Page 186 - Digital Analysis of Remotely Sensed Imagery

Image Geometric Rectification 151

planned one is covered in a yawed image. Thus, its effect is very
similar to that of (X, Y).
Velocity
In order to obtain imagery of a high geometric fidelity, the plat-
form must be moving at a constant velocity during data acquisi-
tion. Any inconsistency in its velocity will lead to image distortion
along the flight direction. If the velocity is faster than the norm,
then the image is stretched, otherwise it is compressed. This gener-
alization also applies to across-track scanning. Such an inconsis-
tency in platform velocity is random and its impact on the acquired
imagery cannot be eliminated.

5.1.4 Nature of Distortions
All distortions mentioned above fall into two broad categories in
terms of their nature: systematic or random. Systematic errors behave
in a predictable manner. Usually, they can be precisely described
mathematically. In other words, they can be completely eliminated
through image rectification. By comparison, random errors are non-
systematic and unpredictable. Their haphazard behavior in the imag-
ery means that these errors cannot be completely removed, though it
is possible to suppress them to an acceptable level through image
rectification.
Both Earth rotation and curvature cause systematic distortions
that can be completely eradicated. So can the distortion caused by
inconsistency in scanning mirror velocity. By comparison, most of the
errors related to the orientation and position of the sensor, and the
velocity of the platform are random. They exert a residual effect on
geometrically rectified images.

5.2 Projection and Coordinate Systems
Projection refers to the systematic manner in which the approximately
sphere-shaped surface of the Earth is consistently transformed onto
2D media according to predetermined mathematical equations. Many
kinds of projection are in use, each having its own unique features
and being applicable to different parts of the world. All these projec-
tions share one commonality in that distinctive global patterns of par-
allels and meridians are first transferred onto an easily flattenable
surface such as a cylinder or a cone. These surfaces are then flattened
to form the planar coordinate system. During the transfer all features
on the curved surface may be distorted in shape, area, distance, or
direction. In one projection it is impossible to preserve all of these
properties. At most one or two properties are preserved along certain
lines. The achievement of fidelity in these properties is termed,
respectively, conformality, equivalence, equidistance, and true direction.
Conformality refers to the preservation of shape of an area during