In the bare earth DEMs where there are few survey points (i.e. bare-earth surfaces in heavy timber, where there are few ground reflections), TINing 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.
Top surface DEMs where project areas meet may have different vegetation heights. Survey projects are flown during winter leaf-off season; therefore adjacent project areas may be 1 or more years apart. Since vegetation is in a state of constant change it is expected to have differing vegetation heights 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.
User 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.Acknowledgement of the originating agencies would be appreciated in products derived from these data.
Internal Consistency: Data are split into swaths (separate flightlines), a separate surface is constructed for each flightline, and where surfaces overlap one is subtracted from another. Where both surfaces are planar, this produces a robust measure of the repeatability, or internal consistency, of the survey. The average error calculated by this means, robustly determined from a very large sample, should be a lower bound on the true error of the survey as it doesn't include errors deriving from a number of sources including: 1) inaccurately located base station(s), 2) long-period GPS error, 3) errors in classification of points as ground and not-ground (post-processing), 4) some errors related to interpolation from scattered points to a continous surface (surface generation).
Conformance with independent ground control points: Bare-earth surface models are compared to independently-surveyed ground control points (GCPs) where such GCPs are available. The purpose of the ground control evaluation is to assess that the bare earth DEMs meet the vertical accuracy specification in the PSLC contract with TerraPoint:
"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)."
During this step, the bare earth DEMs were compared with existing survey benchmarks. The differences between the LiDAR bare earth DEMs and the survey points are calculated and the final results are first summarized in a graph that illustrates how the dataset behaves as whole. The graph illustrates how close the DEM elevation values were to the ground control points. The individual results were aggregated and used in the RMSE calculations. The results of the RMSE calculations are the measure that makes the data acceptable for this particular specification in the contract.
The PSLC has 4 different products available:
1. Bare earth DEM - these are in ArcInfo interchange format (.e00). These files are a representation of the ground surface. All vegetation and man-made structures have been removed. These files are about 35 MB compressed and about 110 Mb uncompressed.
2. Top surface DEM - these are in ArcInfo interchange format (.e00). These files are a representation of the top surface when the area was flown. You can see vegetation, buildings, bridges, etc. These files are about 35 MB compressed and about 110 Mb uncompressed.
3. Bare earth ASCII data - these files are plain text files with X,Y,Z values. The points in this file are all the returns classified as a ground return. The bare earth DEMs are derived from these ASCII files. These files are about 35 MB compressed and about 110 Mb uncompressed.
4. All-returns ASCII data - these files are plain text files with X,Y,Z values and also additional values such as GPS time, return number, etc. These files are very large, about 2 GB per USGS quarter quad.
The All-returns ASCII and bare earth ASCII files are available upon request to the Puget Sound LiDAR Consortium. This data is too large to put online, but it is still in the public domain and therefore interested users may obtain it free of charge. Depending on the amount of data requested, the user would receive a CD-ROM or a DVD-ROM. Other arrangements are also possible and will be evaluated on an individual basis.