A computing device connects with a measurement machine. The measurement machine includes a raster ruler and a measurement unit. The computing device reads environmental temperatures from a temperature sensor. Compensation coefficients of the standard workpiece and compensation coefficients of the raster ruler are calculated. The computing device calculates total compensation coefficients of the standard workpiece and the raster ruler. When the measurement machine measures an object workpiece, the computing device calculates a total error of the object workpiece according to the total compensation coefficients of the standard workpiece and the total compensation coefficients of the raster ruler. Coordinates of the object workpiece are calculated according to the total error and mechanism coordinates of each axis of the measurement machine.