The ASTM D5199-12 (2019) standard specifies a test method for measuring the nominal thickness of geosynthetics. This method is suitable for various types of geosynthetics, such as Geotextiles, geomembrane, etc., to ensure that these materials have appropriate physical performance in engineering applications. The standard details the instruments, equipment and operation steps used in the measurement process to ensure the accuracy and repeatability of test results. This standard can effectively evaluate the thickness of geosynthetics, which in turn affects their application performance in civil engineering, including tensile strength, durability and Stability. This provides a reliable technical basis for Manufacturers, quality control agencies and engineering designers.