U.S. AVHRR MOSAIC


    The mosaic of the conterminous United States was prepared using 16
images from the Advanced Very High Resolution Radiometer (AVHRR) sensors
on the meteorological satellites NOAA-8 and NOAA-9. The images were
acquired between May 24, 1984 and May 14, 1986. Standard image
processing techniques were used to enhance the detail and the data were
resampled to a resolution of 1000 meters (1 km) per picture element
(pixel) and cast into an Albers Equal Area projection. The satellite
image was then further enhanced by digitally introducing terrain shading
based on USGS digital elevation models. The image size for the U.S.
AVHRR mosaic is 2874 lines by 4632 samples. 

    The satellites launched by the National Oceanic and Atmospheric
Administration (NOAA) in Sun-synchronous polar obits have an orbital
period of approximately 102 minutes. The orbital height is 805 km with
a swatch width of 3000 km at the nadir. The orbital tracks repeat every
8 days, thereby providing more frequent coverage than the Landsat series
of satellites. 

    Two channels of data were used in the false-color infrared mosaic
image. Channel 1 (0.58-0.68 um) was used to control the amount of blue
and green in the composite, and Channel 2 (0.72-1.1 um) was used to
control the amount of red. Channel 1 is in the red portion of the
visible spectrum, which is sensitive to daytime clouds and vegetation.
Channel 2 is in the reflected infrared (near-infrared) part of the
spectrum and is used for shoreline and vegetation discrimination. The
image simulates a false-color infrared composite as it would appear
using these AVHRR channels to display on an 24-bit color monitor
typically found on larger image processing systems. This composite was
processed as a single image file and its color palette containing an
optimum subset of 255 colors. This subset is from the thousands of
colors need to show the original two channel composite as displayed on
the 24-bit monitor.  This optimum subset allows display of color
composite images representing multiple AVHRR sensor channels on PC's
having an 8-bit VGA graphics capability. 

    On false-color infrared mosaics, vegetation appears in various tones
of red instead of green. The "redness" indicates vegetation density and
type and whether growing on dry land or in a swamp (a mixture of reddish
vegetation and dark blue surface water produce dark tones). Grasslands
appear light red, deciduous trees and croplands appear red, and
coniferous forests appear dark red or maroon. Desert areas appear white
and urban areas (pavement and buildings) appear bluish green. Lakes,
rivers, and oceans appear in various shades of blue, dark blue for deep
water and light blue for shallow or turbid water. Exposed bedrock
generally appears as a dark bluish-green or other dark tone. 

    Further information on the image enhancement techniques and the
identification of the specific images used in the mosaic may be obtained
from the U.S. Geological Survey, EROS Data Center, Customer Services,
Sioux Falls, SD 57198. A two-sided 19" x 23" color map of the mosaic
is available for a nominal fee by requesting the "United States
Satellite View" from the U.S.