22 
tion they consider important, but on the other hand they tend to consider 
important only information for which they have sound administrative pro- 
cedures. The quality of the available records is assumed to be high until the 
contrary is proven, something that creates many headaches to data base con- 
structors. It was found, for instance, that the land use information available 
for the Paris data bank was of only mediocre quality (Butzbach and Tizianel 
1972). On the other hand, when aerial photography was used to update the 
real estate tax base of a small town in the USA, the total tax revenue in- 
creased by $400,000 for a survey investment of only $26,000 (Avery 1968). 
Photography, therefore, could be very useful in checking the quality of 
the data to be entered in the data base, to supply missing data, and to ensure 
consistency in spatial definitions and classifications, even where it is not used 
as a direct data source. It is a pity that this is often not realised, so that maps 
are used for this purpose instead of photography (Ackerknecht 1969). 
Checking of the data base with photography might have attractive juridical 
aspects as well. The data of unauthorised construction, for instance, can be 
defined quite objectively, which could be of especial interest in conservation 
areas. 
Not all input for the design process, or for planning models, may be 
available directly from information systems. Data on environmental factors 
that are not parcel-based will often not be included. Remote sensing data can 
then be used. They seem particularly suited to these models, they are suffi- 
ciently accurate, and they are consistent and quick to obtain. An example of 
a highway location study, using remote sensing data and grid processing, is 
the Pembrook-Marshfield study (Wulff 1973). Another is the study of the 
accessibility of public services made by Robertson (1972) with a grid system. 
Robertson uses map data for his study, but his results could have been 
achieved more elegantly if photography had been used. 
Also, photography might supply data on areas outside the boundaries 
of the urban information system, for instance when site evaluation for new 
residential areas is to be carried out (de Bruijn and Dieudonné 1970). 
In conclusion it can be said that in those urban regions where the ad- 
ministration is able to maintain and update a sophisticated data base, aerial 
photography will serve mainly as an additional source of information, espec- 
ially useful in consistency control, for checking, for juridical purposes and