Page 117 - Digital Analysis of Remotely Sensed Imagery

88 Cha pte r T h ree

3.2 Storage Media
Over the years tremendous advances have been made in data storage
media. In the mid-1980s ½- and ¼-inch magnetic tapes were commonly
used for data distribution and even as media of permanent storage.
At 1600 bytes/inch, these media do not have a large capacity, and data
access is quite slow. Besides, they are highly instable, with a maximum
shelf-life expectancy of about 10 years under normal storage
conditions. In the early to mid-1990s, these bulky tapes were gradually
replaced by more compact magnetic tapes such as 8 mm, Exabyte,
and DAT tapes. These media had a much larger storage capacity (e.g.,
up to 1Gb) than ½- or ¼-inch tapes, but had one limitation in common:
they required a specific tape drive to read the data. These tape drives
are increasingly difficult to find these days as their use has been
gradually phased out. Replacing these tapes are much more advanced
and reliable media, such as compact disk (CD), digital versatile disk
(DVD), and memory sticks. These storage media are not only more
powerful with a larger capacity than magnetic tapes, but also have
improved reliability and flexibility.

3.2.1 CDs
At present there are three major types of CDs in use: CD read-only
memory (CD-ROM), CD recordable (CD-R), and CD rewritable (CD-
RW). CD-ROM is a write-once-read-many storage medium that is
compact, indelible, and highly reliable. These disks have a life
expectancy in excess of 30 years in a normal storage environment,
much longer than the 10 years for ½-inch tapes. CD-ROM is an ideal
media for storing digital image processing software and raw satellite
data permanently. One outstanding advantage of CD-ROM is that
data stored in it cannot be erased accidentally as they are protected.
CD-ROM is the ideal choice for permanent storage, not only because
of its reliability but also because of its huge storage capacity. As a type
of optical disk, a CD-ROM is capable of storing data up to 1 Gb, even
though the most common size is 670 Mb. At this capacity one CD-
ROM can hold 15 SPOT (Le Systeme Pour l’Observation de la Terre)
multiple linear array scenes. Compared with other storage media, CD-
ROM has the advantages of being inexpensive, compact, portable, and
efficient in access. Since it is a random-access medium, data can be
accessed very efficiently at a speed that is generally much higher than
magnetic tapes’. Most of all, CD-ROM is universally compatible as all
desktop computers are equipped with a CD drive. Since all CD-ROMs
conform to a standard size and format, they can be read by all CD
drives, an advantage that cannot be enjoyed by other magnetic tapes.
Recently, two types of CD have been invented, CD-R and CD-RW.
CD-R disks are blank compact disks that allow data to be recorded to
them. With a CD-write drive and proper software, remote sensing
data and processed results of image analysis can be saved to a CD-R,