The "measuring bridge plate" we usually use is usually an adjustable "measuring bridge plate" (hereinafter referred to as "bridge plate"), as shown in Figure 1. The material of this "bridge plate" in Figure 1 is generally cast iron that is not easily deformed. In the measurement of flatness (straightness), the purpose of ensuring that the "bridge plate" is not easily deformed and selecting the correct structural form of the "bridge plate" is to ensure the accuracy of the original data collection.