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In about 1989, after national coverage has been attained, the provisional maps will be converted to standard maps.

Of the approximately 9,900 7.5-minute quadrangles still to be completed in the conterminous United States, about 8,200 will be produced as provisional edition maps. There are about 1,600 7.5-minute quadrangles mapped at 1:24,000 scale but published at 1:62,500 scale to be authorized for updating and series conversion; about 1,000 of these will be published as provisional maps.

The procedures and techniques for the preparation of provisional maps are expected to result in a 15- to 20-percent decrease in overall costs compared to standard mapping. The majority of the cost savings occur in the final scribing phase (90-percent decrease) and the editing phase (70-percent decrease).

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Orthophotoquad Program

Orthophotoquads are one-color distortionfree photographic products produced in standard quadrangle format with selected place names added. Basic marginal information such as the Universal Transverse Mercator grid, scale, quadrangle name, and survey date are shown. Standard orthophotoquads do not show contour lines, but they may be added for special purposes. Orthophotoquads are used primarily as interim map products for unmapped areas or as supplements to line maps. Produced mainly at 1:24,000 scale in the conterminous United States and at 1:63,360 scale in Alaska, they have proven valuable to engineers, surveyors, foresters, and scientists.

Orthophoto quads are available for about 44 percent of the 48 States.

The National HighAltitude Photography Program

The aerial photograph was one of the first remote sensing products available for studies of the surface of the Earth. Since early in this century, millions of aerial photographs have been taken to provide geographical and other information for a wide range of uses such as military intelligence, mapping, environmental studies, inventory and development of resources, planning, and education. Improvements in

aircraft and remote sensing technology have made it economically possible to acquire black-and-white and infrared-color photographs from 40,000 feet above the terrain being photographed.

In 1978, the Geological Survey began coordinating a National High-Altitude Photography Program specifically designed to maximize the annual coverage contracted with Federal funds and thus to better satisfy Federal and State needs.

By 1982, contracted coverage totaled about 1,767,000 square miles, or approximately 59 percent of the conterminous United States (fig. 4). Complete coverage is anticipated by 1986, when a 6-year cyclic rephotography program will begin.

Photographs from the program consist of 9- by 9-inch bIack-and-white panchromatic and color-infrared film exposed at 40,000 feet above mean ground level. The blackand-white film is exposed in a precision aerial camera that produces photographs at the scale of 1:80,000 (1 inch to about 1.25 miles); each bIack-and-white exposure covers nearly 130 square miles of terrain. The color-infrared film is exposed in a second aerial camera that produces photographs at the scale of 1:58,000 (1 inch equals about nine-tenths of a mile) and covers nearly 68 square miles of terrain.

The need for information about the Earth and its environment will continue to grow as competition for resources accelerates in the decades ahead. The National High-Altitude Photography Program is providing the basic information needed to evaluate and understand the land and its temporal changes.

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Maps have traditionally played a key role in earth science analysis. Because earth scientists and other map users are automating their analysis processes, the map information must also be automated. Therefore, the U.S. Geological Survey is building and maintaining a national digital cartographic data base to make its publishedmap data also available in digital form.

The primary effort is devoted to the building of a data base containing the basic data categories shown on 7.5-minute published topographic quadrangle maps (fig. 5). Work is authorized on the basis of needs expressed by users. The data, available in two forms, are the digital line graphs and digital elevation models. The digital line graphs are graphic data digitized from published maps. The digital elevation models are digitized elevations collected at regularly spaced intervals throughout a quadrangle. Other data

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Figure 4. — Status of National High-Altitude Photography coverage.

being collected include the planimetric features from the 1:2,000,000-scale sectional maps of the National Atlas of the United States of America and selected 1:500,000scale State base maps, elevation data from the 1:250,000-scale map series, land use and land cover data, and geographic names.

Digital elevation data from the 1:250,000scale maps are available for the United States. All 21 sheets of the 1:2,000,000scale National Atlas series have been digitized and are available to users.

In fiscal year 1982, the U.S. Geological Survey added 1,176 digital elevation models to the data base for a total of about 8,000. Each model describes one 7.5-minute quadrangle area.

Currently, there are five categories of digital line graphs: public land net, boundaries, drainage, transportation systems, and other culture. Any one or all of these categories may be digitized from an existing 7.5-minute quadrangle map. Based on user requests through fiscal year 1982, nearly all digital line graph production has been that

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