2.3 CADASTRAL MAPS 
action. The cadastral data (see figure 2(a)) are available in digital 
  
  
  
  
  
  
north- form with each property border represented as a vector. lt is 
lear in- assumed that these vectors do not cross each other but end 
of crop at each junction. In the case of very long borders, we divide 
them into smaller ones by cutting into half. Furthermore, in- 
formation about the spatial location normally is available al- 
though errors, especially shifting errors, cannot be completely 
ubpixel excluded. 
ch has 
| if the 3 LINE SEGMENT MATCHING 
ures Is Matching [5, 10, 13] is to be performed between lines in 2 ge- 
ometries (one of the image, one of the map) which are slightly 
nt sig- distorted relative to each other. One-to-one correspondence 
border cannot be expected because: 
p e some of the property boundaries in the cadastre are not 
nid visible as edges in the image due to same or similar land 
of the use in neighbouring parcels, and 
| to be e edges in the image may not have corresponding bound- 
mages aries in the cadastre, if, for instance, different crops are 
lerived planted on one parcel of land. 
Consequently, the matching method must be robust against 
at im- | | additional lines in both data sources. A matching by un- 
Wher- IT^» : | | structured search is time consuming and may yield incorrect 
riginal , Ham i PR matches, if distortions are of the order of line segment spac- 
 Spec- ings. 
orders (a) intagg with cadastral boundaries superimposed in white Line segments are represented in vector form by 4 parameters 
occur (compare Fig. 3): Starting from the origin of the coordinate 
bright system, the normal to each line (segment) is constructed. 
This normal may cross the line outside the segment. The 
NES parameter Ÿ is defined as the angle of the normal and the | 
ordinate. The middle point M of the line segment has the | 
| pro- coordinates x and y, while a gives the distances from the | 
of our middle point to the end point of the line segment. Ii 
3] and Il 
laries. | 
arallel | 
to the 
rings, a 
lough 
| per- M (x,y) 
all de- a 
As it 
entify 
arcep- 
d are 
romi- Ü 
iction 
oplied (b) image with extracted line segments superimposed in black 
iptual 
lines, Figure 3: representation of a line by 4 parameters 
tified. ps Sue Sng Tjian withifcadastral The four parameters of a line segment have different sig- 
corre- nificance and reliability since they are affected by noise and 
5, but weakness of line extraction algorithms to different degree: the 
1, Le. angle 9 determining the direction of the line (segment) usu- 
other ally is known with high accuracy. Point M is accurately given 
grey in a direction perpendicular to the line. Its position along the 
pe on line, however, is less reliable since long line segments can be 
119 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996