In this paper

a new approach for automatic extracting the local maximum(or local minimum) lines from the first horizontal derivative images of geophysical fields is proposed. The basic procedure of this method is as follows: First

the directional profile method is applied to recognizing local maximum (or local minimum) pixels from the field images. Then

the rotation transformation method is applied to searching maximum (or minimum) line endpoints in all directions. Finally

a vector map for the local extreme lines of the first horizontal derivative images of geophysical fields is obtained through that

the closely spaced lines are merged into broken lines in line with the line merged regulations

and that all of short lines are eliminated. The aeromagnetic map of Shandong peninsula was served as the experimental image. The experimental results showed that the local extreme lines both extracted automatically and interpreted manually are basically identical. It proved that the method is effective.