Link closest points accross altitudes in Matlab, problem with Delaunay Triangulation/ Nearest neighbor finding

I have a 3d matrix with scattered points (Nx4 matrix, x-y-z-data). My aim is to link the closest points together and register each chain in an Kx4 array (x, y, z, data), K being the chain length. The total number of chains depends on the points... A particularity is that these lines only go upwards (z+), I don't want to link points on same z, or go down.

I have been trying different strategies so far, one being with another array shape (Mx4xNz - basically meaning the values were stacked per z's instead of being all on a 2d matrix):

1. pick a point at level Zn

2. go to level Zn+1, look for the closest point in a range of coordinates x,y using delaunayTriangulation and nearestNeighbor

3. register the point into a vector

(I suspect there are other possibilities using nearestNeighbor with the Nx4 matrix, but i can't think how to 'direct' the search upwards and chain the successive points... )

I find myself with the following problem : The finding of nearest point upwards seems to work well but in 1 horizontal direction only!!

<http://i59.tinypic.com/154fvo2.png> (linking that doesn't work)

<http://i62.tinypic.com/1zewcno.png> (linking that works)

Anyone has an idea as to why this is happens? I suspect the problem might be with assigning correctly the nearest neighbor found, but I don't see how the extraction I did in my code below would be wrong?? Thanks for any help!!

Note - During the loop I have the warning : Duplicate data points have been detected and removed. The Triangulation indices are defined with respect to the unique set of points in delaunayTriangulation property X.

Lign=zeros(max_iter,4,s);

for i = 1:s;

pp_id=i; 

for  n=1:max_iter-1; 

      Wn=W(:,:,n); % W is the data 3d-matrix Mx4xNz, as attached
      Wnn=W(:,:,n+1);

      Point_n = Wn(pp_id,:); 

      xn= Point_n(1);
      yn= Point_n(2);
      zn= Point_n(3);
      vn= Point_n(4);

      if xn==0|yn==0|zn==0|vn==0;
          break
      end

      % Look for nearest neighbour at next level
      DT=delaunayTriangulation(Wnn(:,[1:2]));
      [pp_id, d]=nearestNeighbor(DT,[xn,yn]); 

      % limit range
      if d>10 
          break
      end

      % extraction of values at row pp_id 
      Point_n=Wnn(pp_id,:);

      % register point in line
      Lign(n,:,i)=Point_n;

end

end

      figure;
      clf;

        hold on;

        plot3(W(:,1),W(:,2),W(:,3),r*','markersize',2,'linewidth',1);

    for i=1:s
    plot3(Lign(:,1,i),Lign(:,2,i),Lign(:,3,i),'k','linewidth',1);
    end