National Geographic : 1969 Jan
identify plants in the way it picks out different min erals. He nodded. "Sometimes the infrared curves of wheat, oats, rye, and barley look almost alike. But there is a trick we can play. We can make pho tographs in several different wavelengths of both visible light and infrared simultaneously, using fil ters on a multispectral camera with several lenses, or with several cameras. Then we can superimpose those pictures, and the different crops often show up in different colors." With still other techniques and the aid of a com puter, scientists at Purdue's Laboratory for Agri cultural Remote Sensing have made identifications with 95 percent accuracy (pages 60-61). Taking me into his laboratory, Dr. Colwell showed me an optical combiner that he and his students have devised. It is an arrangement of four projectors, each with a wheel of different filters that can be positioned in front of the lens, and each aimed at the same screen. Dr. Colwell put four images of Silver Lake, part of a test area in the northern Sierra, in the combiner and projected them simultaneously on the screen. As he substituted filters, trying first one, now an other, pictures of striking but unrealistic colors flashed on the screen (preceding pages). In one, groves of hardwoods showed up in strong yellow, conifers in dark brown. In another, granite stood out clearly in comparison to other kinds of rock. In a third, roads and trails were heavily marked, although they had been almost invisible in many other combinations. Almost any feature the researcher might want to study can be made to jump out of the screen with the right combination. Limitless possibilities offer themselves with such use of multispectral photog raphy and projection. Radio Waves Ferret Out Icebergs Infrared, of course, is only one part of the electro magnetic spectrum. Other wavelengths, although not as adaptable or as well explored, may also be useful for remote sensing: MICROWAVE These shorter wavelengths of the radio region (which account for the frying-egg sound one hears between stations while tuning a radio) radiate more from ice than from water. Odd ly enough, therefore, ice shows up hotter than wa ter in microwave receivers. The U. S. Coast Guard ice patrol takes advantage of this fact, and micro wave detection devices have become highly efficient iceberg spotters along the shipping lanes of the North Atlantic.* Microwaves can also map heavy rain-bearing clouds in weather studies, and soil-moisture and water-distribution patterns in hydrological and *See "Tracking Danger With the Ice Patrol," by William S. Ellis, NATIONAL GEOGRAPHIC, June 1968. 64 World w eat her Innocent wisps on film, cloud masses thousands of miles long (right) swirl over the North ern Hemisphere. ESSA-V, a satellite sent aloft by the Environmental Sci ence Services Administration, records the weather with a TV camera some 850 miles up. Here it reads atmospher ic conditions for September 14, 1967: a cold front over the Midwest, bottom center, Hurricane Doria off New Eng land, and Hurricane Beulah in the Caribbean. Digesting the satellite's output, a computer prints out a daily world-wide weather mosaic, a basic source for forecasting. Sure hands of a technician, seen through a magnifying glass, clean the delicate workings of IRIS, an Infra red Interferometer Spectrometer, at Texas Instruments Incorporated in Dallas. Designed for mounting in sat ellite or balloon, it will send back temperature soundings of earth's at mosphere to aid in weather analysis. Because the longer IRIS looks the more it sees, it is designed to stare at a single point for 10 seconds, then jump its eye ahead to the next target area.