International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
3. INITIAL VALUE OF THE BLOCK ADJUSTMENT
To start the block adjustment, at first one must calculate the
initial value of the orientation parameters of every image and
the terrain coordinates of the tie points in the block. From the
equation (1), it need at least five control points for each high-
resolution satellite image to calculate the eight affine
parameters and one slantwise angle. However, it is difficult to
acquire many control points in a block. Therefore, one must
find some way to reduce the necessary control points for the
orientation of the images.
Firstly, when calculating the initial value of the orientation
parameters of the image, one can suppose that the slantwise
angle is zero approximately. Therefore, only four control points
is necessary for each image to calculate the initial value of the
orientation parameters.
Secondly, the control points can be set in the overlap zone of
the adjacent models in order to reduce the necessary control
points. In Figure 1, the control point 3 and 4 are the mutual
control points of the adjacent models. Then only six control
points are necessary for the initial orientation of the two models.
O
O O 5
3
O O
2 4 o
6
Figure 1: Distribution of control points
Thirdly, a block usually is composed of many images in several
orbits. The test block shown in Figure 2 is composed of 15
images in three orbits. In each orbit, one can combined the
images into two images to form a bigger model. For example,
as shown in Figure 2, there are fifteen images in the block. In
order to calculate the initial values:
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711772 710/720
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Figure 2: Test Block with 15 IKONOS images in
three orbits
In the first orbit, the image 681 and 680 can be combined into
one big image and the image 691 and 690 can be combined into
another big image.
In the second orbit, 712, 711 and 710 can be combined into one
big image and 722, 721 and 720 can be combined into another
image.
In the third orbit, the image group (732-730) and image group
(740, 741) can be combined into two big images respectively.
Then for each orbit, only four control points are necessary for
the orientation of the combined bigger images. If we set four
control points in the overlap zone of the adjacent orbits, then
only eight control points are necessary for the block.
After the orientation of these six combined images, the initial
value of the terrain coordinates of all tie points can be
calculated by equation (3).
From the above discussion, the initial orientation parameters of
the HRSI in the block can be calculated in the below step:
|. For each orbit, combine the images in the orbit into a
bigger stereo model and use four control points to
calculate the orientation parameters (not include the
slantwise angles) of the two bigger combined images.
2 Use the orientation parameters of the bigger combined
images in each orbit to calculate the initial value of the
terrain coordinates of the tie points in the block.
3. Use the initial value of the terrain coordinates of all tie
points to calculate the orientation parameters of each
actual image in the block.
4. TEST RESULTS OF THE BLOCK ADJUSTMENT
4.1 TEST RESULTS OF IKONOS BLOCK
The first test block has 15 IKONOS images in three orbits. The
photograph area is about 1995 km? (35*57 km). The block has
86 GCPs in all. The location of the GCPs within the test block
is shown in Figure 3. All the GCPs are in the UTM coordinate
systems. The terrain height variation is about 600 meter in the
block.
The block adjustment based on the new strict geometric model
has been tested for all the 86 GCPs, 20 GCPs and 8 GCPs three
cases respectively. The test results are listed in the Table 1. The
results of the block adjustment based on RPC model (the forth
cases in the Table 1) are also listed for the purpose of
comparison.
Table 1: The block adjustment results of the IKONOS imager
Check points (m)
0, Control points (m)
(Pixel) N Mx My Mz N Mx My
0.32 86 0.37 0.44 0.74
0.31 20 0.21 0.33 0.62 66 0.59 0.58
0.32 8 0.19 0.20 0.56 78 0.56 0.72
0.33 5 0.39 0.32 0.40 81 0.87 0.85
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