Abstract: Apparatus for measuring and indicating the settlement of the floor of a large liquid containing vessel such as an oil storage tank. The apparatus comprises sonic echo sounding means positioned at or movable between each of a multiplicity of spaced points disposed within the vessel and above its floor in known relationship one with another, and ranging means for furnishing for each point, and upon the basis of the echo sounding means output, a measure of the distance from the point to the vessel floor and an indicator (preferably visual) for indicating from the ranging results the settlement of the floor below those points. In preferred embodiments, the surface of the liquid within the vessel is utilized as a datum and the arrangement may be adapted to provide an indication of the volume of liquid within the vessel.