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5.5 Computer network 
The usual computer network techniques can be used for 
data transfer. Errorfree connections over long distances 
can be established. But the speed e.g. of the common 
ethernet with 10 MBits/s is not high enough for live video. 
Another disadvantage for image transfer is that a com- 
puter is needed on the camera side for converting the 
data formats. There are so called PC-cameras available 
with a complete computer built-in, but they are quite ex- 
pensive. 
In complex configurations with more than one controlling 
computer a network can be used for coordination of op- 
erations and collecting results, even over long distances. 
Itis also possible to link the whole system into an existing 
network so that access to data and functions of the 
measurement system is available in the administration 
area. 
5.6 Wireless connections 
As mentioned before, certain camera positions that are 
very useful for a configuration cannot be reached with 
cables. One example is a type of crane found very often 
in industrial halls, which may be the only possibility to get 
upper views of an object. Because of its motion a wired 
connection to cameras mounted on it cannot be realized. 
A wireless data transmission can solve this problem. 
The following carriers come into consideration: 
e ultrasound 
e infrared light 
e laser 
e radio 
Acoustic data transmission seems to be not usable for 
the desired purposes, because the realizable bandwidth 
is to small for efficient video data transmission. It also can 
be assumed that it is very sensitiv against noise in indus- 
trial environments. 
Data transmission with modulated light is used for exam- 
ple for wireless network links. The main disadvantage is 
that a direct sight connection is needed and that the 
beam adjustment has to be very precise, especially when 
a laser is used. This fact reduces the flexibility of such a 
system. 
A radio wave based system appears to be the optimal 
solution. The positions of transmitter and receiver are 
nearly free and the operating range is mainly a question 
of output power. Several distant stations can be switched 
to send their data alternately, or parallel when using dif- 
ferent channels. 
Video transmission devices are available at acceptable 
prices for security systems or home video. The main 
problem is to receive geometrical correct images, while 
the industrial environment produces disturbing electro- 
magnetical fields and interferences. The Institute for 
Photogrammetry in Hannover has started an investiga- 
tion, which deals with different methods of transmissions 
as well as hard- and software-based error corrections. 
6. MEASUREMENT SYSTEM EVALUATION 
The market of video equipment is steadily growing, and 
the number of available photogrammetric video meas- 
urement systems is increasing. Even for a photogram- 
metrist it is difficult to have an overview over all compo- 
nents and possibilities. As mentioned above, there is no 
formalized method to compare different solutions effi- 
ciently. This problem has been recognized and in Ger- 
many a team consisting of members of institutions and 
industry will work on this subject. Some basic considera- 
tions will be made below. 
6.1 Ready-to-use systems 
Ready-to-use systems have a standard set of hardware 
and software and are adaptable for different objects. The 
quality and accuracy can be proved at already installed 
equipments or in test configurations, if the layout for the 
desired task does not differ too much. 
6.2 Individual configurations 
In the case of individual configurations where new hard- 
ware components have to be used and a very specialized 
software package has to be developed, usually no testing 
and comparing possibilities for the complete layout are 
given. 
6.3 System parameters 
The performance of a measurement system is defined by 
a lot of hard- and software parameters, e.g. point meas- 
urement accuracy, camera resolution, synchronisation 
stability, etc. Very important is to include configurational 
aspects like image scale, ray intersection angles, redun- 
dancy, etc.. 
A catalog of comparable system parameters may have 
the following structure: 
e a grouping of parameters in a hierarchy of im- 
portance 
e a diagram of dependencies between parame- 
ters 
e standardized testing procedures 
e standardized configurations 
e guidelines for configuration layout 
Independency from existing equipment is not completely 
possible because of certain limits defined by hardware. 
Therefore the connections between parameters have to 
be updated periodically. 
6.4 Evaluation methods 
The following methods can be used to evaluate system 
parameters or a whole configuration: 
single parameter determination 
individual test of component groups 
standardized testing object 
testing at real object 
simulation 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996