The earliest time of the old platform is the reinforcement of the platform is exposed to the outside, after many years of practice, now do cast iron plate reform into box type, reinforcement plate two. The working face still continues to use the standard manual scraping process, the precision requirements are too high will use the manual grinding process or processing on the grinder, but the larger size of the cast iron plate can not be processed by the grinding process, and the upper grinder processing is also limited in size, so most of the use of scraping. The cast iron platform is used for the detection or marking of the workpiece of the plane reference instrument, can be machined on the working surface T-groove, V-groove, U-groove, according to the installation should be adjusted to the level, the load evenly distributed on the fulm, the ambient temperature (20±5℃) should avoid vibration when used. In the original flatness data, the unit rotation is obtained by using the coordinate transformation and the mathematical processing method of binary first order equations, and then the flatness error is calculated. This process is called algebraic computation. The calculation steps are as follows: 1. Set Zoo-Z0n as the X-axis, Zoo-Zon as the Y-axis, the unit rotation quantity of rotation around the X-axis is Y, and the unit rotation quantity of rotation around the Y-axis is X, and use the point selection principle to judge and select 4 extreme points. 2. When two points (saddle surface) or three points (concave or convex surface) become equivalent according to the evaluation criteria, binary primary equations are established to solve the unit rotation X and Y of the cast iron platform. 3. Calculate the rotation increment of each measuring point Zij from the unit rotation quantity X and Y, and add Z to this increment to obtain the transformed coordinate value Z'ij. 4. Judging whether it meets the evaluation criteria from the transformed data. If it meets the criteria, the difference between the maximum value and the minimum value is the flatness error value of the measured surface cast iron platform.