Page 191 - Digital Analysis of Remotely Sensed Imagery

156 Cha pte r F i v e

5.3 Fundamentals of Image Rectification
5.3.1 Common Terms
Image geometric projection has been described using various terms.
The first term, image geometric rectification, refers to the process during
which geometric distortions inherent in the input image (e.g., distor-
tions caused by the sensor, the platform, and the scene itself) are
removed from the output image. Through geometric modification
the output image is made to have the desired projection, a uniform
scale, and a proper orientation. Image geometric rectification empha-
sizes the elimination of distortions. In practice, this term is not with-
out flaws as the behavior of random distortions is not exactly known.
Therefore, these distortions cannot be realistically eradicated. For this
reason the second term georeferencing is preferred by some image ana-
lysts. It simply means reprojection or transformation of an image
from a local coordinate system, in which coordinates are expressed in
rows and columns, into a global one in easting and northing. During
image transformation, systematic distortions are eliminated while
random ones are suppressed to an acceptable level. Another term
commonly associated with image rectification is called coregistration,
the process of registering the coordinate system of one image to that
of another, even though both images could have a local coordinate
system. Image coregistration differs from geometric rectification and
georeferencing in that only the image coordinate system is altered
while no distortion is dealt with if both input images contain geomet-
ric distortions. If the master image is geometrically reliable, the geo-
metric distortions in the slave image are removed or suppressed
through coregistration. However, any geometric uncertainty in the
source image also propagates into the coregistered image. Image
coregistration is usually undertaken for multitemporal images of the
same geographic area when no ground control exists. It commonly
finds applications in change detection (see Chap. 13). Wherever
ground control is available, image coregistration should be avoided
to prevent the geometric inaccuracy in the input images from propa-
gating to another image.

5.3.2 Image Geometric Transformation
Two-dimensional remotely sensed imagery, either spaceborne or air-
borne, is imaged at a height as much as hundreds of kilometers above
the Earth, much higher than the topographic relief on the Earth’s sur-
face. This 3D surface can be approximated as a flat one safely in appli-
cations in which geometric positioning is not a critical concern, such
as in thematic resources mapping. Therefore, only their horizontal
position measured in easting (E) and northing (N) in a ground coor-
dinate system needs consideration in image rectification. The pixel