For different superimposition systems there are a 3. TESTS FOR SUPERIMPOSTION SYSTEMS 
number of variables which influence the 
superimposition accuracy: 
1. In the analytical plotter superimposition can be 
realized directly in the neighbourhood of the 
photocarrier or nearby the ocular(s). 
If superimposition has been realized nearby the 
photo then rotation of the Dove prisms and 
zooming will have the same effect on the su- 
perimposed image and on the photo image. A 
disadvantage can be that when zooming in, the 
pixels of the image displayed on the screen are 
seen more clearly and only the centre of the 
screen will be used for display. 
When superimposition is realized near the ocu- 
lar(s) always the whole screen can be used for 
superimposition and the size of the pixels can 
be kept small. However, when zooming and 
when rotating the Dove Prisms the screen 
image has to be geometrically adapted. 
2. Superimposition systems can be equiped with 
small flat screens or curved screens. When 
using a curved screen the image has to be cor- 
rected for screen curvature. Further the screen 
can be a raster type or vector type screen. 
3. If the database image is projected on one photo 
only this is called mono-superimposition. When 
the database is projected on two photographs 
which form a stereomodel then the database 
can be viewed in stereo. Stereo superimposi- 
tion is important when digital height data have 
to be checked. 
In this research only mono superimposition 
systems have been used. 
3.1. Introduction 
By using two calibration grid plates a practical si- 
tuation has been simulated. 
The simulation date are: 
Photoscale 1:3000, ¢ = 150 mm, 60% overlap. 
The number of grid points in the stereomodel is 77 
(see figure 2). 
  
  
Figure 2. The flat stereomodel. 
In order to get height differences in the stereo 
model the terrain model has been tilted two grads 
around the x- and y-axis. 
The characteristics of some systems are given 
in table 1. 
  
  
  
  
  
  
  
  
  
  
  
I.M.A KRISS RIVS Videomap 
Il 
mono or stereo mono mono/ mono/ mono/ 
superimposition stereo stereo stereo 
monochrome or colour colour monochr. monochr. monochr. 
raster or vector raster raster raster vector 
screen 
screen update per 60 60 60 60 
second 
resolution in photo 20-5 25/50 10 40 
in microns 
superimposision nearby ocular photo photo photo 
photo or ocular 
memory space in Mb - 4-16 4-8 1-2 
  
  
  
  
  
Table 1. Characteristics of some superimposition systems.