Page 232 - Digital Analysis of Remotely Sensed Imagery

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image rectification in which the spatial variation in terrain is taken
into account through a DEM. DEM is particularly favored for geo-
metric rectification when GCPs are not easily obtainable, such as
when it is prohibitively expensive to collect ground control in inac-
cessible areas or when it is difficult to identify them on imagery
(e.g., radar imagery) as a result of topographic shadow and radar
layover.
By comparison, direct georeferencing makes use of orbital
parameters of the platform instead of relying on GCPs in rectifying
images. It is the only method available in areas where the selection
of quality GCPs is hampered by the absence of distinct landmarks
(e.g., the coastal area). Even without the need for ground control,
direct georeferencing enables the computation of 3D positions of
pixels that appear in the FOV of the sensing system. Elimination of the
necessity to establish ground control saves a large amount of labor
and time, and minimizes the cost of georeferencing images. Besides,
it is also possible to georeference images in real time. This capability
is particularly useful in emergency situations in which ground
control is almost impossible to obtain instantaneously, such as fire
fighting, oil spills, or leaking pipelines. Image direct georeferencing
is especially important in mobile mapping where the scene keeps
changing constantly. Nevertheless, image direct georeferencing is
limited by its complexity in converting GPS/INS’s orientation
parameters to the parameters of the sensor. It is also computationally
intensive. This problem is less severe with the advent of powerful
and faster computers.
At present, image direct georeferencing is associated only with
aerial mapping in which a block of airborne photographs are geore-
ferenced simultaneously. Through incorporation of the exterior
orientation parameters of overlapping photographs in the aerotrian-
gulation adjustment, a much more accurate rectification is achieved
for many pairs of stereoscopic images quickly and efficiently than
rectification of individual images. Requiring the focal length of the
camera, image direct georeferencing is suitable for frame photo-
graphs taken with an analog or digital camera. Direct georeferencing
is applicable to very high resolution spaceborne imagery if it is
obtained by scanning the CCD plane. With such frame-based images,
it is possible to achieve excellent results. However, accuracy will be
much lower if the satellite images have a poly-central perspective
projection, such as those obtained via pushbroom scanning. Besides,
it brings out fewer advantages in comparison with stereoscopic
aerial photographs since satellite imagery contains minimal overlap.
Each image has to be georeferenced individually rather than in a
block of tens or even hundreds of images. This disadvantage explains
why no direct georeferencing systems have been developed for
satellite images yet.