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Photogrammetry and Visualisation 
The traditional output of photogrammetry 
is a 2-dimensional plan. While this is 
adequate for topographic mapping, it is 
definitely insufficient for non- 
topographic mapping. Findings from 
cartographic research suggest that many 
non-mappers have problems perceiving  3- 
dimensional objects in 2-dimensional 
planes. A typical two dimensional plan 
of a traffic accident survey is shown in 
figure 1. 
In traffic accident mapping, for example, 
the users of plans are usually lawyers 
and judges, professionals who are more at 
home with words than contours. They might 
not be totally comfortable with the idea 
of interpreting 2-dimensional plans. 
Hence, it is not just desirable, more 
logical to marry photogrammetry with 
computer visualisation so that the 
captured 3-dimensional accident scene can 
be displayed in a "3-dimensional" world. 
Apart from the ease and clarity with 
which visualization systems convey ideas 
and messages to involved persons (such as 
judges and traffic policemen), there is 
one more compelling reason as to why it 
is in fact sometimes necessary to employ 
visualization systems for traffic 
accident mapping. Because of the 
limitations imposed by metric 
photography, there is an increasing trend 
in traffic accident mapping to use 
convergent non-metric photographs for 
data acquisition (see Waldus and Kager, 
1984). 
But convergent photographs do not make 
for good stereoscopic models. Hence the 
construction of traffic accident scenes 
must be done analytically. A 3- 
dimensional visualization system must, 
therefore, work in cooperation with 
analytical photogrammetry for displaying 
the restituted 3D traffic accident model. 
The PMCAD II system 
The PMCAD II system represents one 
successful attempt at developing a low- 
cost photogrammetric-based visualization 
system for traffic accident mapping. The 
acronym PMCAD stands for Photogrammetric 
Mapping through Computer Aided Drafting. 
A detailed description of PMCAD II can be 
found in Koo and Aw (1991). Here only the 
gist of the paper is presented. 
PMCAD II is a refinement on PMCAD (Koo, 
1989) in that it allows shaded renderings 
of the traffic accident scene. The system 
was designed for use by non- 
photogrammetrists. Using off-the-shelf 
cameras, PMCAD II is able to reconstruct 
traffic accident scenes in a microCAD 
System from "random" pictures taken of 
the accident scene. Although PMCAD was 
originally conceived for traffic accident 
mapping, the resultant system is also 
eminently suited for the re-creation of 
any wire-frame or pseudo-solid (shaded) 
model from 2D imagery. 
The concept of PMCAD II is illustrated in 
figure 2. Essentially, the system 
marries analytical photogrammetry with 
microCAD (microComputer-Aided- -Design). 
Two software bridges, a pre- processor 
suite and a post-processor suite, work in 
cooperation with the Direct Linear 
Transformation (DLT) of Karara and Abdel- 
Aziz (1974) to build up the solid model 
piecewise from 2D images obtained from 
enlarged non-metric photographs. 
Control points for the DLT solution 
To solve for the DLT parameters, the 
photo and object space coordinates of at 
least six well distributed homologous 
points must be known. PMCAD solves the 
problem by placing four calibrated range 
poles arranged 90° apart on the 
circumference of a prescribed ring within 
which the debris are to be mapped, see 
figure 3. The co-ordinates of the four 
points on which the calibrated poles rest 
are surveyed on an arbitrary coordinate 
System and only after the accident debris 
has been cleared. This will ensure that 
traffic flow will not be held up 
unnecessarily. 
The described arrangement of calibrated 
range poles will give rise to an "array" 
(more than the required number) of six 
control points. Every identifiable point 
on the range pole can serve as one 
control point. The plan coordinates of 
each point on the range point (X,Y) is 
the same. Only the height dimension (2) 
changes. And since the pole is 
calibrated, each point's (X,Y,Z) is easy 
identifiable. 
THE PMCAD SYSTEM 
  
| Non-Metric Camera ] 
  
  
  
  
  
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Object Wire Solid ; s 
Co-ordinates Frame Models Animation 
(X. Y.Z) Diagram 
  
Fic. 2. The concept of the PMCAD Il system.