A fourth method, graphical displays by cathode ray tube, is available 
only to users of very advanced installations. 
There is no limit to the variety of drawings which can be produced 
using one or other of these methods and in the case of flat-bed machines the 
engineer's customary style and layout can be reproduced complete with titling 
and all annotations. Centre line plans, route plans, long sections, terrain 
and design cross sections, grade lines, channel lines, mass diagrams and 
perspectives are all produced as a matter of course in several design systems. 
CHOICE OF SYSTEM 
When neither the road engineer nor the air survey contractor has a 
house system ready-made, a choice must be made from the many systems 
now available which were developed by others. The choice is critical and the 
task made difficult by the natural enthusiasm of each promoter for his own 
system. All claims must therefore be examined very carefully lest the user 
find himself the guinea pig. In general, it is better to employ the integrated 
system of one contractor than the specialist services of several, because the 
latter course can give rise to problems of interface and will certainly demand 
much more time spent on liaison. 
Amongst the questions which must be asked are: - 
l. Is the system fully developed, proved and documented? 
2. Does it provide the type of service required (i,e. routine 
computing or consultancy)? 
3. Does it provide the range of services required? 
4. Can it take into account all the design constraints? 
5. What restrictions does it place on the design? 
6. What data is required for the programs? In what form must 
it be presented? How much labour is involved in putting the 
data into this form, and who does it? 
215.