Paper related to accuracy of digitizalization of maps

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BaikComparison Raster data and Rasterized Vector Data with �Saw Tooth Line 
Algorithm� for Verifying the Accuracy of Digital Map 
Songhoon Baik 
Member of Technical Staff, GIS Service Development Team, Outside Plant 
Technology Laboratory, Korea Telecom, Republic of Korea 
It has been essential to develop a proper methodology for quality assurance in 
digital map (or vector map, hereinafter referred to as digital map) producing 
process since GIS has been recognized as useful in utilities, national agencies 
and municipalities and so on. 
The purpose of this study is to develop computer-based quality assurance tool 
which can be adapted in producing digital map. Usually, there are three ways to 
produce digital map. Those are manual digitizing, screen digitizing, and mapping 
with analytical stereo plotter. In Korea, screen digitizing is a prevailing 
method in producing digital map. 
Considering the above mentioned conditon, the devoloped tool is intended for the 
digital map with screen digitized through scanning paper map. 
Getting accuracy of base map database is one of the most important factors to 
ensure successful Korea�s National GIS construction. The conventional methods 
for quality assurance, however, have depended on inaccurate and �time-consuning� 
manual works which may cause potential problems in national wide GIS deployment. 

This study has started to develop a software tools which can be used to verify 
accuracy of digitized maps automatically and effectively. 
Inaccuracy and errors may occurred either in every step from scanning paper map 
through digitizing, or may caused by individual workers� mistakes. Considering 
all anticipated inaccuracy and errors throughly for each step in the whole 
process, "Similiarity Measuring Theory" was introduced to the system 
development. By using �Brensenham Algorithm� and �Saw Tooth Line Algorithm�, 
digitized vector maps are converted into raster maps automatically and 
overlapped over the original scanned raster maps, which is used for the source 
of vectorization for the verification of accuracy. Completing the verification, 
the system automatically report the differences between the original scanned 
raster data maps and the rasterized vector maps. 
Figure1 shows the examples of raster data map (a) through scanning the original 
paper map, and the rasterized vector data map (b). The symbol O marks the 
differences between the two types of data map. O are the errors which are missed 
in process of digitizing the raster data map. As shown, raster data map has some 
noises. But these are not errors. Therefore these noises are eliminated by 
Figure 1: Raster data map with scanning paper map (a), and rasterized vector 
data map (b) 
Figure 2: XOR operating (a), and filtering (b) over two types of data map 
The system also provides the method for positional accuracy measurement of point 
features in digitized maps. A digitized vector map, which is a DXF (Drawing 
eXchange Format) file, in TM coordinate system has control points which will be 
compared with corresponding points on the original paper map. 
This verification tool has been reflected by the examples of Ordnance Survey and 
Nation Joint Utilites Group. 
This tool will be expected to replace the conventional verifying method which 
depends on inspector�s manual work, and to save the man-hour and cost. In 
addition, this tool will be expected to make great contribution not only to GIS 
projects in Korea Telecom including 
TOMS (Telephone Outside-plant Management System) but also to National GIS 
project which is being propelled by government.