applied to this kind of work.
The assembling of image parts into a photo mosaic causes errors
of about + 7 mm in the adjusting and pasting process.
Summarizing these single influences there results an accuracy of
about + 15 mm on the average, also in reference to the object.
This evaluation is not independent of suppositions such as scale,
camera angular field or the object itself. But the value was con-
firmed by measuring check points in some final orthophotomaps of
different scales.
The examination of the accuracy obtained by stereoplotting of the
same objects amounted to + 15 mm as well. So stereorestitution
has no advantage against orthophotography in accuracy.
Drawing on overlay sheets can be done within an accuracy of + 0.15
mm independent of scale, which means about + 10 mm in the object,
assuming a tracing scale of 1:70 in accordance with the figures
given above. Enlargement of the orthophoto and tracing in a
larger scale can reduce the drawing errors in relation to the
object.
The accuracy of about + 15 mm which can be obtained by orthophoto-
map-production, stereoplotting or direct tracing normally meets
the requirements of the architect or the art historian.
h. Comparison of times
The time which has to be spent for the production of architectural
orthophotos is dependent on the size of the model, the profiling
distance and the scanning speed. Because these parameters vary
from object to object, it is not possible to give general fiaures.
From experience, which refers to a mean model area of 270 cm? in
the ZEISS Planimat, to a scan width of 2 mm and an on-line profiling
speed of 2.5 mm/sec the following times result:
Preparations 1.5 hours
Scanning (projection) 1.5 hours
Total time
In off-line operation with the ZEISS Planimat-GZ | system the
scanning time amounted to 1.5 hours on an average, which also
represents a mean scribing speed of 2.5 mm/sec.
Projection time in off-line operation depends on electrical inter-
polation facilities and can take several more hours than profile
scribing.
