Because the main part of the wheel tread is a slope of 1:40, in order to make the top surface of the rail evenly stressed under the load of the wheel with a tapered tread, the rail should not be laid vertically on a straight line, but should be tilted inward appropriately. This inclination of the rail is called the rail bottom slope. The rail bottom slope measuring instrument is a measuring instrument used to measure the rail bottom slope and angle of the rail. This product has a fast measurement speed and is easy to use. It can be used during the day and at night. It is an ideal measurement tool for engineering and construction departments.