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1989-03-24
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GEOPHYSICS OF NORTH AMERICA - AVHRR IMAGE DATA avhrr.doc
Eight files of Advanced Very High Resolution Radiometer (AVHRR) data are
provided in image format as follows:
JUN86.CH1: Channel 1 for June 1986
JUN86.CH2: Channel 2 for June 1986
JUN86.CH5: Channel 5 for June 1986
JUN86.NVI: Normalized Vegetation Index for June 1986
NOV86.CH1: Channel 1 for Nov. 1986
NOV86.CH2: Channel 2 for Nov. 1986
NOV86.CH5: Channel 5 for Nov. 1986
NOV86.NVI: Normalized Vegetation Index for Nov. 1986
Each file is a one week composite of the Global Coverage (GAC). The two
time periods chosen for inclusion were selected by George Stephens
(NOAA/NESDIS/Office of Satellite Data Production and Distribution, In
teractive Processing Branch, (E/SP22), Washington DC 20233) for best
cloud-free coverage. The days used in these composites are listed below:
June, 1986: Julian days 162-168
Nov., 1986: Julian days 316-322
The Normalized Vegetation Index (NVI) is computed as: NVI = (ch2 - Ch1)
/ (ch2 + ch1). For a detailed description of the NVI, refer to the
"Global Vegetation Index Users' Guide" available from:
NOAA/NCDC/SDSD/ECC6
Satellite Data Services Division
World Weather Building, Room 100
Washington, DC 20233
Tel: (301) 763-8111
Note: the values in the NVI and channel 5 are numerically reversed in
the SDSD format).
All files are stored as 2-byte binary image files with values ranging
from 0 to 255, increasing numerically with brightness/emittance
("positive" images). Each grid has 1080 columns by 451 rows. Each row
exactly fills the space between longitude 170-deg. E and 10-deg. W. The
first row is oriented with the lower-left corner of the first pixel at
longitude 170-deg. E, and latitude 75-deg. N. The last row therefore is
oriented with the lower-left corner of the first pixel at longitude 170-
deg. E and latitude 0-deg. N. The right hand edge of the last pixel of
each row is at longitude 10-deg. W.
The following format information can be used for display:
image title : (all 8 AVHRR image files)
data type : integer
file type : binary
rows : 451
columns : 1080
minimum : 0
maximum : 255
cell width x : .16666660000
cell width y : .16666660000
Processing of AVHRR data:
Source files: The SDSD "Plate Carree" tapes were used as source data in
latitude/longitude projection on a world grid of 2500 columns by 904
rows. The Plate Carree grid began at Longitude 180-deg. W and Latitude
75-deg. N, and ends at Longitude 180-deg. E and Latitude 55-deg. S.
Sequencing was from West to East, and by latitude row from North to
South.
Re-sampling to 10-min grid: The "Plate Carree" files were re-sampled
using a simple algorithm which aggregated the original pixels into their
corresponding 10-min by 10-min cells within the North American window
(170-deg. E to 10-deg. W, and 75-deg. N to 0-deg. N). The brightness
values for Channels 1,2 and 5 were aggregated by averaging. The Normal
ized Vegetation Index values were aggregated by choosing the maximum
value of those which mapped directly into each new grid cell. Because
both input and output grids were the same projection, and because the
output grid was only slightly larger than the input, most output values
represent a single input value, however there can be up to four with a
lesser frequency throughout the grid. No input point can be represented
in more than one output value, however.