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Graphic Preprocessing 
Because most originals for scanning are not so ide- 
al, so the linework scanned from the orignals may exist 
many broken lines, stains and voids, etc. For this rea- 
son the thinning result may distort. So it is necessary to 
carry out preprocessing, which mainly include the fol- 
lowing contents: automatic joining of broken lines, 
stain removing, void filling and line smoothing etc. All 
of the above mentioned preprocessing are automatic. 
Although the preprocessing can reduce a lot of editing 
work, it can not replace the interactive editing com- 
pletely. 
Interactive Graphic Editing 
Because the graphic preprocessing can not deal with 
large area line adhesion and line broken, additionally it 
can not add lines and texts too. So the system must 
have some interactive editing functions. As long as the 
system have more powerful graphic editing functions, it 
can reach more practicable. 
The graphic interactive editing software in this scan 
digitizing software system include two parts. one is the 
graphic interactive editing for raster data, the other is 
for vector data. In this software package we have devel- 
oped two interface softwares for Paintbrush software 
package, and the graphic editing for raster data is per- 
formed by Paintbrush. As known as the Paintbrush has 
very powerful image editing functions such as image 
zoom in, zoom out, overall roaming, addtionally it can 
add and delete lines, curves and texts. Besides the 
above functions it can also do image enhancement, im-^ 
age filtering and noise removing and so on. As long as 
using the interface software converts the original data or 
the raster data after line thinning to the graphic formats 
that Paintbrush can read such as PCX and TIF graphic 
formats, and then the raster graphic can be edited in 
Paintbrush. After editing using another interface soft- 
ware can convert the edited graphic to raster data. So 
the graphic interactive editing for raster data in the sys- 
tem is very powerful and practicable. Besides that in the 
system the vector graphic are edited with AutoCAD. 
For this reason we developed some standard interface 
software for generating graphic file DWG of AutoCAD. 
using the interface software can convert the vector 
graphic to DWG file, and then vector graphic can be 
edited in AutoCAD. Because this software system adopt 
two standard software package Paintbrush and Auto- 
CAD to do graphic editing, they make the graphic edit- 
ing functions more powerful. 
Line Thinning 
Because the scanned lines have a certain width be- 
fore line thinning. In order to do line following, the 
731 
scanned lines must be thinned frist. The lines after 
thinning are the single connected lines, which usually 
maintain the central lines of their original lines. No 
matter what kind of method we take to do the line thin- 
ning process, whose basic principle is that the thinning 
process is carried out symmetrically from four directions 
of up, down, left and right and the process must main- 
tain the continuous states of the original lines. In our 
software system a knowledge based line thinning 
method is adopted, which first needs to set up expert 
knowledge and decision table for line thinning. Then 
the decision system can determine whether a point is 
deleted or sustained according to the situation of its 
eight neighbors. Using this method can greatly increase 
the efficiency of line thinning. The land use map after 
thinning is shown in Fig. 6, whose scale is 1:10,000. 
Automatic Raster to Vector Conversion 
Generally the graphic data can be divided into two 
classes such as raster data and vector data. Both of 
them have advantages and disadvantages. For instance 
the rasterized graphic data are easy to do display and 
overlay, but it is difficult to do editing and it usually 
need more space to store, on the contrary the vector da- 
ta are easy to edit and only need less space to store. Ad- 
ditionally the raster to vector conversion is needed in 
both of the land use statistics and the automatic scan 
digitizing of contour line map. Therefore the raster to 
vector conversion software is one of the key softwares 
in our package. Raster to vector conversion software 
performs the line following from the thinned raster da- 
ta, and finally it can convert the rasterized graphic data 
to vector data. The first thing to do in the process of 
raster to vector conversion is to determine the nodes, 
then the line tracing starts from one node and finds the 
next point on the line through searching its eight neigh- 
bors, as shown in Fig 4, until reaches the end point of 
  
  
  
  
  
  
5 4 3 
6r P 2 
| 
7 8 1 
Fig. 4 Eghit neighboring points of point P 
the line. So a series of direction codes of the vector lines 
are generated , because the land use map and contour line 
map containing many curves, In this kind of maps stor- 
ing vector data in direction codes can save a lot of disk 
space than storing x, y coordinates directly. So in our 
system the direction codes is used to represent vector