A meridian convergence is the angular difference between grid north and true north. Therefore the true north can be determined by putting a compass on a map with the indication of the meridian convergence showed as a mean value. Currently, in all varieties of topographic maps published in Taiwan, the meridian convergence, generally referred as a mean value, is marked on the declination diagram. The mean value of the meridian convergence could be located in the middle part of the map but map user could not be able to figure out the distribution of its varieties in the whole map. Hence, in this paper the accuracy of the mean value is analyzed and manipulated to derive the relative accuracy in different scales of topographic maps using transverse Mercator projection. Besides, from 6° to 2° zone mapping the detailed strict equation is analyzed and its practical equation proposed. It is concerned that this research is contributive to the topographic map compilation.