2014/07/16 Council Agenda Packet

APPENDIX C: LIDAR DISCLAIMER The following excerpt has been extracted from the metadata for the Puget Sound LIDAR Consortium 2000 dataset13: Considerable care has been taken to ensure that these data are as accurate as possible. We believe most of the data is adequate for determination of flood hazards, for geologic mapping, for hydrologic modeling, for determination of slope angles, for modeling of radio -wave transmission, and similar uses with a level of detail appropriate to a horizontal scale of 1:12,000 (1 inch = 1,000 feet) or smaller and vertical accuracy on the order of a foot. Locally, the data is of considerably poorer quality. In the bare earth DEMs [Digital Elevation Model] where there are few survey points (i.e. bare - earth surfaces in heavy timber, where there are few ground reflections), TINing [Triangular Irregular Network] the points produces large triangular facets where the surface has significant curvature. Similar, though finer, textures are evident where vegetation reflections are incompletely filtered. Elevations are likely to be less accurate in these areas. LiDAR data values for water surfaces are not valid elevation values. LiDAR surveying produces few survey points on water. Mirror-like surfaces fail to scatter the laser beam and unless the beam is perpendicular to the surface, no light is reflected back to the detector. Or intense reflections may lead to negative blunders, points that are too low. Interpolation between the nearest on -land points and sparse water points produces large triangular facets that may not accurately reflect the water -surface elevation. Where the water surface is surveyed adequately, adjacent swaths may be flown at different tide stages, producing swath -parallel cliffs. Ideally, LIDAR topography would be clipped to eliminate all open -water areas, but at present this is very labor-intensive. Users should carefully determine the place -to -place accuracy and fitness of these data for your particular purposes. For many purposes a site- and use -specific field survey will be necessary. The accuracy specification in the contract between the Puget Sound LIDAR Consortium and TerraPoint is based on a required Root Mean Square Error (RMSE) 'Bare Earth' vertical accuracy of 30 cm for flat areas in the complete data set. This is the required result if all data points in flat areas were evaluated. Because only a small sample of points is evaluated, the required RMSE for the sample set is adjusted downward per the following equation from the FEMA LIDAR specification (adjusted from the 15 cm RMSE in the FEMA specification to 30 cm to accommodate the dense vegetation cover in the Pacific Northwest). The Seattle and Bremerton LIDAR are part of the PSLC 2000 project. The Tacoma LiDAR is part of the PSLC 2004 project. t3 Full metadata is available on the PSLC website: http://pugetsoundlidar.ess.washington.edullidardata/metadata.html L-27 Appendix L: Climate Change Background