Monitoring the coastline and its surroundings necessitates frequent sampling and mapping to understand the coastal processes and provide guidance for management decisions. Obstacles include continuous tide and wave influence on the morphology as well as the stability of permanent control monuments. The challenges of mapping and monitoring the coastal terrain given the dynamic nature of the system have evolved over the years, with increased spatial and temporal sampling.
Early techniques relied on repeated, cross-shore transects using a survey transit and level rod. The electronic transit integrated with a distance meter (total station theodolite) increased the number of measurements. This was followed by the development of the satellite-based global positioning system (GPS) and LiDAR units, capable of high point-density, three-dimensional (3D) geospatial data. Also known as laser scanning systems, LiDAR has since been coupled with position and orientation sensors, enabling mobile mapping systems (MMS) utilized on both aerial and terrestrial platforms.
Aerial LiDAR surveys mounted on a helicopter or airplane achieve the greatest spatial coverage but are currently limited in point density when compared to the terrestrial counterpart due to the sampling speed. Aerial surveys are typically more costly, requiring more planning and logistics (e.g., fuel and environmentally favorable conditions). Static, tripod-mounted systems are less expensive and output more detailed maps but are limited in spatial coverage. Mobile terrestrial systems offer a balance between the two aforementioned techniques and are able to provide high-resolution data at lower operational costs while being rapidly deployable.
The use and accuracy of mobile terrestrial systems has been well documented in urban and coastal environments. It serves well to complement airborne LiDAR surveys by resolving more detailed structure such as building sides or foredune faces with the denser point spacing and oblique scanning angles.
Mobile, terrestrial-based LiDAR systems offer the benefits of traditional, stationary high-resolution scanning and given a high-precision navigation system, allow for large regional surveys to be conducted within hours at comparable resolution. The Coastal and Hydraulics Laboratory, Field Research Facility (FRF), utilizes this technology to monitor beach elevation throughout the year on a seasonal scale as well as before, during, and after extratropical, subtropical, and tropical storms or hurricanes. The observational data are critical for understanding spatial and temporal trends of beach morphological evolution within the time scale of a storm event, seasons, years, and ultimately decades. The data can also be used to improve model predictions of coastal inundation and vulnerability during storms, quantify during-storm morphodynamics, monitor beach nourishment projects, and assess large-scale sediment transport and morphology evolution.
A typical mobile survey begins with initialization of the GPS/INS system to acquire stable position, attitude, and velocity data through a series of automated leveling and satellite locking routines. Once there is a signal lock on at least seven GPS satellites and the GPS Azimuth Measurement Subsystem calculates a fixed solution for the two GPS antennas’ separation, the calibration procedure for GPS/INS system can begin. The vehicle operator drives around a calibration site where figure-eight turns and sudden accelerations and decelerations are used to move and level the sensitive gyroscopes contained within the IMU. Once the attitude (roll, pitch, and yaw) accuracy values reach the desired minimum threshold based on the instrument’s precision (0.02 deg for the POS-LV 220 system), the navigation system is sufficiently calibrated to begin field data collection.
LiDAR data are collected at a peak pulse repetition rate of 300 kilohertz (kHz). This results in 122,000 measurements per second with an angular resolution of 0.08° in the vertical along a narrow transect at 90° to the vehicle. As the survey progresses, the surrounding topography is scanned and a 3D point cloud is built from sequential two-dimensional linescans. A typical survey will start with the vehicle positioned parallel to the dune near the dune toe with the scanner pointed orthogonally off the passenger side and scanning offshore. The radar system is also operated while LiDAR scanning.
This work was done by Nicholas J. Spore and Katherine L. Brodie for the Army Engineer Research and Development Center (ERDC). For more information, download the Technical Support Package (free white paper) below. ERDC-0010
This Brief includes a Technical Support Package (TSP).
Collection, Processing, and Accuracy of Mobile Terrestrial Lidar Survey Data in the Coastal Environment
(reference ERDC-0010) is currently available for download from the TSP library.
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