[A, R] = geotiffread('...tif'); %reading .tif image
[row, col] = size(A); % row = 7081, col = 7811
N_small = 1000;
small_size = [11 11];
min_row = (small_size(1)+1)/2; % min_row = 6
max_row = row-min_row+1; % max_row = 7076
min_col = (small_size(2)+1)/2; % min_col = 6
max_col = col-min_col+1; % max_col = 7806
% A_small is a matrix of size 11-by-11-by-1000, each slice of
% which will be a random 11-by-11 selection from A:
A_small = zeros(small_size(1),small_size(2),N_small);
for kk = 1:N_small
% max_row-min_row+1 = 7071
% -> randi returns a random integer between 1 and 7071
% -> i0 is a random integer between 6 and 7076
i0 = randi(max_row-min_row+1)+min_row-1;
% max_col-min_col+1 = 7801
% -> randi returns a random integer between 1 and 7801
% -> j0 is a random integer between 6 and 7806
j0 = randi(max_col-min_col+1)+min_col-1;
% select from A the 11-by-11 matrix centered at i0,j0
% and assign it to the kk-th slice of A_small
A_small(:,:,kk) = A(i0-min_row+1:i0+min_row-1,j0-min_col+1:j0+min_col-1);
end
