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index composites.
The raw data are packaged as continuous
orbital segments of the afternoon AVHRR
acquisitions. The orbital segments are
produced by joining several consecutive
observations, obtained along an orbit by the
various ground stations, into a single
continuous data segment for that orbit. The
size of the orbital segments vary with the
amount of land data covered by the orbit.
Orbits beginning over Australia and
continuing over Indonesia and Asia are
nearly pole-to-pole. The data volume of a
pole-to-pole segment is nearly 250
megabytes. The data volume of the
shortest segments, such as one over the
Alaska peninsula, is approximately 50
megabytes. There are 14 orbits per day
with an average total daily volume of 2
gigabytes. The raw data are distributed in
NOAA Level-1b format (Kidwell, 1991).
The 10-day vegetation index composites
are multiband, georegistered, global data
sets. The 10 bands of the composite
product are comprised of: AVHRR channels
1-5, NDVI, satellite zenith, solar zenith,
relative azimuth, and date of observation.
The five AVHRR channel bands are stored
as 16-bit. Each of which has a data volume
of nearly 1.4 gigabytes. The other five
bands are stored as byte data and have a
volume of nearly 695 megabytes each. The
total data volume of a single 10-band
composite is approximately 10.5 gigabytes.
Fifty-four 10-day composites will have been
produced by the end of 1996, and the
combined archive of orbital segment and
10-day composite data will exceed 4.0
terabytes.
Compression technique for global 10-
day composites
Data compression is an important
91
management tool for image data, especially
lossless compression (Nelson, 1991). Data
compression reduces the size of files to be
transferred on the network, and allows for
more data to be placed on physical media.
In addition, it reduces the storage capacity
required to maintain the data online or
near-line at the distribution site and the
user's site. However, decompression is
usually necessary before the data can be
analyzed using traditional image processing
and analysis systems. The decompression
step can be time consuming and require
that the processing or analysis system store
and manipulate the full data set.
The data volume of a multiband, 10-day
global composite, at 1-km resolution, in the
interrupted Goodes homolosine map
projection (Steinwand, 1994) is 10.5
gigabytes. Although most users may not
need or use all ten bands of a 10-day
composite, the practical use of the NDVI
data as a time series would require
approximately 25 gigabytes for a one-year
time period.
A data compression technique has been
developed specifically for use in the
storage and distribution of the global 10-
day composite data (Kess et al., 1994).
The technique functions on a single band at
atime. Each band is divided into a matrix
of 128 by 128 pixel blocks. Blocks that are
entirely a single value such as ocean, or
the interrupted area created by the map
projection, are compressed with run length
encoding. The land data blocks are
Huffman encoded and ordered so that each
resolution level can be progressively
decompressed. The geographic
coordinates for each block and all of the
information necessary to decompress the
data in that block to different resolutions is
stored in a header file.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996