This fact can be less important because projection time only occu- 
pies the orthoprojector but not the operator. 
Direct tracing and stereoplotting require much more time than the 
production of photomaps. An estimation would be an increase by 
a factor of 5 to 10, depending upon the quantity of details to 
be mapped. The same increase in time occurs in topographical 
applications of the orthophotomap. 
Providing the same density of detail representation direct tracing 
on overlay sheets takes only one third of the time needed for con- 
ventional stereoplotting. Corresponding results originate from 
cartographical tests of Commission D of the OEEPE (Organisation 
Européenne d' Etudes Photogrammétriques Experimentales). 
So the most economical method of representation is the photomap, 
at least as far as production time is concerned. The advantage 
is diminished by higher instrumentation costs, but remains never- 
theless significant. 
b. Present experiences and results 
Some institutions have carried out differential rectification 
applied to architectural objects. Comprehensive experiences, 
however, exist at the moment only at the Photogrammetric Insti- 
tutes of Cracow, Poland, and Stuttgart, West Germany. A few 
examples of architectural orthophotos are given in the figures 
h to 9. Figure 3 contains the data belonging to these representa- 
tions. 
Importance is attached to the mention of various orthoprojection 
systems and orthophoto producers. The examples show that it is 
possible to obtain architectural orthophotos by quite different 
instrumentation. 
For objects having great discontinuities special arrangements 
such as changing the slit width, the profiling speed or the degree 
of model approximation become perhaps necessary. 
A modification of the instruments by the manufacturers cannot be 
expected for this field of application. Therefore one must manage 
for the present with the available instrumentation. 
6. Applications of architectural orthophotography 
Orthophototechnique can be applied in architectural photogrammetry 
to objects which are curved or extended in depth. The advantage