2015 OLC Topobathy Lidar DEM (Voids): Middle Fork Willamette River, OR

Airborne topobathymetric lidar derived digital elevation model (DEM) with voids in areas where no bathymetric points could be obtained, collected for the Middle Fork of Willamette River, Oregon in 2015.

In September 2015, Quantum Spatial (QSI) was contracted by the Oregon LiDAR Consortium (OLC) to collect topobathymetric Light Detection and Ranging (LiDAR) data in the fall of 2015 for the Middle Fork Willamette River site in Oregon. The Middle Fork Willamette River area of interest stretches between Eugene/Springfield and Dexter Dam and Fall Creek below Fall Creek Dam. Traditional near-infrared (NIR) LiDAR was fully integrated with green wavelength return data (bathymetric) LiDAR in order to provide seamless and complete project mapping.

Data were collected to aid OLC in providing a complete topobathymetric dataset to be used in analyses of the long term impacts associated with sediment deposition on downstream habitat and channel morphology.

Digital elevation model incorporating both topographic ground measurements as well as bathymetric bottom points for the area of interest in Willamette river.

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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be aware that temporal changes may have occurred since this data set was collected and some parts of this data may no longer represent actual surface conditions.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.

Absolute accuracy was assessed using Fundamental Vertical Accuracy (FVA) reporting designed to meet guidelines presented in the FGDC National Standard for Spatial Data Accuracy3. FVA compares known ground check point data collected on open, bare earth surfaces with level slope (<20 degrees) to the triangulated surface generated by the LiDAR points. FVA is a measure of the accuracy of LiDAR point data in open areas where the LiDAR system has a high probability of measuring the ground surface and is evaluated at the 95% confidence interval (1.96 * RMSE). The mean and standard deviation (sigma ) of divergence of the ground surface model from ground check point coordinates are also considered during accuracy assessment. These statistics assume the error for x, y and z is normally distributed, and therefore the skew and kurtosis of distributions are also considered when evaluating error statistics.

For the Middle Fork Willamette River survey, 30 ground check points were withheld in total resulting in a fundamental vertical accuracy of 0.060 meters at the 95% confidence interval (1.96 * RMSE).

Additionally, 45 bathymetric (submerged or along the water’s edge) check points were also collected in order to assess the submerged surface vertical accuracy, resulting in an average absolute accuracy of 0.012 meters at the 95% confidence interval (1.96 * RMSE).

QSI also assessed absolute accuracy using 555 ground control points. Although these points were used in the calibration and post-processing of the LiDAR point cloud, they may still provide a good indication of the overall accuracy of the LiDAR dataset, and therefore have been provided - please see the dataset report for more details.

The specified depth penetration range of the Riegl VQ-820-G sensor is one secchi depth; therefore, bathymetry data below one secchi depth at the time of acquisition is not to be expected. To assist in evaluating performance results of the sensor, a polygon layer was created to delineate areas where bathymetry was successfully mapped.

This shapefile was used to control the extent of the delivered clipped topo-bathymetric model and to avoid false triangulation across areas in the water with no returns. Insufficiently mapped areas were identified by triangulating bathymetric bottom points with an edge length maximum of 4.56 meters. This ensured all areas of no returns (> 9 m2), were identified as data voids. Of the areas successfully mapped, 46.50% had a calculated depth of 0 – 0.5m, 38.00% had a calculated depth of 0.51 -1.0 m, 13.89% had a calculated depth of 1.01 – 1.5m, and the remaining 1.61% had a calculated depth between 1.51m and 6.5m (Table 9, project report).