2022 WA DNR Topobathy Lidar DEM: Nooksack River, WA

Create custom data files by choosing data area, product type, map projection, file format, datum, etc. A new metadata will be produced to reflect your request using this record as a base. Change to an orthometric vertical datum is one of the many options.

Bulk download of data files in GeoTiff format, WA State Plane South NAD83(HARN) US survey feet coordinates and orthometric heights in feet.

The Data Access Viewer (DAV) allows a user to search for and download elevation, imagery, and land cover data for the coastal U.S. and its territories. The data, hosted by the NOAA Office for Coastal Management, can be customized and requested for free download through a checkout interface. An email provides a link to the customized data, while the original data set is available through a link within the viewer.

Link to the NV5 Geospatial, Inc. Technical Lidar Report from the Washington Lidar Portal.

Planning:

In preparation for data collection, NV5 reviewed the project area and developed a specialized flight plan to ensure complete coverage of the Nooksack River Lidar study area at the target combined point density of greater than or equal to 15 pulses/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, channel flow rate conditions, tidal ranges, and water clarity were reviewed.

The turbidity is often high in the Nooksack River, which decreases the depth penetration of Lidar. Because of this, special care was made to ensure that Lidar acquisition occurred when turbidities were below 10 NTU. In this area, preliminary data from Washington DNR illustrated that in order to have turbidities less than 10 NTU the flow rates had to be less than 4,000 cfs. The flow rate was around 2,000 cfs during the time of acquisition. Low turbidities and flow conditions typically occurred in the winter months when the weather was not freezing. Therefore, NV5 acquisition occurred during February 2022 during leaf off conditions as per contract specifications. Washington DNR also determined that acquisition should occur during the Mean Lower Low Water (MLLW), which refers to the lowest daily tide. Secchi depths were recorded close to the time of acquisition during the MLLW.

Turbidity Measurements and Secchi Depth Readings

In order to assess water clarity conditions prior to and during lidar and digital imagery collection, NV5 collected turbidity measurements and secchi depth readings. Readings were collected at three locations throughout the project site between February 24th and 25th, 2022. Turbidity observations were recorded three times to confirm measurements. A true Secchi depth reading is where the Secchi depth reaches extinction (the point at which you can no longer see the Secchi disk). Please note that the Secchi depth for the second sample location reached the bottom of the riverbed . The Silt substrate made obtaining a Secchi depth reading impractical in the third location. Silt causes several problems in trying to obtain a Secchi depth reading such as creating unsafe wading conditions by causing field crew members to sink, creating a false Secchi depth due to silt particulates getting stirred up into the water column, and creating very shallow depths at the MLLW (when turbidity values were collected) that doesn't allow the Secchi disk to be accurately deployed.

Ground Survey Points

Ground survey points were collected using real time kinematic (RTK) and post-processed kinematic (PPK) 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. PPK surveys compute these corrections during post-processing to achieve comparable accuracy. RTK and PPK surveys record data while stationary for at least five seconds, calculating the position using at least three one-second epochs. 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.

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 equitably distributed throughout the study area.

Base Stations

Monuments were used for collection of ground survey points using real time kinematic (RTK) , and post processed kinematic (PPK) survey techniques.

Base station locations were selected with consideration for satellite visibility, field crew safety, and optimal location for GSP coverage. NV5 Geospatial utilized one permanent real-time network (RTN) base station from the Washington State Reference (WSRN). NV5 also used two existing monuments for the Nooksack River Lidar project. NV5's professional land surveyor, Evon Silvia (WAPLS#53957) oversaw and certified the ground survey.

NV5 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 was triangulated with nearby Continuously Operating Reference Stations (CORS) using the Online Positioning User Service (OPUS1) 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 Nooksack River 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.

Airborne Survey

The lidar survey was accomplished using a Riegl VQ-880-GII green laser system mounted in a Cessna Grand Caravan. The Riegl VQ-880-GII 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 (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-GII contains an integrated NIR laser (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. systems can record unlimited range measurements (returns) per pulse, however a maximum of 15 returns can be stored due to LAS v1.4 file limitations. 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 60% (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 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, NV5 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. NV5 refracted water column points using NV5's proprietary LAS processing software, Las Monkey. The resulting point cloud data was classified using both manual and automated techniques. Processing methodologies were tailored for the landscape. Brief descriptions of these tasks are shown below.

Lidar Processing Steps

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. Software used - POSPac MMS v8.5

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. Software used - RiProcess v1.8.5, Lidar Launcher v1.1 (NV5 proprietary software ), Las Monkey v2.6 (NV5 proprietary software )

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. Software used - Bays-StripAlign v2.19

Import calibrated points into manageable blocks for editing. Software used - TerraScan v.19

Apply refraction correction to all subsurface returns. Software used - Las Monkey v2.6 (NV5 proprietary software)

Classify resulting data to ground and other client designated ASPRS classifications (Table 9). Assess statistical absolute accuracy via direct comparisons of ground classified points to ground control survey data. Software used - TerraScan v.19, TerraModeler v.19

Generate bare earth models as triangulated surfaces. Generate highest hit models as a surface expression of all classified points. Export all surface models as GeoTIFFs (.tif)) format at a 3.0 foot pixel resolution. Software used - Las Product Creator v3.6 (NV5 proprietary software), ArcMap v10.6

The NOAA Office for Coastal Management (OCM) downloaded 18 raster DEM files in GeoTiff format from the Washington Lidar Portal. The data were in Washington State Plane South NAD83(HARN), US survey feet coordinates and NAVD88 (Geoid12B) elevations in feet. The bare earth raster files were at a 3 feet grid spacing. No metadata record was provided with the data. This record is populated with information from the NV5 Spatial, Inc. technical report downloaded from the Washington Dept. of Natural Resources Washington Lidar Portal.

OCM performed the following processing on the data for Digital Coast storage and provisioning purposes:

1. Used internal an script to assign the EPSG codes (Horizontal EPSG: 2927 and Vertical EPSG: 6360) to the GeoTiff formatted files.

2. Copied the files to https.