A topography measurement system comprising a radiation source configured to generate a radiation beam, a spatially coded grating configured to pattern the radiation beam and thereby provide a spatially coded radiation beam, optics configured to form an image of the spatially coded grating at a target location on a substrate, detection optics configured to receive radiation re-fleeted from the target location of the substrate and form an image of the grating image at a second grating, and a detector configured to receive radiation transmitted through the second grating and produce an output signal.