We previously [1] reported on the characteristics and capabilities of flatness gage IP-4-gp, which is designed to measure the flatness of rolled products on hot-strip mills. The instrument has significantly better performance characteristics than existing devices based on triangulation-type laser height gages. Thus, the gage presents new possibilities in the manufacture of rolled products. In particular, due to its high sensitivity and geometric resolution, as well as algorithms developed specifically for this purpose, the instrument is the only flatness gage that can measure not only the flatness of strip during rolling, but also the shape of rolled plates as they move along roller tables. Features of the Measurement of Plate Flatness. Plate flatness gage IP-4-l was built on the basis of gage IP-4-gp, which is designed to measure the distribution of the relative elongations (RED) of strip during rolling. The RED is a characteristic that is identical to the nonuniformity of the distribution of the metal along the roll gap. Thus, it can be used directly to regulate the flatness of the strip. Examples of measured REDs are presented in [1]. However, when the gage is used to measure the flatness of plates, for several reasons the results must be represented in the form of measurements of the plates’ shape. In general, plate shape is taken to mean the position of points of the surface of the plate relative to a base plane. In the application being discussed here, it is necessary to introduce several definitions of plate shape. The natural shape of the plate ‐ the hypothetical shape that internal stresses would cause the plate to assume in a weightless environment after the removal of all external forces. The shape of the plate on a test slab ‐ the plate shape obtained as a result of the interaction of the plate’s natural shape and the reaction forces associated the slab’s support of the plate. This shape cannot be accurately reproduced when a given plate is placed on the same slab several times in succession, due to the distortion of the plate’s natural shape during each such placement and friction at the support points of the plate. The shape of the plate on the roller table ‐ the plate shape obtained as a result of the interaction of the plate’s natural shape and the reaction forces associated with the support of the plate by the roller table as the plate moves through the measurement zone. This shape may be different from the shape of the plate on the test slab due to differences in the number of support points and their positions. Instability of plate shape ‐ plate shape is destablized by the fact that, over time, external effects (temperature, bending, impact, vibration) decrease the internal stresses created during rolling, straightening, and other processing operations. As a result, the natural shape of the plate may be changed. The IP-4-l can be used to measure plate flatness in one of two variants: Monitoring of plate flatness on production units. In this variant, the gage is installed on hot-strip mills or various straightening machines. The measurement results are used to correct the process regime or to perform secondary straightening, i.e., steps are taken to significantly improve the quality of the product. In this case, it is best if plate flatness is represented in the form of an RED. Such an approach allows the shape of an actual plate moving along a roller table to be represented in the