A method and apparatus for determining a base line of a measurement curve wherein after preparing a template having an arc upward convex, a CPU of the data processing system moves, without rotating, the template with the arc of the template always contacting the measurement curve and determines the base line as the envelope of the arc of the template. During moving of the template, the system may move the template horizontally under the measurement curve; calculate, at every horizontal point at a given horizontal position of the template, the vertical distance between the arc of the template and the arc of the measurement curve; detect the minimum of the vertical distance at the given horizontal position of the template; and move the template vertically at the given horizontal position by the minimum vertical distance. To determine the base line, the CPU initializes data of the base line by the smallest value and judges whether the arc of the template after moved is superior to the arc before moved at every point of the arc after moved. The data of the base line is updated at the points where the arc after moved is superior to the are before moved by the data of the arc after moved.