Abstract: The profile of the upper surface of the material being processed in a blast furnace is determined by pulsing a laser beam at the surface while incrementally changing the angle of the emitter to the surface, along a predetermined path, and sensing reflections of the beam using a detector having a relatively small angle of view compared with the area of the surface, the detector being located a fixed distance from the emitter. Points along the profile are calculated by triangulation, from the emitting and detection angles, and the known distance between the emitter and detector. In making the calculations, the amount that the emitting angle has changed may be determined by finding the amount that the reflection of the beam has deviated from a datum location on the detector. Ways of simplifying this calculation by making first approximation assumptions are disclosed.