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Show A transparent tracing- cloth base map of the Colorado River Basin was laid over the appropriate Regional Aeronautical Charts ( Department of Commerce), which show topographic relief at 1,000- foot intervals. Provisional life zone boundaries, based on the foregoing tabulated data, were drawn on the tracing cloth, using the contours shown on the aeronautical charts as general guide lines. The topographic accuracy of these charts, which are on a scale of 16 miles to the inch, is for the most part quite high even for the more remote, unsettled regions. Contact prints of the completed tracing- cloth map, which measured 34 by 57 inches, were then taken into the field and compared, area for area, with conditions on the ground. Nearly all of the field checking was done in a two- place light plane at altitudes varying between five hundred and three thousand feet above ground level. Planes of this type, which can cruise up to 6 hours without landing and give approximately 16 miles per gallon of fuel are receiving increasing use in many types of regional surveys and certainly will have an important and permanent place in such work. Corrections were made as carefully as possible on the field copy of the life zone map while in the air, together with any pertinent notations. Occasionally the changes also were indicated on a set of Sectional Aeronautical Charts having a scale of 8 miles to the inch. Approximately 10,500 linear miles were flown on this reconnaissance project, which pretty thoroughly covered the basin. ( Plate 2, in pocket.) Ground studies are an indispensable supplement to air surveys and should follow them for purposes of verification, but the air surveys should come first because they provide at the outset a visual and mental grasp of the entire region which could be attained only by months or years of ground work. This is particularly true of large unfamiliar regions having a rugged and complicated topography which can be glimpsed only intermittently and at long range by ground travel, but it holds good in any type of country. A disadvantage of air surveys is that some landscape features, such as potential development sites, appear deceptively small and are in danger of receiving inadequate consideration. With practice, however, supplemented by a checkup on the ground, one learns to make the proper correction for scale. Moreover, the required correction becomes less as one makes the surveys from lower altitudes. Other disadvantages of air observation, such as the impossibility of making stationary observations are relative ( light air craft usually being easy to maneuver in small circles), and with practice can be greatly reduced. However, the viewpoint of persons on the ground with respect to scenery, and the feeling for the type of recreational use to be made of the area by persons confined to the ground can best be appreciated while one is on the ground. This probably represents the most important advantage of ground studies over air surveys. The air surveys showed that for many areas the provisional life zone map was surprisingly correct even in minor details, which demonstrates the accuracy of the aeronautical charts and the published ecological studies already mentioned. In a few localities actual conditions on the ground differed widely from assumed conditions on which the map had been based. This was particularly true of the Upper Sonoran Zone in the Little Colorado River Basin, which on the first draft of the map was taken from the life zone maps of Merriam ( 1890) and Swarth ( 1914) instead of from the altitude table ( pp. 220 and 221), with which the maps were in conflict. These early life zone maps had been drawn up before the life zone concept had been fully developed and before detailed studies had been made of this particular basin. Apparently, these writers based the lower limit of this zone on the lower limit of the pinon- juniper belt in the basin, but subsequent studies have shown that this is not the true lower boundary of the zone in this particular area. Lack of moisture is the operating factor at the lower boundary of the pinon- juniper belt in the Little Colorado River Basin, and on the basis of temperature conditions the Upper Sonoran Zone actually extends to a much lower level in this area, as demonstrated by the presence of other Upper Sonoran life zone " indicators" of more elastic moisture requirement. Some errors were found to have resulted through having taken data from some of the more recent vegetation type maps, but these were few. They arose, no doubt, from the physical impossibility encountered by the type mappers of covering all 222 |
