Two light beams from respective light-emitting devices (e.g., lasers or lamps) cross each other and strike a surface (e.g., of a fluid) at respective oblique orientations relative to the surface (e.g., oblique but nearly vertical orientations that are equal and opposite to each other). A camera captures the surface scattering of the beams in a photographic "double-beam" image containing two respective photographic forms corresponding to the two respective surface scattering locations. The measured distance between the two photographic forms is trigonometrically indicative of the height and slope of the surface in the vicinity of the two surface scattering locations. Some inventive embodiments effect "single-beam" images that are trigonometrically indicative of height only. Plural (e.g., numerous) individual or paired light-emitting devices can be arranged so that a camera snaps an instantaneous photograph containing corresponding forms that are mathematically informative of a surface's configuration at plural (e.g., numerous) locations.