thinned figures of fig3 b closest to the specified points in fig3 a then at the step 29 the shortest path in the thinned figures joining these points is determined the use of the shortest path here eliminates the erroneous passage such as one between the points 1 39 and 2 39 through the letter a like portion of the fig a so that the unique identification of the line piece specified is possible thus the ties such as the letter a like portion of the fig a and the vertical portion of the fig b can be removed in this embodiment provided that the points are specified correctly when the figures contain cuts between the two sequential points such as between the points 2 39 and 3 39 4 39 and 5 39 and 5 39 and 6 39 the end points of the figures containing each of these points are joined together by straight lines as shown in fig3 c so that the cuts can also be removed in this embodiment then at the step 30 the connected line piece shown in fig3 c obtained at the step 29 is symbolized in the manner of vector symbolization by linear approximation i e the straight lines are fitted to the connected line piece and the connected line piece is represented by the coordinates of the junctions between the straight lines as shown in fig3 d the result of this symbolization is displayed on the crt display 7 for inspection by the operator at the step 31 when an error in the symbolization is detected in the inspection by the operator the operator can select the menu symbolization cancel in order to correct the error the step 32 determines whether this is the case.