2023 WA DNR Lidar DEM: Asotin County, WA and Lewiston, ID

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Bulk download of data files in GeoTIFF format, in WA State Plane NAD83 (HARN) US feet coordinates and elevations in NAVD88 (Geoid12b) feet.

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Link to the NV5 Geospatial Technical Lidar Report

Link to the Washington Geologic Survey QA Report

Planning:

In preparation for data collection, NV5 Geospatial reviewed the project area and developed a specialized flight plan to ensure complete coverage of the Asotin County, Washington and Lewiston, Idaho lidar study area at the target point density of greater than or equal to 8.0 points/m2 (0.74 points/ft2). 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 and potential air space restrictions were reviewed.

Ground Survey

Ground control surveys, including monumentation and ground survey points (GSPs) were conducted to support the airborne acquisition. Ground control data were used to geospatially correct the aircraft positional coordinate data and to perform quality assurance checks on final lidar.

Base Stations

Base stations were utilized for collection of ground survey points using real time kinematic (RTK), fast static (FS), and total station (TS) survey techniques.

Base station locations were selected with consideration for satellite visibility, field crew safety, and optimal location for GSP coverage. NV5 Geospatial utilized three permanent real-time network (RTN) base stations from the Washington State Reference Network (WSRN) for the Asotin County, Washington and Lewiston, Idaho Lidar project and one existing mag hub nail. NV5 Geospatial also established five new monuments using 6 inch mag hub nails with orange survey washers. NV5's professional land surveyor Steven Hyde (WAPLS#22028821, L-10235) oversaw and certified the ground survey.

NV5 Geospatial utilized static Global Navigation Satellite System (GNSS) data collected at 1 Hz recording frequency for each base station. During post-processing, the static GNSS data were triangulated with nearby Continuously Operating Reference Stations (CORS) using the Online Positioning User Service (OPUS) for precise positioning. Multiple independent sessions over the same monument were processed to confirm antenna height measurements and to refine position accuracy.

Monuments were established according to the national standard for geodetic control networks, as specified in the Federal Geographic Data Committee (FGDC) Geospatial Positioning Accuracy Standards for geodetic networks. This standard provides guidelines for classification of monument quality at the 95% confidence interval as a basis for comparing the quality of one control network to another.

For the Asotin County, Washington and Lewiston, Idaho Lidar project, the monument coordinates contributed no more than 5.6 cm of positional error to the geolocation of the final ground survey points and lidar, with 95% confidence.

Ground Survey Points (GSPs)

Ground survey points were collected using real time kinematic (RTK), fast-static (FS), and total station (TS) survey techniques. For RTK surveys, a roving receiver receives corrections from a nearby base station or Real-Time Network (RTN) via radio or cellular network, enabling rapid collection of points with relative errors less than 1.5 cm horizontal and 2.0 cm vertical. FS surveys compute these corrections during post-processing to achieve comparable accuracy. RTK surveys record data while stationary for at least five seconds, calculating the position using at least three one-second epochs, while FS surveys record observations for up to fifteen minutes on each GSP in order to support longer baselines. All GSP measurements were made during periods with a Position Dilution of Precision (PDOP) of less than or equal to 3.0 with at least six satellites in view of the stationary and roving receivers.

Forested checkpoints are collected using total stations in order to measure positions under dense canopy. Total station backsight and setup points are established using GNSS survey techniques.

GSPs were collected in areas where good satellite visibility was achieved on paved roads and other hard surfaces such as gravel or packed dirt roads. GSP measurements were not taken on highly reflective surfaces such as center line stripes or lane markings on roads due to the increased noise seen in the laser returns over these surfaces. GSPs were collected within as many flightlines as possible; however, the distribution of GSPs depended on ground access constraints and monument locations and may not be equably distributed throughout the study area.

Airborne Survey

The lidar survey was accomplished using a Riegl VQ-1560ii-S system mounted in a Cessna Caravan. Table 4 summarizes the settings used to yield an average pulse density of greater than or equal to 8 pulses/m2 (greater than or equal to 0.74 pulses/ft2) over the Asotin County, Washington and Lewiston, Idaho project area. The Riegl VQ-1560ii-S laser system can record unlimited range measurements (returns) per pulse, however a maximum of 15 returns can be stored due to LAS v1.4 file limitations. The typical number of returns digitized from a single pulse range from 1 to 10 for the Asotin County, Washington and Lewiston, Idaho 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 greater than or equal to 55% (greater than or equal to 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 orientation of the aircraft to the horizon (attitude) 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, NV5 Geospatial 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. Processing methodologies were tailored for the landscape. Brief descriptions of these tasks are shown below:

Resolve kinematic corrections for aircraft position data using kinematic aircraft GPS and static ground GPS data. Develop a smoothed best estimate of trajectory (SBET) file that blends post-processed aircraft position with sensor head position and attitude recorded throughout the survey.

Calculate laser point position by associating SBET position to each laser point return time, scan angle, intensity, etc. Create raw laser point cloud data for the entire survey in *.las (ASPRS v. 1.4) format. Convert data to orthometric elevations by applying a geoid correction.

Import raw laser points into manageable blocks to perform manual relative accuracy calibration and filter erroneous points. Classify ground points for individual flight lines.

Using ground classified points per each flight line, test the relative accuracy. Perform automated line-to-line calibrations for system attitude parameters (pitch, roll, heading), mirror flex (scale) and GPS/IMU drift. Calculate calibrations on ground classified points from paired flight lines and apply results to all points in a flight line. Use every flight line for relative accuracy calibration.

Classify resulting data to ground and other client designated ASPRS classifications. Assess statistical absolute accuracy via direct comparisons of ground classified points to ground control survey data.

Generate bare earth models as triangulated surfaces. Generate highest hit models as a surface expression of all classified points. Export all surface as Cloud Optimized GeoTIFFs at a 1.5 foot pixel resolution.

The NOAA Office for Coastal Management (OCM) downloaded 1860 GeoTIFF files from the Washington Lidar Portal. The files were in WA State Plane South NAD83 (HARN) US feet coordinates and the elevations were in NAVD88 (GEOID12b) US feet. The bare earth raster files were at a 1.5 ft grid spacing. OCM assigned the vertical EPSG code (6360) and the horizontal EPSG code (2927).

No metadata record was provided with the data. This record is populated with information from the NV5 Geospatial technical report downloaded from the Washington Dept. of Natural Resources Washington Lidar Portal. The technical report is available for download from the Washington Lidar Portal. The link is provided in the URL section of this metadata record.

The RIEGL VQ-1560 II-S system has a rotating mirror design and high-output dual laser technology, allowing for data collection at a higher altitude and faster speed on large-scale mapping projects and corridor work. This system provides more dense, high-precision data with uniform point distribution and faster acquisition.