2002 Puget Sound LiDAR Consortium (PSLC) Unclassified Topographic LiDAR: Puget Sound Lowlands Washington

TerraPoint surveyed and created this data for the Puget Sound LiDAR Consortium under contract. The area surveyed is approximately 730 square miles and covers the lowlands for Mason County. It also includes the Chehalis River in Grays Harbor County and a section of the Dugeness River in Clallam County. All data was collected during ideal conditions (leaf-off).

The LAS files were derived from all returns ASCII files that contain the X,Y,Z values of all the LiDAR returns collected during the survey mission. The GPS time, scan angle and return numbers of each point were preserved in the conversion from ASCII to LAS. Futher details are available in the Process Steps below. No ancillary data, such as intensity, is available for this data.

Initially this dataset was flown in 2002 with a 50% overlap between flight lines. Due to the overlap the nominal point spacing of the dataset is 1.0 meter.

The LAS files can be used to create DEMs and also to extract topographic data in software that does not support raster data. Other surface features can also be extracted with custom applications. This high accuracy data can be used at scales up to 1:12000 (1 inch = 1,000 feet). LiDAR data has a wide range of uses such as earthquake hazard studies, hydrologic modeling, forestry, coastal engineering, roadway and pipeline engineering, flood plain mapping, wetland studies, geologic studies and a variety of analytical and cartographic projects.

10784

LiDAR points in LAS 1.2 format

none

Any conclusions drawn from the analysis of this information are not the responsibility

of Terrapoint, PSLC, NASA, USGS, NOAA, the Office for Coastal Management or its partners.

This data can be obtained on-line at the following URL:

https://coast.noaa.gov/dataviewer/#/lidar/search/where:ID=2489

;

None

Users should be aware that temporal changes may have occurred since this data set was collected

and some parts of this data may no longer represent actual surface conditions. Users should not use

this data for critical applications without a full awareness of its limitations. These data

depict the heights at the time of the survey and are only accurate for that time.

Not applicable for pure elevation data: every XY error has an associated Z error.

Puget Sound Lidar Consortium evaluates vertical accuracy with two measures: internal consistency and conformance with independent ground control points.

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.; Quantitative Value: 0.30 meters, Test that produced the value:

Root mean square Z error in open, near-horizontal areas, as specified by contract. Our assessment suggests that all data meet this standard. Accuracy may be significantly less in steep areas and under heavy forest canopy.

Elevation data has been collected for all areas inside project boundaries.

Puget Sound Lidar Consortium evaluates logical consistency of high-resolution lidar elevation data with three tests: examination of file names, file formats, and mean and extreme values within each file; internal consistency of measured Z values in areas where survey swaths overlap; and visual inspection of shaded-relief images calculated from bare-earth models. File names, formats, and values: All file naming convention and file formats are check for consistency.

Internal Consistency Analysis: This analysis calculates and displays the internal consistency of tiled multi-swath (many-epoch) LiDAR data. The input for this analysis is the All-return ASCII data, but it only uses the first returns. The data is divided into swaths, or flightlines, and they are compared with each other. Since the contract specifications require 50% sidelaps, it means that all areas should have been flown twice. The results of this analysis is to verify that the data was generally flown to obtain the 50% sidelaps, that there are no gaps between flightlines and also that overlapping flightlines are consistent in elevation values.

Visual inspection of shaded-relief images: During the visual inspection, hillshades are derived from the bare earth DEMs. The hillshades are examined for any obvious data errors such as blunders, border artifacts, gaps between data quads, no-data gaps between flight lines, hillscarps, land shifting due to GPS time errors, etc. The data is examined a scale range of 1:4000 to 1:6000. During this process we also compare the data to existing natural features such as lakes and rivers and also to existing infrastructure such as roads. Orthophotos area also used during this phase to confirm data errors. If any of these data errors are found, they are reported to TerraPoint for correction.