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. . ..~ prints were Purchased for nearly all irrigated areas of the basin. Where NHAP was unavailable, 1:i24,000 scale U.S. Forest Service color photography or Bureau of Land Management eIR photography was purchased. NHAP prints ranged in acquisition date from 1982 to 1984. Experienced photointerpreters delineated every individual irrigated field, or potentially irrigated field, to create a detailed irrigated lands framework. Photointerpreters established the difference between riparian vegetation and irrigated pasture through vegetation textures and signatures, the presence of ditches, native growth patterns, microclimatology, and soil types. Polygon delineation was made manually on in4ividual mylar overlays of the prints. Most photointerpretation was aChieved monosc~picallY, because identification of irrigated fields on CIR prints is a straightforward' task. Stereo viewing was employed for small areas where special discrimination problems were found. .... ') el w Transfer Transfer is necessary to put photointerpreted field delineations in the context of an earth coordinate system for digitization. All available u.s. Geological Survey orthophoto quadr~ngle maps were purchased to support transfer of the field boundaries to stable base' mylar overlays for standard U.S. Geological Survey 7.5-minute quadrangles, theimap base selected for this project. orthophoto quadrangles ensured planimetric accu~acy by keeping interpretations in agreement with field boundaries, roads, hydrograpby, and section lines appearing on the orthophoto quadrangles. Where orthophoto quadrangles were not available, NHAP stereo pairs were photointerpreted and delineations tra~sferred to the map base in a single step using a stereo zoom transfer scope. All pho~ointerpreted information was checked for polygon and label accuracy and for matching 'with adjacent quadrangles. Digitization FOllowing transfer of photointerpretation data to mylar overlays, the field delineations were manually digitized using GES (Geographic Entry System) software (ESL, Inc., 1987p). Digitization was performed on a Calcomp 9000 digitizing tablet linked to a Tektronix 4014 display screen. Software and peripherals were run by th~ HP 3000 Series III computer. The digitizing procedure had a resolution of 0.0085 inches or 17 feet on the ground, a resolution appropriate to the photointerpretation limits for 1:24,000 scale photography. This resolution was set as a result of the GES software options ~lected, not hardware limitations. The project area was divided into two areas, or geoblocks, to facilitate the digitizing process. The north-south dividing line between the two geoblocks was located in approximately the middle of the basin with few adjacent irrigated lands. The two areas were digitized concurrently with exact matching between geoblocks to prevent slivers or overlapping. Overlays were set up in each geoblock for each information theme to be digitized. The overlays were: control points, basin (hydrologic unit) boundaries, counties (Delta, Montrose, Hinsdale, Saguache, and Gunnison), sections (numbers 1 through 36), and irrigated land polygons. The basin and county overlays were digitized at 1:24,000 scale after transfer of boundary lines from 1:500,000- and 1:l00,OOO-scale maps. Basin boundaries were refined to be consistent with topographic contours appearing on the 1:24,000-scale maps. Section and Irrigation overlays were digitized on a quad-by-quad basis. Common lines between overlays and geoblock boundaries were digitally copied to eliminate duplicate lines and ensure consistency between themes. Upon completion of.digitization, a 1:24,000 black-ink plot on mylar was made for each quad. Every plot was checked on a light table against the original transfer map base from which it was digitized. _L_