convert image into 3XN matrix

I have a dem data(an image of 6000X5200). I need to convert the data and create a 3XN matrix with row number, coloum number and Z value (pixel value of the image represent the Z value).

    orgim = imread('dem');   % read DEM data
    nentry = 0;
        for ii = 1:size(orgim,2)
          for jj = 1:size(orgim,1)
                nentry = nentry +1;
                mydata(nentry, :) = [ii, jj, orgim(jj,ii)];
                clear X Y Z;
        end
      end

This works but takes time. Is there any optimum way to convert the multi-dimentional 6000X 5200 matrix as 3XN matrix, without loop

[Andrei Bobrov]

[x,y] = ndgrid(1:size(orgim,1),1:size(orgim,2));
mydata = [y(:),x(:),orgim(:)];

[Jan]

"clear X Y Z"?! This line can obviously omitted.

Answer 1

[Andrei Bobrov]

[x,y] = ndgrid(1:size(orgim,1),1:size(orgim,2));
mydata = [y(:),x(:),orgim(:)];

or

mydata = [fliplr(fullfact(size(orgim))), orgim(:)];

Answer 2

[Jan]

Although Andrei's solution is smarter, I suggest the pre-allocation as general programming schema:

 orgim = imread('dem');   % read DEM data
 mydata = zeros(numel(orgim), 3));  % Pre-allocate!!!
 nentry = 0;
 for ii = 1:size(orgim,2)
   for jj = 1:size(orgim,1)
     nentry = nentry +1;
     mydata(nentry, :) = [ii, jj, orgim(jj,ii)];
   end
 end

This can be noticably faster already.

Answer 3

[bes]

Thanks all. Your comments helped me a lot.