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Fig. 1: Several photos 
  
  
  
  
  
  
  
  
  
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Fig. 2 : Two photos 
  
  
  
  
  
2.1.1 Working with several Photographs. Leica and 
Rollei are offering systems working with several 
photos which are thought of for non cartographic 
applications. Elcovision (Leica) and Multi Image 
Restitution (Rollei) use a very accurate digitizing 
tablet with a cursor having a magnifying glass. The 
software performs, as a first step, a block adjustment 
by bundles. Knowing the exterior orientations of all 
photographs, a point is measured in all images and its 
space coordinates are computed and plotted by 
means of a CAD system. Both systems recommend to 
use small format photography of reseau metric 
cameras. For this reason the attainable accuracy is 
very high between 1: 20 000 and 1: 40 000 
(Luhmann, 1991). 
2.1.2 Working with two photographs. This 
possibility can be very interesting in the field of map 
revision, especially in developing countries where 
monoscopic plotting, as recommended by Sowton 
(Newton et al., 1991) and Warner ( 1992 ), is not 
possible because Digital Terrain Models are not 
available. 
The software for inner, relative and absolute 
orientation, and that of communication with the tablet, 
are not difficult to write. The software for plotting can 
be written in a language accepted by a CAD system 
(AUTOLISP in the case of AUTOCAD ). In this way, 
after measuring separately the left and the right image 
of a point with the cursor, its space coordinates can 
be computed and plotted. Thus, point by point, the 
map, or the features which need to be updated, can be 
completed. As the files generated by the most popular 
CAD systems can be read directly by several GIS, the 
map revision can be a straightforward task. 
2.2 Using a procedure partly monoscopic and 
partly stereoscopic 
There is a drawback, however, in the procedure 
described in the last paragraph: only points which can 
be clearly identified in both images can be used. This 
drawback can be overcome if a stereoscope and a 
parallax bar are used. By means of the stereoscopic 
Vision the two homologue points can be determined 
and, by placing the cursor successively on the left and 
395 
the right mark of the bar, the coordinates of both 
points can be measured. This implies a mixed 
procedure: Homologue points are found 
stereoscopically with the parallax bar and the 
measurements of the two marks with the cursor are 
made monoscopically. 
The photographs to be employed need not to be 
standard aerial pictures taken by a metric camera. 
With due considerations regarding the needed 
accuracy, small format photography could be also 
used, either aerial or terrestrial. Terrestrial photos, 
suitable to be treated stereoscopically or not, could be 
employed in hilly areas. A newly built road, for 
instance, could be mapped in this way to update a 
GIS or an old map. 
In developing countries, where mapping at medium 
scales is far from being complete, this procedure can 
be used by civil engineers to obtain a first 
cartographic document with which to prepare a 
preliminary sketch for planning roads, water suply, 
irrigation, etc. 
2.3 Using a stereoscopic procedure 
This possibility implies the construction of an 
instrument similar to the so called Image Space 
Plotter ( Petrie 1992 ). These simplified instruments 
are based on a scanning mirror stereoscope and a 
mechanical measuring unit. Unlike the high 
performance Analytical Plotters, they do not provide a 
parallax free model. The Y parallax has to be 
eliminated manually and map features have to be 
plotted “point by point". Contours can be drawn in an 
indirect way. 
A Photogrammetric Plotting System employing a 
personal computer, digitizing tablets and a CAD 
system is being developed at our Institute and a 
description of it is given in a paper which will be 
published elsewhere. Here only a brief description will 
be given : The system resembles the 
photogrammetric plotters known as Image Space 
Plotters , whose mechanical measuring unit, has been 
replaced by one or two digitizing tablets. 
Working stereoscopically requires either one tablet 
and two cursors (fig. 3), or two tablets (fig. 4). The 
arrangement of two tablets (fig. 4) was prefered for 
the time being in order to avoid the complication of 
building the additional hardware implied by the 
selection box (fig. 3), something necessary to send 
alternatively the signals of both cursors. 
AI 
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Selection box 
  
  
  
  
  
  
Fig. 3: One tablet and two cursors 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996