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UNITED STATES DEPARTMENT OF THE INTERIOR NATIONAL PARK SERVICE WASHINGTON ADDRESS ONLY THE DIRECTOR, NATIONAL PARK SERVICE February 10, 1938 Memorandum for Mr. Wirth: In searching for material to assist in the completion of the geologic report on the Big Bend National Park (Proposed) we found two aerial photographs of portions of the Rio Grande valley. One of these, taken over the Mesa de Anguila, proved exceedingly useful in supplementing field observations. By means of it, Mr. Rothrock was able to map the rock outcrops with greater accuracy than had been done by previous reconnaissance field parties. This fact is illustrated by the attached drawings, which show in Figure 1 a portion of the map made on a topographic base, and in Figure 2, the map of the same area constructed from the aerial photograph. The advantage derived from using the aerial map is self-evident. The justification for photographing the entire area proposed for the national park is found in the following considerations: 1. The photographs will form the basic data from which maps of various kinds and adaptable to a great many uses can be prepared at any time. The types of most useful to park work are: a. Ownership maps showing the exact location of boundary fences, roads, and buildings. This information is most important in Texas where conflicting surveys have caused almost endless litigation. b. Vegetative type maps. c. Land use maps. d. Topographic maps. e. Geologic type maps (see Figure 2). f. Erosion control studies. g. Water-supply surveys. h. Road and trail surveys.
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