Various techniques and devices are used to determine the shape errors of large-sized parts of technological units. This is important for the correct assignment of parameters for further reduction processing by special machines and machine tools. The acquired errors arise for several reasons: 1) large dimensions and weight of rotating parts, 2) the instability of the axis of the rotating part mounted on two support rollers, 3) the available initial and acquired shape error in the cross section of the part. It is important to define the types of form errors of the surfaces, error analysis, prediction and calculation of deviations from roundness in the cross section of the bandage, and the evaluation of cylindricity. Modeling of the process of operation and diagnostics of the unit, development of methods for determining errors, calculation of real values of deviations from the original contour on the basis of reconstruction of the contour of the section becomes important and necessary. These processes are performed using modern CAD systems and software. Obtaining data arrays in the process of measurements and their processing with the help of a special software module that performs an interactive calculation of shape errors with different geometric parameters of the cross-section of the bandage, provides the ability to obtain information about the state of the outer surfaces and parts of the supports on the operating unit. This article presents a method for determining the geometric parameters of the shape in the cross section of the rotation part, algorithms and software for reconstructing the contours of the cross sections of the part and calculating the shape error of the outer cylindrical surfaces