of latitude and another matrix of longtitude for the location of an area pixel to pixel. I have a location value (longtitue and latitude) that I want to find the position of the pixels which this value will be located in. How can I find this?

Look at below to the exemplary matrix of Latitude.

There is another matrix like this for longtitudeThe point that I am looking for is Latitude=35.6886 and Longtitude= 53.6113

[David Hill]

Can't you just use the find() command?

[Honey]

I tried this but it doesnt work.

Answer 1

[DGM]

Consider the simple example:

% two orthogonal grids
[x y] = meshgrid(linspace(0,1,10))

x = 10×10

0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000 0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000 0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000 0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000 0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000 0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000 0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000 0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000 0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000 0 0.1111 0.2222 0.3333 0.4444 0.5556 0.6667 0.7778 0.8889 1.0000

y = 10×10

0 0 0 0 0 0 0 0 0 0 0.1111 0.1111 0.1111 0.1111 0.1111 0.1111 0.1111 0.1111 0.1111 0.1111 0.2222 0.2222 0.2222 0.2222 0.2222 0.2222 0.2222 0.2222 0.2222 0.2222 0.3333 0.3333 0.3333 0.3333 0.3333 0.3333 0.3333 0.3333 0.3333 0.3333 0.4444 0.4444 0.4444 0.4444 0.4444 0.4444 0.4444 0.4444 0.4444 0.4444 0.5556 0.5556 0.5556 0.5556 0.5556 0.5556 0.5556 0.5556 0.5556 0.5556 0.6667 0.6667 0.6667 0.6667 0.6667 0.6667 0.6667 0.6667 0.6667 0.6667 0.7778 0.7778 0.7778 0.7778 0.7778 0.7778 0.7778 0.7778 0.7778 0.7778 0.8889 0.8889 0.8889 0.8889 0.8889 0.8889 0.8889 0.8889 0.8889 0.8889 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000

% the point you're trying to locate
targetpt = [0.3333 0.5555]; % [x y]
tol = 0.001; % tolerance

% the row and column where the point is found
[tprow tpcol] = find(abs(x-targetpt(1))<tol & abs(y-targetpt(2))<tol)

tprow = 6

tpcol = 4

Don't expect simple equality tests to work with floating point numbers like this. You'll have to test matches to within some defined tolerance.

[Honey]

Thanks. It is great but I can't find any answer even with different amount of tolerances. What should I do?

tprow =

0×1 empty double column vector

tpcol =

0×1 empty double column vector

[DGM]

Provide an example of the matrices you're trying to use and the values you're trying to find.

[Honey]

I attached them here in an excel file. [X,Y] is the matrices I am trying to use and [X',Y'] is what I am trying to find.

[DGM]

Ah yeah. Those aren't really close to the mesh at all. It depends on what you want to do. If you want to keep working on the fixed mesh and just find the nearest vertices, you can do something like:

X = xlsread('X,Y.xlsx','X');
Y = xlsread('X,Y.xlsx','Y');
yx = xlsread('querypoints.xlsx');

% X and Y are meshgrids; don't really need all that 
Xv = X(:,1).';
Yv = Y(1,:);

% find the subscripts of the nearest match
[~,idxx] = min(abs(Xv-yx(:,2)),[],2); % index along dim 1
[~,idxy] = min(abs(Yv-yx(:,1)),[],2); % index along dim 2

% bear in mind the vector orientation
% here, "x" and "y" refer to the names from the spreadsheet
% not the array dimensions

% for example, look up the closest match for a selected query point
k = 1; % pick a point
yx(k,:)

ans = 1×2

53.6114 36.6867

X(idxx(k),idxy(k))

ans = 36.6550

Y(idxx(k),idxy(k))

ans = 53.6510

I had to rename the second file only because the web-version refused to recognize the filename. Otherwise it worked fine on desktop.

[Honey]

Great ! It works and now with 'find()', I can have the pixel numbers too