PHOTOGRAMMETRIC PLOTTING BASED UPON DIGITIZING TABLETS AND A CAD SYSTEM 
José E. Juliá, Universidad Nacional de Tucumán, Argentina 
Commision IV , Working Group 3 
KEY WORDS: Mapping, Revision, Developing Countries, GIS Updating, Image Space Plotter, Digitizing Tablet 
ABSTRACT 
Several possibilities of performing photogrammetric plotting using digitizing tablets, personal computers and a 
CAD system are proposed. Some procedures ranging from a simple one, consisting merely in laying two or more 
photographs in a digitizing tablet and observing monoscopically homologue points, to another that makes profit 
of stereoscopic observation and implies the construction of an instrument similar to an Image Space Plotter, are 
described. These systems can be very useful for map revision or for feeding a GIS, especially in small private or 
state oganizations which can not afford to buy expensive photogrammetric equipment. In developing countries, 
where huge unmapped areas at scales 1: 50 000 and even 1: 100 000 exist, these procedures can be an 
interesting tool to obtain a first map for preparing preliminary sketches for civil engineering projects. 
Considerations about the attainable accuracy are also made. 
1. INTRODUCTION 
In the past, there was no possible way to make spatial 
plotting, or even to obtain the three coordinates of 
isolated points, without expensive equipment. 
Everybody remembers the high cost of Analogue 
Instruments. Even the so called Approximate 
Instruments ( Zeiss Stereotop, Officine Galileo 
Stereomicrometer ), which had a rather limited 
accuracy mainly because they were based upon an 
incomplete mathematical model, had a price by no 
means low and were therefore out of reach for many 
users. The use of the parallax bar to obtain height 
differences between two points yielded good results 
only if the points were very close. Thus, this procedure 
was restricted to measuring the height of trees, towers 
or buildings. 
The arrival of computers initiated the reign of 
Analytical Photogrametry. Analytical Plotters meant 
not only important improvements in accuracy but 
made also possible, among other advantages, the 
digital treatment of the resulting map within a CAD 
System. For the above mentioned Approximate 
Instruments “there sounded the death knell “ (Petrie, 
1992), and they were replaced by the so called Image 
Space Plotters like the Sterecord from Zeiss and the 
Stereobit from Galileo. The prices of Analytical 
Plotters are similar to that of Analogue Instruments 
and the cost of Image Space Plotters does not differ 
too much from that of Approximate Instruments, so 
the impossibility of its use by many users still 
remains. 
The future belongs doubtless to Digital 
Photogrammetry. One of its promises is user friendly 
and economical systems. Nowadays, however, the 
cost of most accurate digital systems is too high, even 
for large photogrammetric companies, and although 
less accurate systems can be more accessible, they 
are not still easily available for many users. Thus, the 
digitizing tablet appears as an interesting tool allowing 
the realization of several photogrammetric tasks. One 
of these tasks, for flat terrain, is rectification, which 
was implemented in several GIS as a data capture 
technique, and recently in version 12 in AUTOCAD. 
Another proposed task is map revision using a single 
photograph and a digital terrain model ( Newton et 
al.,1991), which can be a good solution in developed 
countries. In developing countries, however, Digital 
Terrain Models are non existent and huge unmapped 
areas exist at scales 1:50 000 or even 1:100 000. 
Thus, revising or feeding a GIS can be a real problem. 
Therefore, a low cost system allowing 
photogrammetric plotting would be welcomed in 
several places like small cartographic offices in 
provinces, municipalities and research centers. 
2. PHOTOGRAMMETRIC POSSIBILITIES OF 
DIGITIZING TABLETS 
Digitizing tablets working with personal computers 
offer interesting possibilities for many users lacking 
photogrammetric equipment. Besides the above 
mentioned analytical rectification and monoscopic 
plotting with the help of DTMs, which are becoming a 
common practice, there can be considered a 3 D 
plotting system working either with monoscopic or 
stereoscopic observation. Eventually, a mixed 
procedure, partly monoscopic and partly stereoscopic, 
could also be used. 
2.1 Photogrammetric plotting with monoscopic 
observation 
In this case only one digitizing tablet will be needed 
and two or more photographs can be used (fig. 1and 
2). This possibility is the easiest one to implement. 
Working with two photographs requires a software for 
inner relative and absolute orientation. Working with 
more than two photographs implies the use of a 
bundle block adjustment software. 
394 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
  
  
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