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system maintained by the vendor. 
At the other extream, the vendor would supply the user with 
an executable code only. It offers some obvious benefits. 
The vendors’ copyrights are better protected. It is easy to 
deliver the revisions. The tailoring aspect requires, 
however, special attention. 
A certain degree of flexibility is achieveable by using a 
wide set of options for selecting the desired actions. 
There is however a danger that the user will be overloaded 
with the options. User manuals are too thick to be studied. 
Carefully selected default values for parameters, on-line 
help facilities, etc. can reduce the risk. A more suitable 
approach is to include a system generation phase in which 
all the essential parameters for the site are defined. 
The flexibility in the approach above is limited: all the 
features have to be built in and a user can only select a 
subset of them. Further flexibility is difficult to achieve 
without letting the user to do some reprogramming or 
program extensions. ; 
An intermediate approach where a user is supplied with a 
library ‘of compiled, relocateable modules is in many 
aspects optimal. I offers some of the benefits of the total 
system but still lets the user to build his own system. 
However, special attention is required in the system design 
phase: rather than taking as a goal to build Aa single, 
complete system, the goal has to be on building reusable 
system components. The reusable components can be used not 
only by the vendor to build the total system but also by 
the user as building blocks for designing a system for his 
specific needs. In many aspects the system design has to be 
a bottom-up process where functionally increasingly complex 
objects are built. 
8. DISCUSSION 
Making recommendations of the tools and techniques to be 
used in software development for photogrammetry is not an 
easy task. Software engineering as a professional field 
seems to be an area that relies very much on heuristics, 
rules of thumb and solutions that just have emerged, 
without any goal-directed development work. Some of the 
adapted techniques become standards, very often de facto 
standards without approval by any standardization 
organisation. Unix operating system and C language are 
examples of those. 
It is somehow interesting to note that Unix systems are 
especially popular within university and research 
environments while "others" mostly rely on the operating 
systems supplied by the computer manufacturers. 
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