Cartography: The Moon: Rougher than You Think

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Viewed with the naked eye or the world's biggest telescope, the moon looks flat. Because of its great distance, the sharpest irregularities on its surface show only because of the shadows that they cast in slanting sunlight. But the moon is more rugged than Afghanistan; when earthly astronauts land there, they will need the best possible contour maps to guide them through the precipitous mountains that hide just over the lunar horizon. Last week NASA's moon pioneers were beginning to plot their first explorations, using an entirely new set of maps made by the Army Corps of Engineers that shows the jagged surface in astonishing detail.

Better Than Shadows. For Chief Cartographer Albert L. Nowicki of the Army Map Service, the traditional method of measuring lunar mountains by their shadows is not accurate enough. It works well only in the center of the moon's visible disk; off toward the edges it fades into uselessness. So the Army has turned to stereo photo-mapping in order to take advantage of the fact that the moon wobbles slightly but predictably at intervals of 3½ to seven years as it orbits the earth. This means that pictures taken of the moon at different times are like pictures taken of earthly objects from different directions. By careful triangulation and infinitely detailed examination, the differences in the photographs can be used to measure distances and altitudes.

To get the most valuable angle-shots, the Army's mapmen visited observatories in the U.S. and Europe, collected more than 4,000 pictures dating as far back as 1890 and showing the moon at different stages of wobble. By intricate calculation they were able to deduce lunar elevations from slight differences in matched pairs of photographs. Only a few years ago the job would have taken too long to be practical, but the Army's Honeywell computer raced headlong through thousands of bristling equations. Gradually the map of the moon's visible disk, which has just about the same area as North America, filled with measured mountains and crater rims.

Arbitrary Altitude. The moon has no sea level to use as base elevation (its so-called seas are waterless plains), so Nowicki selected Mösting A, an easily identified crater near the center of the disk and gave the bottom of its crater the arbitrary altitude of 7,000 meters (23,000 ft.) to serve as reference for all other elevations. The finished map, which is 4.5 ft. in diameter, includes more than 5,000 surface features, giving elevations in hundreds of meters. One version of the map shows high and low areas in colors: the highest mountains (14,000 meters) are light yellow, the lowest depressions (3,000 meters) dark blue. The chromatic result looks like a scientific contribution to the highly unscientific field of abstract art.