There are many advantages of this 
technique to the product distribution 
process. The technique results in 
approximately a 10:1 reduction of data, 
reducing the near-line storage 
requirements. It provides for spectral, 
spatial, and geographical  subsetting 
capability to the user. The spectral 
subsetting results from the compression of 
each band separately, spatial subsetting 
comes from the capability to decompress at 
selected resolutions, and geographical 
subsetting is possible because the image is 
stored as separate 128 by 128 blocks. The 
user can specify geographical coordinates 
of any rectangular subset. The 
decompression interface determines which 
array of 128 by 128 pixel blocks is required 
to cover the subset, and only those blocks 
are decompressed. This technique also 
minimizes the amount of data that is 
retrieved from the file server, and thus 
improves access times. The spatial 
subsetting is most useful if the user wants 
to browse the composite at coarser 
resolution before selecting a full resolution 
data set. It is also useful for many global 
studies where coarser resolution data 
would suffice. 
Network Access to the Data 
Network access and distribution of data has 
become very dynamic. New modes of 
access, such as the World Wide Web 
(WWW), provide a creative opportunity for 
sharing data and information. One of the 
ways the EDC DAAC is providing access to 
the global 1-km data set's orbital segments 
and 10-day vegetation index composites is 
through DAAC's WWW home page. As the 
orbital segment and 10-day composite 
products are produced, they are staged to 
the DAAC's Data Distribution System (DDS) 
for network access. The DDS includes a 
92 
digital linear tape (DLT) robotic data 
storage library that is configured to hold 
264 10-gigabyte cartridges or 
approximately 2.6 terabytes of storage. 
Access to these data are provided through 
a global 1-km data set WWW home page. 
The URL is: 
(http://edcwww.cr.usgs.gov/landdaac/1KM 
/1kmhomepage.html). 
Using this WWW access path, users can 
read documentation and general 
information about the global 1-km AVHRR 
data set and the products that are 
available. The WWW interface offers a 
simple HTML form that allows users to 
select and request a 10-day composite or 
an orbital stitch. 
The orbital segment HTML form provides 
documentation on the orbital segment 
production process and an explanation of 
how to identify the approximate geographic 
coverage of a segment. The orbital 
segment data are organized in directories, 
sorted by satellite (NOAA-11, NOAA-14), 
year, month, day, and hour. A user can 
quickly find the appropriate timeframe and 
orbits of interest. Since orbital segments 
are delivered in standard NOAA level-1b 
packed format (three 10-bit values in a 
32-bit word), very little data compression of 
level-1b data can be achieved. 
The global 10-day composite HTML form 
provides for selection of specific 10-day 
periods, bands, resolution and, optionally, 
geographic area. When the user submits 
the form, the DAAC's WWW server runs a 
program that calculates and reports the 
actual size and geographic coordinates of 
the data that will be extracted and a rough 
estimate of the amount of time necessary to 
retrieve the data. The user can then 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996 
chc 
dat 
req 
adc 
ha 
trai 
cor 
red 
19 
har 
for 
mo 
Sul 
The 
de\ 
dat 
are 
the 
spe 
The 
effi 
the 
of | 
dis 
to |