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I I 1506 I ,. Results I SlakeoUl Surveys The average errors in the LI DAR elevation data are reported in Table I as two values: (I) the average elevation offset between the L1DAR point elevations and the surveyed ground elevations and (2) the average elevation error (accuracy) of the L1DAR data at a particular study area (determined by averaging absolute elevation difference within a study site). The stakeout spot survey data for all bare ground surfaces except rock ledges indicate that (I) the L1DAR elevations are generally higher than the ground survey elevations, (2) the RAMS elevation data are consistently more accurate than the ALMS elevation data, (3) the accuracy of the RAMS elevation data ranges from 0.12-0.23 m, except at RM 46 and RM 59 where accuracies are 0.77 m and 0.53 m. respectively, and (4) the accuracy of the ALMS elevation data ranges from 0.14-0.60 m. except at RM 45 where accuracy is 0.79 m. Both the RAMS and ALMS systems claim an absolute vertical accuracy of 15 em. The stakeout survey showed that the RAMS met or exceeded the contract specification of 15 cm at only two sites, while the ALMS met this specification at only one site. The cumulative elevation errors for all study areas for bare sand surfaces, boulder and cobble surfaces, and for rock ledges revealed some counterintuitive results. One would expect that the uneven boulder/cobble topographic surface would create greater vertical errors than the unifonn. flat sand beaches. However. our findings show that the elevation errors for bare sand and boulder-cobble surfaces have similar ranges and similar average error values (0.35 m for bare sand and 0.31 m for boulder-cobble surfaces). The rock ledges at study area RM 59 have a higher elevation error (0.67 m) than the other bare ground surfaces, which is anributed in part to the higher elevation errors found at study area RM 59 for all types of surfaces (0.53 m for bare sand and boulder-cobble surfaces; Table I) and in part to uncertainties in the exact horizontal location of the L1DAR pulse returns on the ledges with overhangs. Sandbar mapping Surveys Comparisons between sandbar mapping ground survey data and RAMS L1DAR data for the five long-tenn monitoring areas were made by creating x-y scatter plots and frequency distributions of elevation differences. An example is shown in Figure 2 for Site RM 2.8. The x-y scatter plots show that the RAMS LI DAR point elevations are generally parallel to the ground survey elevations. except at study areas RM 59 and RM 62.7, and that on average the RAMS L1DAR elevations are higher than the ground survey elevations (0.09-0.28 m; Table 2). The average elevation errors for the five study areas range from 0.11 m to 0.28 m (Table 2), with only one site meeting the contract specification of 15 cm. Study area RM 59 has the sparsest sandbar mapping ground survey data and the highest elevation error (0.28 m), however. this elevation error is much less than that found by the stakeout spot surveys conducted in the study (0.53 m; Table I), The ALMS elevation data for bare ground surfaces were similarly evaluated. The point distribution in its x-y scatter plots show a more scattered distribution than that shown by the RAMS data. In addition, unlike the RAMS data. the ALMS data is not generally parallel to the ground survey data and therefore could not be corrected with a simple offset adjustment. For all study sites except one (RM 30 is offset by -0.06m; Table 2). the ALMS elevations are higher than the ground survey data (0.06-0.75m; Table 2). The average elevation errors for the five study areas range from 0.33m to 0.84 m (Table 2), with no sites '\leeting the remtract specification of 15 cm. The ALMS elevation errors are significantly larger than the RAMS errors, which correspond to the findings of the stakeout survey. 5