-4- 
During this process certain geometric corrections are applied which make 
the picture equivalent to a perspective picture of a flat stationary Earth. 
The separate black-and-white bands are photographically enlarged to scale 
1:1,000,000 and combined to produce the tonal response of color infrared 
fi lm. 
In its more than 2 years of operation ERTS-1 has returned about 
140,000 scenes, each recorded in 4 spectral bands. Cloud-free coverage 
has been obtained for about 80 percent of the Earth's land mass. Many 
areas have been recorded numerous times in all seasons of the year. 
However in recent months the tape recorder has been progressively de 
teriorating, and pictures can now be obtained only for those areas which 
are within range of the receiving stations in Alaska, Canada, the 
United States, and Brazil. 
Tests performed on the early images received demonstrated that the 
actual resolution is about 250 m for low-contrast images on both the RBV 
and the MSS (3). 
After the failure of the RBV system, cartographers were faced with 
the problem of attempting to make maps with the MSS records. Built into 
the NASA ground data handling system is an element called the Precision 
Processor. In this instrument the original ERTS image is compared with 
a library of preselected ground control point images. By automatic cor 
relation the ERTS image coordinates of the control points are measured. 
The computer then determines the constants of a polynomial describing 
the image distortions. On a second stage the original image is rescanned 
in 25 mm square blocks and reprinted in the correct position. A single 
injage of Lake Tahoe, Calif, acquired by the MSS two days after spacecraft 
launch was processed in this manner and reproduced lithographically. This 
proved to be a rather slow and cumbersome procedure and adversely affected 
the resolution of the final product. In addition it was found that the 
geometric integrity of the original MSS images was far better than had 
been expected. Precision Processing has now been virtually abandoned. 
The next approach was simply to match the images to an existing line 
map. For small map scales this worked surprisingly well. The Soil 
Conservation Service prepared a 1:1,000,000-scale mosaic of the entire 
United States on the Albers equal area projection. The individual images 
were subjected to transformation in a rectifier to obtain the best fit to 
the existing base map (4). No accuracy check was made on this map. 
A similar technique was applied to produce a standard 1:250,000-scale 
quadrangle of the Phoenix, Ariz., sheet on the Universal Transverse 
Mercator (UTM) projection. Parts of 5 ERTS frames were mosaicked to 
produce the photoimage base which met NMAS for planimetric position.' 
Although Arizona is one of the most photogenic areas in the country, the 
resolution was not really adequate for this scale, and similar efforts at 
1:250,000 scale in less photogenic areas were considered unsatisfactory.