Page 123 - Digital Analysis of Remotely Sensed Imagery

94 Cha pte r T h ree

(a) (b)
FIGURE 3.6 Comparison of a 734-by-454 image that has been compressed with
JPEG (quality: 60, standard deviation: 2). (a) Original image, (b) image that has been
compressed with JPEG. See also color insert.

and the acceptable loss to the quality of the compressed image. A
compression ratio of around 60 percent usually results in the optimum
balance between the quality of the compressed image and reduction
in its size (e.g., the compression will not cause too much loss of quality
with a reasonable reduction in file size) (Shannon, 1997). As for the
quality of the compressed image, there is hardly any noticeable
degradation to the naked eye. As shown in Fig. 3.6, the image saved
in the JPEG format has a size of only 49.8Kb instead of 1.14Mb in the
ERDAS IMG format. A compression ratio of 96:1 is achieved in this
case. With the current compression standard such as the coding
systems mentioned above, the JPEG format does not allow the original
quality of an image to be fully recoverable from the compressed file
because it is intentionally designed so (Fig. 3.6). For this reason, JPEG
should be avoided in storing raw remote sensing data and all products
derived from them.

3.3.4 TIFF and GeoTIFF
Developed by Aldus Corporation (1988), TIFF has become the industry
standard for image interchange. It supersedes all existing graphics or
image file formats by incorporating enough flexibility to eliminate the
need of or justification for proprietary image formats. However,
proprietary information can still be stored in a TIFF image without
violating the intent of the format. TIFF is characterized by three
distinctive features of being extendable, portable, and revisable. New
image types can be added without invalidating older types. Besides,
the addition of new informational fields to the format will not affect the
ability of older applications to read the images. This format, independent
of the platform and operating system, can be used as an internal one
for image editing and swapping.
This all-purpose format has a rich and variable structure that is
more complex than many of the proprietary image formats it
supersedes. A unique tag is used to identify individual fields that can
be present or absent. All TIFF images are made up of three components: