A depth gauge includes an upright elongate support (1) and several spaced emitters (2) and spaced detectors (3) mounted on the elongate support. The emitters (2) and detectors are electrically connected to a processor that sends signals to the emitters and receives signals from the detectors.In a first aspect, at least two sets of emitters or at least two sets of detectors are mounted on the elongate support. The sets may be directed in different respective directions.In a second aspect, each emitter may emit radiation, and/or each detector may detect radiation, selectively only within a selected range of frequencies, or emit/detect radiation outside the selected range only at a comparatively reduced level. Different emitters may emit radiation, and/or different detectors may detect radiation, only within different ranges of frequencies, or emit/detect radiation outside those ranges only at comparatively reduced levels.