Applies to tanks above 8 m in diameter with cylindrical courses that are substantially vertical. Provides a method for determining the volumetric quantity contained within a tank at gauged liquid levels by optical triangulation using theodolites and laser beams. The measurement required to determine the radius may be made internally or externally. The external method is applicable only to tanks that are free of insulation. For tilted tanks with a deviation of up to 3 % from the vertical a correction as described in ISO 7507-1 may be applied.