white square at the center of each cross. The squares were
measured repeatedly at a Wild AC1 analytical plotter with an
estimated accuracy of 2 - 3 um.
Figure 1. Grid plates for geometric tests and calibration.
4. TESTS AND CALIBRATION OF DTP SCANNERS
Using the above test patterns the following five scanners were
tested: Agfa Horizon, Agfa Arcus II, UMAX Mirage D-16L,
UMAX PowerLook, Sharp JX-610. The first scanner belongs to
our Institute and was tested over a few years, the remaining ones
were tested at companies or were lent. In all tests a resolution of
600 dpi was used (exception: 400 dpi for Mirage). The same test
patterns, data measurement and analysis, and calibration
procedures were used for all scanners. A difference exists for the
grid plates. The 25.4 cm wide plates could not be put flat on the
A4 scanners (Arcus II and PowerLook) and a part of the plates
could not be imaged. Latter is important for the on-line plate
because one of the border lines was totally missing. This plate
positioning (one side of the plates was lying on the scanner frame
around the scanner glass plate, less than 1 mm higher than the
scanner glass plate) caused imaging displacements which could
not be modelled by an affine transformation (as used in the
interior orientation). For all geometric tests Least Squares
Template Matching (LSTM) was used to measure the grid
crosses. The standard deviation of the matched positions was
0.03 - 0.04 pixels, i.e. 1.3 - 2.6 um, for the 600 dpi and the 400
dpi scans respectively. In the following x-direction is the
direction of the CCD line (horizontal), y-direction is the direction
of the scanning movement (vertical).
4.1. Geometric accuracy without calibration
Table 2 shows the geometric accuracy of the scanners. For all
scanners, except the Arcus, two scans were made. The results
were similar for both scans, however here the worst of the two
results is shown. For this test all grid lines were measured by
LSTM and an affine transformation was computed between these
values and the reference values (as measured at the analytical
plotter). As control points either all points were used, or four
corner or eight points. In the last two cases the remaining points
were serving as check points and their errors are shown in Table
2. The versions with all points as control show the global
geometric accuracy of the scanners. Only for Horizon the
accuracy is worse than 60 um, for the Mirage it is even close to
20 um! The maximum errors are bounded and correspond to ca.
2.5 - 3.5 RMS. The errors are generally larger in x, indicating
large lens distortions. Using only 4 control points the errors of the
check points increase. This is natural because the corner points
have larger errors than points, let's say in the middle of the
scanner stage, and thus the estimated affine parameters have
larger errors. The big systematic errors introduced by the errors
16
of the corner points are also indicated by the large mean errors,
which ideally should be zero. A version with 8 control points (4
corners and 4 points at the middle of the borderlines) was also
tested. The results were better, in some cases significantly.
The above mentioned scanner accuracy may be sufficient for
some applications. Consider for example a scanner with 100
microns geometric error, used to generate hardcopies of digital
orthoimages in scales 1:24,000 and 1:12,000, using 1:40,000
scale input imagery scanned with 25 microns, and an orthoimage
pixel size of 1 m (equal to the footprint of the scan pixel size).
The scanner error translates to a planimetric error of 4 m in the
digital orthoimage, and 0.17 mm and 0.34 mm in the 1:24,000
and 1:12,000 hardcopies. This approximates the measuring
accuracy in topographic maps, and may be acceptable for many
users.
Table 2. Statistical values (in um) of geometric accuracy without
calibration. Errors (residuals) after an affine
transformation.
Control/ RMS Mean Max absolute
Scanner | check
points X y X y X y
4/621 | 146 | 71 | -5 | -26 | 224 151
Horizon | 8/617 | 147| 67 | -4 | -13 | 223 130
625/0 | 92 | 54 | O 0 220 159
4/621 | 106 | 51 | 67 | -39 | 214 117
JX-610 | 8/617 91 | 42 | 45 | -26 | 182 105
625/0 | 56 | 29 1 0 0 182 91
4/621 35-120 | 24 |: 4 73 56
Mirage | 8/617 32742091929" |*.7 67 54
D-16L | 62510 | 18 | 19| 0 | © 56 51
4/521 85 | 81 | S1 | -69 | 199 151
Arcus 11 "8/5171 76% 1*62 1*36 "46 1“ 180 129
525/0 | 63 | 41 | O 0 216 122
4/546 | 101 | 112 | -66 | 103 | 181 177
Power- | 8/542 | 87 | 77 | -45 | 65 158 138
Look | 5500 | 52 | 43 | 0 | 0 | 185 | 114
4.2. The geometric calibration procedure
The calibration consisted of two stages. In the first stage the
effects of the lens distortion were modelled. Radial lens
distortion caused large displacements in x-direction, and the
tangential lens distortion smaller but significant displacements in
y-direction. The off-line plate was scanned, all points were
measured by LSTM and an affine transformation between these
measurements and the reference values using all points as control
points was computed. The residuals of this transformation were
indicating the occurring errors. These errors were transferred
from the pixel to the scanner coordinate system. There an x-
correction regular grid was interpolated based on the residuals.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
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