International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004
seconds with respect to the ITRF90 (International
Terrestrial Reference Frame 1990) system during the
data take and at least four times before and after
image data acquisition,
e the corrected attitude data with respect to the local
orbital coordinate frame measured by gyros and the
star tracker unit ULS with 8Hz, the data are already
corrected for different effects (Bouillon et al. 2003)
e the look direction table for the 12000 CCD pixel
elements expressed within the sensor coordinate frame
and
e the data used for time synchronization like line
sampling period and scene center time.
According to the ,, SPOT Satellite Geometry Handbook“ (SPOT
IMAGE 2002) Lagrange interpolation of the ephemeris data
and linear interpolation of the attitude data are recommended to
calculate data sets (position, velocity and attitude) for every
scan line. For DEM production the exterior orientation is
transformed to a local topocentric system (LTS) with a
fundamental point located at the center of the image scene. For
orthoimage production the exterior orientation is expressed in
the Earth Centered Earth Fixed (ECEF) WGSS84 Cartesian
frame. The transformed data serve as input for DLR's
processing software.
4. ORTHOIMAGE GENERATION AND ACCURACY
ANALYSIS
To get an impression of the absolute and relative accuracy of
the position and attitude data, and to get an estimation of the
necessity to improve the ancillary data by bundle adjustment or
other methods, orthoimages are derived using an already
derived DEM.
The inputs for the orthoimage production are the interior
orientation (extracted from the meta data file), the six
parameters of the exterior orientation with respect to an ECEF
coordinate frame for each image line (interpolated from the
measured sampling points) and the digital elevation model
(DEM). In the case of Catalonia the reference DEM described
in chapter 3 is used for the orthoimage generation.
The principle of the orthoimage production is based on the
intersection of the actual sensor viewing direction (pointing
vector) with the DEM applying the rigorous collinearity
equation. The orthoimage processor calculates the object space
coordinates of the points within the intermediate local
topocentric system and then transforms them to the desired map
projection of the output image using geodetic datum
transformation parameters (Müller et al. 2002,2003). The DEM
is internally transformed to the same LTS as the exterior
orientation, where an undulation of —18.2 m with respect to the
ED50 geodetic datum is taken into account. Bilinear resampling
to a 10 x 10 m grid has been performed for the final
orthoimage.
After generation of the three orthoimages without any ground
control information, a check of the accuracy using 24 of the
ground control points has been performed. For the quality
assessment the measurements have been carried out in bilinear
enlarged orthoimages to achieve sub-pixel accuracy. Table 1
shows the deviation in x (east) and y (north) direction for the
orthoimages in comparison to the control points.
Table 1: Mean values and standard deviations for the
difference to the orthoimages of 24 ground control points in
meter in UTM EDS0 coordinate system (Catalonia)
x1, y1 - Coordinates in reference orthoimages
x2. y2 — Coordinates in orthoimage from forward looking
x3. y3 — Coordinates in orthoimage from backward looking
x4. y4 — Coordinates in nadir looking image (HMA)
x2—x1 |y2-yl |x3—x! |y3-yl |x4—x1 | y4—
MEAN -9,90 -16,59 -0,36 -11,16 | -24,22 =0,22
3
SIDV. 4,64 8,48 5,72 S d 5,96 3.7
The result shows that even without any ground control, the
absolute georeferencing accuracy of the HRS sensor is in the
order of one to two pixel, less than 20 meter and standard
deviation less than one lpixel. This is expected, since the values
for the absolute pointing accuracy is given by the French
colleagues to about 33 meters with 90% accuracy (Bouillon et
al. 2003, Airault et al. 2003). Only the x-coordinate in the nadir
looking image (x4) shows a slightly higher mean difference
than the stereo channels
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Figure 2: Shifts between the two orthoimages derived
from forward and nadir looking channels of SPOT HRS
and HMA (mean values in a regular grid), Catalonia
test area
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