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Image Geometric Rectification 199

georeferenced already. In this case the AOI method is the more
appropriate choice.
In the image processing flowchart (Fig. 1.2), image subsetting is
presented ahead of image rectification. Apparently, this sequence
applies to the raw image. In case of georeferenced images, image sub-
setting takes place after image rectification. Possibly, image subset-
ting may take place twice in reality. The first time a much larger area
than the study area is delimited. This processing is necessary as the
rectified image may not be oriented properly, introducing void into
the final image after rotation. A second subsetting is needed to make
the final image have a regular shape or one conforming to the outline
of the study area defined by the AOI.

5.9.2 Image Mosaicking
Opposite to image subsetting, image mosaicking is the process of
stitching multiple images or digital photographs of the same area
together to form a larger image. It is activated when the study area
is covered by multiple images. Image mosaicking is quite common
with aerial photographs and hyperspatial resolution satellite
images because they cover a limited ground area per scene. A size-
able study area requires multiple images to cover. They must be
mosaicked to form one image for the convenience of subsequent
analyses.
Image mosaics fall into three major categories: index, uncon-
trolled, and controlled. Index mosaic is created out of analog photo-
graphs that have been properly aligned. Generated for reference
purposes, index mosaic does not involve digital processing to produce,
so it is not discussed further. By comparison, uncontrolled mosaics
are generated from raw images without geometric rectification, but
controlled mosaics must be produced from georeferenced images.
Mosaicking of nongeoreferenced images relies on the spatial conti-
nuity of the same ground features in multiple overlapping images.
Component images are stitched together through visually examin-
ing these features to ensure their uninterrupted continuity across the
border of multiple images. It is usually undertaken in computer sys-
tems that are unable to preserve the geometric properties of images,
such as Adobe Photoshop. In this environment a pair of images are
mosaicked through manually shifting, rotating, and scaling one of
them, usually the slave one. Once a mosaic is created, it is stitched
with the third image in a similar manner. The only difference is that
geometric change can be done only to the new image but not to the
mosaic. This process continues until all images have been inserted
into the final mosaic. The accuracy of the mosaicked image is subject
to the visual acuity of the analyst and the amount of distortions
inherent in the component images. Geometric inaccuracy in any of
the input images other than rotation cannot be eliminated during