tems provide some kind of projective monitoring 
routines based on initial values of the exterior 
orientation. Automatic algorithms, too, have 
been developed for same tasks. Those algor- 
ithms are mainly designed for traditional, point- 
wise methods. Finding the "suitable" features 
and their images on photographs leads to the de- 
termination of exterior orientation of photo- 
graphs in question. Performing a photogram- 
metric triangulation is possible as well, but as it 
is based on the idea of block adjustment, it may 
be too rigid in small map revision projects. 
As the final goal is to produce a map or to im- 
port data for some planning projects, we have to 
find the difference between the existing data in 
the database and the new data determined by 
the intersections performed on several photo- 
graphs. For this we need to find the correspon- 
ding images of each feature on different photo- 
graphs. The same requirement is valid when 
constructing stereo models for mapping in stereo 
mode. Algorithms for this task have been widely 
designed, but only for finding homogenous points 
on several photographs, and we need to use ro- 
bust methods in same sense as in edge detection. 
At first, our aim is to employ a manual approach 
to overcome the problems mentioned earlier. In 
second stage automatic methods will be imple- 
mented, if manual methods prove the capability 
of this procedure in mapping projects, as we be- 
lieve. The flow of the process is described in 
Figure 5. 
  
  
  
      
Photography 
- not necessarily a block 
  
  
  
Digitalization 
  
Solving the 
exterior 
| orientation of 
  
  
  
the photographs 
  
LIS/GIS 
  
Duration of the 
mapping process 
depends highly on 
size of the mapping 
area and on the 
  
  
Performing an 
intersection of a 
+ object feature 
possibility to use 
automatic methods 
  
  
  
  
  
  
  
  
  
  
simultaneously 
on several 
photographs Constructing 
| stereomodels for 
| a possible 
| stereomapping 
| Stereomapping 
| | with digital 
3 plotters 
  
| 
| 
| 
| 
| 
| 
| 
| 
| 
| 
| 
| 
| 
| 
  
  
  
Figure 5 
3. CAPABILITY OF LINEAR FEATURES 
At the Helsinki University of Technology some 
tests have been made to find the capability of 
the use of linear features in photogrammetric 
tasks. All tests have been accomplished with 
simulated data. The means of the accuracy are 
theoretical and show only the magnitude. Tests 
have been done mainly for space lines, circles 
and parametric splines. 
The capability of features in case of intersection 
has been the top interest. Some research about 
the exterior orientation with linear features have 
also been done with few feature types. Variables 
in the tests have been the number of points in- 
cluded in computation, position of the feature on 
a photograph, and the orientation respect to the 
datum. 
504 
A new approach on a mapping process using feature based methods. 
In case of close-range photogrammetry and map- 
ping simulated tests with line features have 
been accomplished. In the mapping example only 
one line feature was determined. In the test 
photographs were added into computation one by 
one and at the same time the number of obser- 
vations was also increasing. The block of aerial 
photography had three flight strips consisting of 
three images each. So, the maximum number of 
images was nine and the maximum number of 
observation points obtained was 63; 11 points of 
the photographs in middle flight strip and 5 of 
the other two strips. 
The results are presented in average standard, 
error ellipsoids. The ellipsoids are supposed to 
construct a probability tube around the trace of 
the curve. Additionally, all figures are scaled