ES IN THE
e solutions
id low cost
le for small
rdware and
advantages
stortions
an efficient
iented and
quality and
es sur des
celles tres
utilisables
autant que
irtographie
istruments
giciel basé
production
alité et de
histicated
tal Stereo
ng SPOT
esented a
he DCCS
innovative
ation, car-
ried out with an almost completely automatic measuring
procedure.
More recently (ISPRS, Washington 1992) the Intergraph
Co. produced a refined Image Station equipped with a
high resolution graphic screen, active polarising filter and
a high performance RISC computer. Many other ad-
vanced projects are now in progress (Leica, Zeiss, ...)
and will surely be presented in Vienna in 1996.
These all concern sophisticated and costly systems,
based on ad hoc designed and built hardware.
Some digital instruments have appeared over the same
period, first at scientific Conferences and then on the
market, using standard PC hardware of a very low cost,
offering poor performances, that are suitable for thematic
mapping, architectural photogrammetry and teaching.
One of the best known is the DVP (Digital Video Plotter),
developed at the Lavall University of Quebec, Canada
and sold by Leica.
Another category of instruments can be conceived: a
professional stereo-plotter that fills, in terms of perform-
ance and cost, an intermediate position between the two
opposite solutions that are already on the market. This
instrument should allow one to obtain performances of a
satisfactory level for the usual cartographic applications,
that can be comparable to that of an analytical plotter.
Therefore by using limited cost standard hardware, it
should be easy to use and should offer many of the ad-
vantages of automation that are of the digital photo-
grammetric approach.
In order to obtain this ambitious objective, it is necessary
to carefully analyse the characteristics of the digital im-
ages that are to be treated and, as a consequence, to
define the acquisition and processing procedures, the
accuracy that one considers necessary and sufficient
during the different steps of the photogrammetric proc-
ess and the configuration of the suitable hardware.
2. LOW COST DIGITAL PHOTOGRAMMETRIC
SYSTEM (LC-DPS)
A low cost digital photogrammetric system (LC-DPS)
should be assembled using mainly standard PC hard-
ware, that is easily available on the market.
The components are:
e a scanning unit for the primary data acquisition from
conventional 9" x 9" aerial photographs;
e a restitution unit for orientation, computer assisted
restitution and automatic DEM production. This re-
quires:
one (or more) CPU based on a 486 or Pentium
processor, large RAM (at least 16 Mb) and con-
venient peripherals for storing and managing im-
ages
one (or more) high resolution screens (at least
SVGA 15" 1024 x 768 dots).
2.1 Image acquisition
In principle a digital plotter is able to manage any type of
49
digital image, directly acquired from satellites or CCD
digital sensors, or obtained from conventional
black/white or colour photos by scanning. Let us exam-
ine this last approach, that will still remain the most
common for several years to come.
The scanner market offers very greatly different solu-
tions. These range from sophisticated scan systems
such as the PhotoScan PS1 (Zeiss-Intergraph) which
cost over 200,000 US$ to a cheap A4 DTP scanner,
which may cost less than 1,000 US$. With the former,
one can obtain perfect digital images from the geometric
point of view (+ 1 um) with a density of up to 4,000 dpi
(pixel size of 7.5 um), with the latter, the density is of
300 dpi (pixel of 85 um) where the geometry of the ac-
quired image must however still be verified (errors of
many 1/10's of a millimetre). One, in practice, finds the
same enormous difference on the scanner market in
term of cost and performances that exists between an
aerial photogrammetric camera and a poor 24 mm x 36
mm amateur camera.
Consistent with the general philosophy of a LC-DPS one
foresees that the standard acquisition scanner should
have medium-low level technical characteristics and
price: numerous tests have been carried out with the
UMAX PS 2400X, an A4 scanner with an optic resolution
of 600 x 1200 dpi (up to 2400 x 2400 by interpolation)
equipped with an illuminator for scanning slides. The
choice of the resolution is tied to the occupation of the
memory problem: the number of Mbytes (i.e. Mpixels)
corresponding to the digitised images for photogrammet-
ric use with different densities, is shown in Table 1.
Pesze| Mbytesfor | ovetep | Mbytesfor | 24bit
dpi | ^cy | bwor&bt | ara | b/wor8bi | odor
[um] | oobr(9'x9) | 6'x9' | oobr(2 x2)
300 85 7.3 4.9 4 1:1
600 | 42 29.2 19.4 1.4 4.3
1200 | 21 116.6 77.8 5.8 17.3
2400 | 10 466.6 310.0 23.0 69.1
Table 1 - Digital images of different densities. The recom-
mended values for a LC-DPS are in bold type.
As a first hypothesis, considering the performances of a
486 or Pentium PC in terms of speed and of available
mass storage, let's limit the size of each digital image to
about 20 Mbytes: the standard scanning density should
therefore be 600 x 600 real dpi (not interpolated) corre-
sponding to a pixel dimension of about 40 um. For the
small format images used in close-range photogramme-
try, the density can also reach 1200 or 2400 dpi, using
the interpolation firmware of the scanner during the ac-
quisition stage.
As a conclusion, the standard performances of a LC-
DPS should be foreseen for (see Table 1):
e 230 mm x 230 mm (9" x 9") aerial images, that are
limited to the 150 mm x 230 mm (6" x 9") overlapping
area:
- black/white or 256 colours (8 bits), 600 dpi
e 60 mm x 60 mm images (such as ROLLEI, about 2" x
2") for terrestrial applications:
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
