After obtaining the moving trace MT, the computing device then performs a clustering procedure to the moving traces MT to obtain a detection zone DZ in step S05, wherein the clustering procedure includes obtaining multiple boundary points based on the moving traces MT, and obtaining the detection zone DZ according to the boundary points. The computing device obtains the detection zone DZ according to the moving traces MT can be based on a probability distribution function. For better explanation of the following description, the following uses the normal distribution function as the probability distribution function as an example. The normal distribution function is preferably a binary normal distribution function, and one dimension of the normal distribution function is the density of the moving traces MT, the other dimension of the normal distribution function is the distribution of intersections between the moving traces MT and a reference line.