Similar number of horizontal stations marked inside the tank at equal distance around the circumference. Vertical stations are taken from the center of the tank to the inside of the wall, 5M apart with the first station at 0.5M from the wall. By placing a leveling staff at the center of the tank and taking reading with horizontal optical plummet at each vertical station, the profile of the tank bottom can be obtained. From this profile the volume can be calculated using the correct formula for the geometrical shape for the floor with irregular configuration. If the tank is in operations and cannot be emptied out, the Zero Quantity is to be taken from the previous calibration chart.