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