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In preparation for data collection, Quantum Spatial reviewed the project area and developed a specialized flight plan to ensure complete coverage of the Chester Morse Reservoir lidar study area at the target combined point density of ≥8 points/m2. Acquisition parameters including orientation relative to terrain, flight altitude, pulse rate, scan angle, and ground speed were adapted to optimize flight paths and flight times while meeting all contract specifications. Factors such as satellite constellation availability and weather windows must be considered during the planning stage. Any weather hazards or conditions affecting the flight were continuously monitored due to their potential impact on the daily success of airborne and ground operations. In addition, logistical considerations including private property access, potential air space restrictions, and water clarity were reviewed.

The lidar survey was accomplished using a Riegl VQ-880-GH laser system mounted in a Cessna Caravan 208B. The Riegl VQ-880-GH boasts a higher repetition pulse rate (up to 550 kHz), higher scanning speed, small laser footprint, and wide field of view which allows for seamless collection of high resolution data of both topographic and bathymetric surfaces. The green wavelength (ʎ=532 nm) laser is capable of collecting high resolution topography data, as well as penetrating the water surface with minimal spectral absorption by water. The Riegl VQ-880-GH contains an integrated NIR laser (ʎ=1064 nm) that adds additional topography data and aids in water surface modeling. The recorded waveform enables range measurements for all discernible targets for a given pulse. The typical number of returns digitized from a single pulse range from 1 to 14 for the Chester Morse Reservoir project area. It is not uncommon for some types of surfaces (e.g., dense vegetation or water) to return fewer pulses to the lidar sensor than the laser originally emitted. The discrepancy between first return and overall delivered density will vary depending on terrain, land cover, and the prevalence of water bodies. All discernible laser returns were processed for the output dataset.

All areas were surveyed with an opposing flight line side-lap of ≥60% (≥100% overlap) in order to reduce laser shadowing and increase surface laser painting. To accurately solve for laser point position (geographic coordinates x, y and z), the positional coordinates of the airborne sensor and the attitude of the aircraft were recorded continuously throughout the lidar data collection mission. Position of the aircraft was measured twice per second (2 Hz) by an onboard differential GPS unit, and aircraft attitude was measured 200 times per second (200 Hz) as pitch, roll and yaw (heading) from an onboard inertial measurement unit (IMU). To allow for post-processing correction and calibration, aircraft and sensor position and attitude data are indexed by GPS time.

Upon completion of data acquisition, QSI processing staff initiated a suite of automated and manual techniques to process the data into the requested deliverables. Processing tasks included GPS control computations, smoothed best estimate trajectory (SBET) calculations, kinematic corrections, calculation of laser point position, sensor and data calibration for optimal relative and absolute accuracy, and lidar point classification. Riegl’s RiProcess software was used to facilitate bathymetric return processing. Once bathymetric points were differentiated, they were spatially corrected for refraction through the water column based on the angle of incidence of the laser. QSI refracted water column points using QSI’s proprietary LAS processing software, Las Monkey. The resulting point cloud data were classified using both manual and automated techniques. Processing methodologies were tailored for the landscape. Northwest Hydro performed all multi-beam sonar acquisitions using a R2 Sonic 2022 High-Resolution Multibeam sonar system and used Hypack Hysweep software to process and edit raw track lines to remove any noise and to evaluate the data for visual anomalies. Northwest Hydro delivered the cleaned sonar data to QSI whose processing staff imported the multi-beam sonar, as class 8 (Model Keypoints), into the existing bathymetric lidar using Bentley Microstation and Terrasolid software. Automatic and manual cleaning algorithms were run on the combined dataset to help ensure a seamless point cloud.