68 
Table 1. Comparison the traditional optical/mechanical approach with today’s softcopy AT processes 
Process 
Optical/Mechanical 
Softcopy 
Film 
Process & create diapositives 
Process 
Scdn 
Not required 
Required 
Point marking 
Avg.points/model 
Time/model 
Accuracy 
Solution Robustness 
Required 
6 
20-30 min 
Operator skill dependent 
minimal 
Not required 
-60 (user definable) 
l-2min 
Superior and consistent 
Significant redundancy 
The relationship of AT to the aerial survey has also 
become very important as GPS 
Technology is today being widely applied during air 
survey missions. The introduction and success of auto 
AT is just starting to be felt, but the implications are 
enormous. By minimizing this often difficult and 
complex task, the doors to photogrammetry open to a 
vast army of potential users, especially as low cost 
personal computer systems can now be used for stereo 
collection and analysis. 
4. DIGITAL TERRAIN MODELS 
Probably the first successful implementation of 
significant automation was in the DTM generation 
process. DTM’s are a key requirement for orthophotos 
and terrain analysis. When used appropriately auto 
DTM generation is now quite successfully applied. 
Generally additional geomorphological features such 
as break-lines, exclude areas and salient point are 
invaluable to the DTM surface formation process and 
are highly recommended. 
The auto DTM generation process utilizes a features 
based matching in a hierarchical process where 
refinement via least squares adjustment of the derived 
DTM improves as the computation proceeds down 
through the pyramids to the lowest level. See Figure 3. 
Despite high automation, post process interactive 
editing is generally deemed necessary for quality 
assurance. The amount of editing and number of 
geomorphological features required depends on the 
purpose for which the DTM is required. If contours are 
desired as final output then detailed collection of break 
lines and salient points would be required to give 
appropriate structure to the software generate contours. 
For orthophoto generation only, often little additional 
terrain structure is required. 
5. ORTHOPHOTO 
The process of ortho rectification for aerial 
photographs can be summarised as the conversion from 
central perspective to orthographic projection. The 
conversion is undertaken generally as a batch processes 
in which a new output image is generated from the 
input image after corrections for terrain height 
displacement (requirement for DTM) and photo tip and 
tilts (requirement for At) have been applied. 
Once images are rectified the step of mosaicking or 
digitally joining one or more images may be 
undertaken in which several steps may be combined 
such as global tonal balance, seam line definition and 
area extraction. 
Generally it is desirable to minimize the number of 
photographs required to “make-up” an orthophoto and 
by using good flight planning and GPS camera control 
resulting in pinpoint exposures. 
Upon completion of the rectification and mosaic 
processes the orthophoto image can easily be recorded 
(permanently) onto compact disc (CD) for distribution 
and archive 
6. LOOKING AHEAD 
There are many challenges ahead, one often talked 
about is automatic feature recognition which is still 
considered some way away from full and robust 
implementation, but like other steps to automation, 
incremental advances involving. Research into 
automated building extraction is well advanced as with 
automatic line following. The challenge is to bring 
these research activities into the main stream 
commercial systems, which can provide as good if not 
better solutions that current interactive techniques 
provide. 
Another area of high interest is at the airborne 
acquisition stage where the potential of combing 
inertial navigation systems (INS) and GPS could 
ultimately result in fully orientated photos at the 
moment of capture. Once this technology is refined and 
combined with say digital aerial cameras, it may lead 
to the elimination of today’s scanning and 
aerotriangulation steps. 
Small format digital cameras are already being 
deployed for aerial imagery capture in conjunction 
with GPS recording of exposure stations capturing 
either color or color infrared imagery. As the CCD 
array sizes expand we can look forward to even more 
utilization of these systems with metric or semi-metric 
cameras.