The age-old approach for finding straight lines (and other easily parameterizable features) in an image is to use the Hough Transform. There is a good article on the HT in Wikipedia.

I've seen the HT used on extremely noisy images with amazing effect. For instance, laser ranging of corner-cube bearing satellites can find the distance to the satellite to a few millimetres. The signal to noise ratio at the detector is rather less than one in a million (the one being a reflected photon; the million being the sky brightness) yet because all the signal falls on a straight line in a plot of photon arrival time against distance along the satellite track, the great majority of the noise is rejected.

Yes, in a previous life I worked in a machine vision lab. With Mike Brady at Oxford, to be precise.

Paul