Two simple rules are also prescribed for 
the photographer. Rule A : frame the 
picture using the zoom lens so that the 
picture captures the calibrated poles in 
three positions: left hand edge, middle, 
and right hand edge. Rule B : the base- 
distance ration should be about 1/5. An 
example of a picture which complies with 
the two rules is shown in figure 4 
N 
res 
SES IN 
xut 
     
| Traffic 
Accident 
Site 
Fic. 3. Suggested arrangements of "minimum" control point configuration. 
  
  
  
Fig. 4. A photograph of a simulated accident site taken with a non- 
metric 35- to 70-mm zoom camera, showing "preferred" range pole 
arrangement. 
Pre- and Post-processing 
After the pictures have been taken, the 
first step is to extract, analyse and 
format image coordinates (x,y) on-line 
from the non-metric photographs for the 
DLT solution. A low-cost commercially 
available digitizer A3-sized (420mm by 
300 mm) digitizer was used for PMCAD II. 
Four postcard-sized pictures of the 
accident scene can be arranged for 
simultaneous digitizing on the A3-sized 
digitizer, see figure 5. The extracted 
(x,y) photo coordinates are now sent into 
the DLT suite for the solution of the 
object space coordinates (X,Y,Z). 
After the DLT suite has been run, the 
output file of the digitized object space 
coordinates become available. This X,Y,Z 
file now forms the basis for re-creating 
the three dimensional computer model. In 
the PMCAD II solution, the 3D computer 
   
    
Fic. 5. Enlarged positives arranged on the digitizer for on-line data (x,y) 
acquisition. 
model is re-created inside a microCAD 
system. This approach of marrying 
microCAD with analytical photogrammetry 
has the important advantage of 
eliminating the tedium of writing graphic 
entities - lines, text, symbols, points, 
splines, 3D faces, 3D line, 3D splines - 
which together make up the 3D model. 
The post-processor suite is PMCAD's 
visualization tool. It is here that the 
component entities of the solid model are 
put together. The post-processor suite 
guides the user into building up the 
solid model using simple and umambiguous 
instructions. The resultant accident 
scene is either a wireframe object 
(figure 6) or a shaded rendering of the 
same (figure 7). 
Conclusion 
The availability of low-cost, small 
format, non-metric close range 
photogrammetry coupled with computer 
visualisation techniques has convinced 
many of the exciting prospects of using 
this technique for traffic accident 
mapping. 
Inspite of the legal impediments to 
accepting traffic accident plans from 
photogrammetry, the Singapore Traffic 
Police is exploring its possible use. The 
project while still in its very embryonic 
stage is both exciting and challenging. 
  
Fic. 6. Wire-framed diagram generated by PMCAD II of the simulated 
accident scene.