Plotting the edge of a beam from prewitt or other method

Hi guys, wondering if you could all help me at all. I have a picture of a beam which I would like to plot the edge of the line of. The picture is below:

http://i52.tinypic.com/29mqa6e.jpg

I have tried to convert it to canny method to see if it could be easier at all, I had to interpolate the image to get more pixels for a good edge detection. Prewitt method seems to works and give me this:

http://i53.tinypic.com/r7r4pl.jpg

But I cannot seem to find a way to plot the edge of this beam. I have tried:

 x=[150 330];
y=[1438 1438];
improfile(ROI,x,y)

To try and get an edge for white intensity but because I have interpolated the image, the results are not very good. Any help is much appreciated, Thanks - Graham

[Graham Boag]

Any thoughts? I can pick out the points in the black and white picture manually but its a very rough idea of the beams edge