RGB -> CMY
100 4
80 7 = = mp
=
& 604 —..—G
= 40 B
8 i
20 +
0 4
1 51 101 151 201 251
RGB (0-255)
Fig 2. Look Up Table, RGB -> CMY
The green forest colours have been created as composites of
Yellow and Cyan, using percentages described in the LUT. The
agricultural areas have been more complicated, due to the
perception of the eye in yellow colours, reducing the amount of
information in those areas. This problem is solved by
calculating a black contrast image and recalculating the values
of the three other colours. Each image pixel is calculated as
follows.
K = min (C,M,Y) "contrast image"
CzC-K
MzM-K
Y=Y-K
Recalculation of the colours (CMY) reduces the darkening
influence of the black colour, which is favorable since it gives
an image with higher contrast, without any darker prints. In the
agricultural areas the yellow colour in combination with the
black contrast image is used.
By a "powerful" contrast stretching, it is possible to use the
B/W-image in urban areas, especially to show the road network.
2.6.4 Printing Originals and Printing. Five printing originals
are produced, CMYK plus, in conformity with traditional
methods, one extra black for text information. In the current
projects, the black original also includes contour lines and
symbols, which allows a simpler production process.
Different raster angles are used on the originals to avoid moiré
effects, causing undesirable, patterns in the printed map. During
this production the angles of each colour is calibrated to
minimize this problem.
The exact fitting between the originals during the printing
process is especially important in the 4-colour technique used in
the current projects. Inexact fitting results in shadow effects and
unhomogeneous colours. Tiny lines, including two or more
colours are particularly sensitive to such negative effects.
2.7 Quality Control
In projects of a magnitude comparable to the Baltic Base Map
projects it is necessary to include quality check points in the
production process. This is done in three steps, described
briefly as follows.
2.7.1 Image Control. This quality check has been performed by
SSC before the images and digital data was sent to the Baltic
states. During the precision correction of the satellite
information, the accuracy of the adjustment is described as an
RMS error and its residuals. The values and directions of the
residuals are checked by the operator before the image
resampling is performed. The satellite information was checked
before deliverance in accordance to expansion and shrinkage
errors, image radiometry and annotation information.
2.7.2 Data Base Control. For each map sheet delivered from
the customers in the Baltic States to the final production of
printing originals in Kiruna, the elements included in the digital
information are checked in accordance with a verification plot.
The check has been done in an iterative way, including still
another control of the revised data before delivery. For obvious
reasons this part of the production check has to be performed by
the interpretators in the Baltic States having local knowledge of
the areas in question.
2.7.3 Printing Control. A verification plot was produced using
the printing originals as input. A check and acceptance of this
proof print by the responsible personnel completes the quality
control of the Base Map printing originals.
2.8 System Design
The configuration of hardware and software packages was
discussed in the initial part of the projects with responsible
personnel from all three countries. The final solutions include
already existing equipment combined with new installations,
configured to optimize the production facilities within the Base
Map Projects.
Workstations and Personal Computers are connected to a local
data net, which allows efficient data transfer. A plotter for
verifications and a tape recorder as backup facility are also
connected to the net. The tape recorder back up was also used
as delivery media in the initial part of the projects, but the fast
development and installation of the Internet allowed more
efficient ways of distributing data between the countries. Vector
data has later been transferred by net to Sweden.
As the data delivery from SSC is made on CD-ROM 5, a CD-
reader is included in the local net. An advantage of using CD's
for storage of image data is the possibility of using the CD-
reader as an external disc, which minimizes data storage
problems on the internal WS-disc.
3. RESULTS and DISCUSSION
The production procedures established in the three projects
include GPS measurements as ground truth data, satellite image
registrations, interpretation, digitizing, data base storage and
digital map production, representing the most up-to-date
technology of mapping.
Orthophoto accuracy. The accuracy of the GPS measurements
in the projects was an RMS of less than 5 metres, which is
sufficient using the satellite orbit adjustment model developed
at SSC Satellitbild. It is concluded that, in comparison with
traditional methods using control points digitized from
topographic maps, including RMS errors of approximately 20
metres, GPS measurements greatly increase the accuracy of the
satellite images positions.
Digital Terrain Models are used to reduce the influence of
terrain variations and non-vertical viewing from the satellite
registration. The maximum magnitude of this influence was
calculated to 53 metres. By using a simple DTM, with a height
accuracy of 25 metres, this error was reduced to a level of 5
metres.
A total RMS of less then 10 metres, GPS + DTM,
corresponding to 0.2 mm on a scale of 1:50,000, satisfies the
required delivery quality of satellite products used for the
production of data bases and maps in similar scales.
Methodology. As the geometry of the satellite information is
more accurate than a cartographic map product, the location of
the elements in these projects are related to the satellite imagery
position. An investigation, performed in cooperation by
Swedish Space Corporation and the National Land Board of
Sweden, has verified that images registered by the SPOT
satellites are accurate enough for interpretation and updating of
topographic maps on a scale of 1:50,000 (Engberg A,
Malmstróm B, 1991).
Satellite images covering 60 * 60 km’ imply advantages
compared to traditional remote sensing using aerial photos due
to the size of the homogeneous area registered, which in
combination with information from different sources during the
production, optimizes the interpretation capability.
A flexible defined methodology allows the most efficient usage
of external sources during the production. Due to the quality of
the non-satellite data it was early decided to use mostly national
information as a complement to remote sensing data registered
by the SPOT satellites. Editing experiences show, in general,
458
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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