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Title
Proceedings of the Symposium on Progress in Data Processing and Analysis

5.1 Digital Image Reproduction and Digital Orthophotos
For some time already, the printing industry has been using digital procedures
for high-quality reproductions in order to obtain better contrast adaptation and
more refined differentiation of colours. These procedures also have considerable
advantages for image assembly and image mounting and are of great interest in
photogrammetry, especially for the elaboration and reproduction of orthophotos.
For example, the company Belfotop (Belgium) provides reproductions of ortho
photos by means of digital procedures on a routine basis. The Kern DSR15, equip
ped with digital cameras, also includes a program system for the production of
orthophotos. The digitizing of photographic images is a problem which has now
been largely solved. It is possible to use an analytical plotter equipped with digital
cameras for this task. Last September, Zeiss presented a specialized scanner for
this task, which they had developed together with Intergraph. The scanners cur
rently available on the market are not adapted to these tasks, as the resolution is
too low for original photographic pictures. If one wants to ensure that the resolu
tion of aerial photographs will really be used, one has to provide a pixel size of
about 5 microns. This requirement is fairly closely fulfilled by the new Zeiss ins
trument.
Efficient instruments are also available for image reproduction, as they were
developed for the printing industry. However, their economic viability is only
assured for reproduction by offset and are rather expensive for limited editions or
the elaboration of unique documents. In the past, photographs have also often
been reproduced by heliography, which requires a rastering of the image.
Nowadays, about the same quality can be obtained by laser printers or electro
static procedures and it is interesting to note that the Ordnance Survey has
already equipped one of its selling offices with an electrostatic colour printer to
reproduce maps on request.
5.2 Digital Photogrammetric Plotters
Progress in digital image processing suggests that purely digital instruments
could also be built to perform image measuring and mapping as such.
Theoretically, this would require only a powerful image memory and a high-
resolution monitor. Stereoscopic vision can be obtained by subdividing the
monitor into two parts, and by using an optical observation system or by working
with a polarization screen. The floating mark is only a brighter pixel directed by a
cursor. In a similar way, lines and symbols could be represented and super
imposed on the aerial photographs displayed on the monitors. Although the
technique is very simple, it seems difficult to obtain a performance with these
instruments similar to that provided by a classical analytical plotter. As already
explained in the previous chapter, an image quality comparable to a photo
graphic film in a plotting instrument can hardly be obtained on a monitor.
Moreover, smooth panning is difficult to obtain on a digital plotter.
5.3 Image underlaying on a Geographic Information System
An interesting derivation from the sketched digital plotter results from the com
bination of a digital plotter with a geographic information system. The role of
information systems has already been discussed in connection with photogram
metric mapping. Planning tasks are increasingly carried out by CAD-systems