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I I 150'1 I Reproducibility oj'LlDAR data . ,. .~. I Table 3 summarizes the reproducibility results for each oflhe five study sites within study area RM 59 as two different measures of the correspondence between pre- and post-spike-flow elevations for bare- ground and vegetated surfaces. The ;;econd column in Table 3 represents the elevation offset between Ihe two L1DAR dala sets. The elevation 91Tsels for all of the bare-ground and vegetated-ground units are relatively small (overall average of 0.1 J m for bare ground and 0.12 m for vegelated surfaces; Table J) and may be eliminated by "grounding" each L1DAR survey data set using stable ground elevation control poinls. The third column in Table 3 shows the precision of the RAMS L1DAR data, which averages 0.23 m for bare ground and 0.63 m for vegetaled terrain. The larger disparities belween pre- and post-spike- flow elevations within the vegetated lerrain relative to those within bare-ground lerrain are reflected in the lower precision oflhe L1DAR data in the vegetated terrain (Table 3). The low precisions for vegetated terrain demonstrate the random nature of L1DAR's penetralion in the canopies. The high proportion of elevation differences in Ihe 1-2 m range in vegetated terrain will make detection of local elevation changes on vegetated sand bars al a 0.25-m level nearly impossible. I Conclusions The vertical accuracy and precision of the L1DAR elevalion data were evaluated in several river reaches within the Grand Canyon 10 detennine if this remote-sensing technique can provide reliable data for change detection of various downstream resources. The results of these evaluations are summarized below: I. Both L1DAR contractors claimed an absolute vertical accuracy of 15 em. Comparison against two different ground elevation surveys at various sites throughout the river corridor showed both L1DAR datasets failed to meet this specification in most cases. Thoughtful scrutiny needs to be given to any claims of accuracy by L1DAR operators. 2. The precision of the L1DAR elevation data is 0.23 m on bare ground and 0.63 m on vegetated ground suggesting that sediment volume estimates using L1DAR data within vegetation may produce high uncertainties in temporal analyses. 3. Average L1DAR elevations are consistently higher than ground elevations at all study sites, but the elevation offset is different at each river reach. At some river reaches. the L1DAR elevation data parallels the ground surface suggesting an adjustment could be made 10 "ground" the data. However, the variable nature of the adjustment along the river corridor would require extensive ground control throughout the river corridor. This apparent characteristic of L1DAR data makes its application almost as limiting as photogrammetry, which also needs ground control panels. 4. The type of ground substrate. e.g. sand vs. boulder. seems to have linle effect upon the accuracy of the LI DA R data, 5. The current state of L1DA R technology does not provide adequate vertical precision and accuracy to replace the current method of conventional total station topographic surveying for monitoring volume change of sediment resources affected by operations of Glen Canyon Dam. 7