9:00 - 10:00 a.m. local time each day.
Contiguous images are acquired every 18 days only over the
North-American continent. For the rest of the world, selected
strips over portions of the orbit track are imaged. They are
chosen on the basis of cloud-cover forecasts and requirements
of NASA-approved investigators. (NASA, NPD 800 2A, 1973)
Contiguous orbits give side overlap of about 101 in equa
torial regions and about 80% in the polar region. Depending on
the latitude of an area, it is possible that a particular place
could be imaged 2 to 6 times in one cycle. The imaging system
views large areas in a fraction of a second, and each computer-
processed image covers an area of 34,225 sq. km. (185 x 185 km.).
One of the two remote sensors on board the satellite, the
multispectral scanner (MSS), provides data in pictorial form
for many users in the world. The MSS scans horizontally along
the orbital track from west to east and records data in four
spectral wavebands. These are currently referred to as Band 4
(visible green - 500-600 nanometers); Band 5 (visible red -
600-700 nanometers); Band 6 (infrared - 700-800 nanometers);
and Band 7 (infrared - 800-1100 nanometers). The last two
bands are in the reflected near-infrared portion of the electro
magnetic spectrum and thus the images are produced from energy
beyond the limits of visible light.
The data for each band are printed as individual black and
white photographs; therefore, for any single scene, there are
4 such images. In order to achieve colour images, three differ
ent spectral bands of the same area are combined. Green, red
and infrared bands are assigned colours and the result is a
false-colour composite. Depending on the colour assigned to the
infrared bands, the dominant colour of the composite may be red
or cyan. Such prints are readily available from the EROS Data
Center (Sioux Falls, South Dakota, U.S.A.) and from the Canadian
National Air Photo Library (Ottawa, Ontario, Canada). The images
require no sophisticated equipment to yield information of
immediate value to earth scientists. These prints in the 18.5 x
18.5 cm. format at a standard scale of 1:1,000,000 provide a
remarkable sequence of uniformly-illuminated, essentially plani-
metric, vertical views of the Earth’s surface that cover very
large areas (34,225 sq. km.).
Like the Gemini and Apollo pictures, these images are in
valuable because of their synoptic aspects: that is, a single
scene covering a wide variety of terrain, geology and land use
types, is presented under near-optimum viewing conditions that
emphasize the contextual relations of these surface features
(Short, N.M. and Lowman, P.D., Jr., 1973). This property of
