The Accuracy of Topographic Maps as GIS Data Sources

There are many accuracy standards for the analog and digital maps, so that map users can estimate the error range in using maps as GIS data sources. In Korea, there are not, however, clear map accuracy standards and the quality of the maps has not been tested. The accuracy of the National Topographic Maps (scale : 1/5,0000 was tested, and that of the aerial photograph was tested as an alternative to using topographic maps. Through indirect leveling and local triangulation, survey data were gathered. After digitizing the maps and stereo-plotting the aerial photographs, the digital data were converted into GIS database. And then they were evaluated in terms of vertical and horizontal accuracys. The major findings were summed up into the followings. Firstly, digitizing error was 0.001" - 0.003", but the maximum transformation error was 0.0153" which was ca. 5 times greater than digitizing error. Secondly, error distribution diagram showed not circular but ellipsoidal pattern whose major axis was Y in the X -Y coordinate plane. The error range to Y axis was less than O.83mm, while to minor X axis it was less than l.56mm. Thirdly, although Root Mean Square Errors of the maps and aerial photographs were almost same, error ranges and standard deviations of aerial photographs were much smaller than those of maps. Considering X and Y axes altogather, the Root Mean Square error of the maps was 1.8mm, and that of aerial photographs was 1.1mm. Fourthly, the RMS error limits made of stereo-plotting and map making processes were 0.5mm for stereoplotting and 0.8mm for map making. Compared with this RMS error limits, the RMS errors of the maps and aerial photographs studied were far from the error limits. Compared with National Map Accuracy Standards(NMAS), the errors drawn from this study didnot satisfy the standards