1. A method for determining the interior shape of a spherical tank for the computation of the liquid volume of the tank, comprising the steps of: establishing a predetermined number of measuring stations within the tank; establishing an arbitrary system of coordinates, the location of said measuring stations being identified with reference to said system of coordinates; placing a series of markers evenly distributed on the inner surface of the tank to form reference points which are measured by measuring instruments at said measuring stations in accordance with said system of coordinates; photographing all said reference points to obtain double coverage of all the inner surface areas of the tank; plotting the measured reference points onto a drawing table of a coordinatograph to establish a horizontal great circle plane of the sphere together with a first interim center of the sphere; feeding the coordinate values of all reference points and said first interim center of the sphere to a computer for the determination of the coordinate values of a second interim center of the sphere; plotting said calculated second interim center of the sphere onto said drawing table; generating a plurality of vertical planes evenly angularly spaced through said second interim center and generating intersecting horizontal circle planes on the drawing table to form selected other points; forming in an autograph stereoscopic models of the inner surface of said tank using the double coverage photographs, said autograph being associated with said coordinatograph with the drawing table; obtaining a series of three-dimensional readings from said stereoscopic models in a predetermined system of coordinates, said readings including the identification of said reference points and said selected other points on the inner surface of said tank; transferring all said three-dimensional readings to an interface unit wherein said readings are registered and converted into field coordinates; calculating on the basis of all three-dimensional readings the final mean radius of the sphere, the coordinate values of the final center of the sphere and the deviation from the mean radius for every point of reading; calculating on the basis of said three-dimensional readings the mean radius of said small circles; and calculating the volume of each of the areas between adjacent small circles to determine the total volume of said tank.