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scenes. Since this is a spatial average, a safer estimate of the accuracy to characterize all <br />pixels would be about 0.5 pixel. <br />Following geocorrection, TM images were merged to form a mosaic to eliminate seams <br />through the study area. This mosaic image was converted to .tiff format and displayed <br />using false color in ArcMap to assist interpretation for mapping the lateral extent of the <br />riparian phreatophyte vegetation. In false color, healthy vegetation is displayed as red to <br />aid the human eye to perceive minute differences in vegetation vigor. This permitted <br />discretion between vegetation receiving supplemental ground or surface water over that <br />receiving only precipitation. False color images are displayed in Figure 4. <br />Figure 4. <br />False color TM images <br />of the study area. Note <br />the change in coloration <br />along the seam between <br />the two upper images. <br />This difference may be <br />due mainly to automatic <br />selection for color <br />presentation in the <br />software and only partly <br />to the timing of plant <br />development. Plant <br />development, however, <br />is more advanced in the <br />upper right hand image <br />taken 11 days later than <br />the other two. <br />As a subsequent step, the DOQQs and the 2001 irrigated lands shapefile were imported <br />into ArcMap to differentiate phreatophyte vegetation from the surrounding lands. <br />Using DOQQs for reference, a polygon was digitized to encapsulate the phreatophyte <br />vegetation while excluding any irrigated agricultural land. Precise determination of the <br />boundary between irrigated land and phreatophyte vegetation was difficult in several <br />locations based on the TM image and the DOQQs alone. When this occurred, the <br />irrigated lands shapefile produced by RTi and oblique air photography were used to aid <br />the placement of the boundary. The riparian phreatophyte vegetation along all major <br />watercourses found inside the alluvial extent shapefile were mapped into the digitized <br />study area shown in Figure 3. An example of the phreatophyte shape file is shown in <br />Figure 5. <br />9 <br />