Between the geometry of an ideal perfect model and the real structure, there are often discrepancies that may occur in different shape and size. There can not be many types of geometrical imperfections completely avoided, but their size is of interest to make a comparison to the tolerable limits can.
For example, the body is a target-actual comparison made (e.g. gap measurements between the doors) in the automotive industry for certain components. In many cases, optical measuring techniques are used for such tasks, which point coordinates in 2D and 3D structures can be captured by photogrammetry. If a CAD model exists for the same structure, it can be calculated between the rendered surfaces, the deviation in the respective coordinate system. The real structure is thus after the manufacturing process with the ideal model compared (reverse engineering).
An application example from the construction industry is optical 3D measurement of a steel cylindrical shell in which the lack of roundness and pre deep bugling should be determined. The measured point coordinates are shown in the figure above.
The lower figure shows the measured deviations of the cylinder surface in comparison to an ideal cylinder model. By means of a best-fit analysis can thus be determined, the deformations to the inside and outside of the cylinder, and eventually compared with the respective limit values.