The application of terahertz time-domain spectroscopy (THz-TDS) and imaging to differentiate flat and curved surfaces in reflection via the THz Gouy phase shift is demonstrated. For a THz-TDS system that is aligned for reflection from a flat surface, the presence of a curved surface displaces the image focal plane from the detector plane, resulting in a Gouy phase shift. The potential of utilizing this configuration for estimating the number and size of curved objects is discussed with particular emphasis on agriculture crop yield estimates.