Figure 2. Distribution of the Areas of calibration in the 5419
scene
In the following we will describe the main characteristics of the
two scenes, the applied method, and the results.
2. SCENE DESCRIPTION
2.1 Configuration of the stereo pair
The geometry of the stereo pair was reconstructed by using the
information contained in the .STE file accompanying the
images. In this file the convergence geometry of the
stereoscopic images is described by three angles: convergence
angle, asymmetry angle, and bisector elevation angle (BIE).
These measure the geometric relationships between two rays
that intersect in a point on the ground which is common to the
two images, and for this reason are called homologous rays.
Two homologous rays define a plane, called convergence plane.
The convergence angle is the angle two homologous rays form
on the convergence plane; the asymmetry angle is the angle
between the projection of the vertical of the point on the
convergence plane, and the bisector of the convergence angle;
lastly, the BIE is the angle between the bisector and its
projection on the horizontal plane (Figure 3). The most
important among the three angles is the convergence angle,
whose value should vary between 30° and 60°. It is better to
have an asymmetrical acquisition, because this allows to
recognize the features on the ground with better accuracy, but
the asymmetry angle must remain below 20°. The BIE angle
determines the amount of parallax which will appear in the
vertical direction after alignment; for this reason it should be
between 60° and 90°(ISD).
Figure 3. Stereo geometry angles
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
On June 17 2011, the WorldView2 level 1B stereo pair was
acquired over the study area. The area covered by the images
contains roughly 100 Km? and is comprised between 9° 3’
30,23° and 9? 17' 57,23'* in longitude, 39? 17' 57,23" and 39°
08’ 11" in latitude. The characteristics of the two images, both
acquires with the sensor in "forward" position, are summarized
in Table 1. For the sake of simplicity, the images have been
renamed according to the acquisition time: 5318 for the first and
5419 for the second.
Parameters
Acquisition date
Local time
View Zenith
View Azimuth
Pixel size
le
Table 1: Characteristic of the WorldView2 stereo pairs images
The three angles defining the geometry of the stereo pair are
reported in the following table:
Angles
Convergence
(beginning — ending)
Asymmetry
(beginning — ending)
BIE
(beginning — ending)
= UE
Table 2: Angles defining the stereo pair
The geometric configuration of the acquisition is slightly out of
the limits described above, both for the convergence and BIE
angles, whereas the asymmetry angle is acceptable. The stereo
pair also has a 93.3% overlap.
The .EPH file, which is also part of the image metadata,
contains the geocentric coordinates of the points of the orbit and
the corresponding accuracies, sampled at constant intervals of
0.2 seconds. The Figure 4 shows the ground projections of the
satellites’ paths as reconstructed from this file.
This configuration has produced two images of different
qualities: while the 5419 image can be considered
radiometrically suitable for the purposes of our work, the same
cannot be said for the 5318 image. In fact, the latter was
acquired in the worst conditions for bathymetric usage, and can
almost be considered as a worst-case scenario.
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