The characteristic factors are important to the gridding of isopleth map. By comparing the applied results of two kinds of the classical mathematical morphological skeleton extraction arithmetic, this paper found that skeleton defined with the maximum digital disk was close to the distribution of the characteristic factors of the isopleth map. In order to improve the deficiencies in the skeleton extracted with the maximum digital disk arithmetic, disposals like noises removing, joint and thinning were used there. Suggestion on shape and size of the structure element were also given.