Scanning the Waters
November 1, 2007
Hydrographic surveying can be dangerous work. As surveying professionals chart near-shore features to help ensure the safety of vessels passing through our nation’s waters, they often encounter weather changes, rough waves, visual hazards and shoreline obstructions.
Traditional methods of hydrographic surveying usually entail a survey vessel equipped with a single beam echosounder or multibeam sonar system along with a position and orientation GPS system to record the location of features. For mission planning, surveying crews use existing charts, aerial LiDAR and available photos as guides to the area to be surveyed. Data collection includes accurate positions and representations of hills, mountains, and even lights and towers that will aid in fixing a ship’s position, as well as aspects of the waters and sea floor. To accurately chart this data, it must be collected under strict rules for evaluation and positioning set out by the National Oceanic and Atmospheric Administration (NOAA).
“There are limitations with traditional methodologies,” explains NOAA’s Lt. Cmdr. Richard T. Brennan. “The field hydrographer, for example, records several positions around the extent of an islet to determine its approximate shape and dimensions. A polyline containing these points is drawn to represent the feature. As you can imagine, this is time-consuming, dangerous and fraught with possibilities for error.” And, Brennan adds, these methods are “insufficient, producing low-resolution data.”
Brennan and others who survey our nation’s shorelines often seek new, more efficient technologies and methodologies to provide a safer environment and to better protect the equipment used on such missions. Brennan himself had the opportunity this year to see how new approaches to hydrographic surveying can do just this. The results were impressive.
The PreparationNOAA’s National Geodetic Survey (NGS) is congressionally mandated to survey the national shoreline, approximately 95,000 miles of coastline. The agency’s Coastal Mapping Program (CMP) works to provide a record of regularly updated and consistent national shoreline data to define America’s marine territorial limits and manage coastal resources. This shoreline is present on NOAA nautical charts and is considered authoritative when determining the official shoreline for the United States.
Through Stewart Kuper Jr., cartographer with the NGS, Brennan was aware of Riegl’s long-range laser technology. Brennan contacted the Riegl USA office in Orlando, Fla., to determine if there was a product to meet his needs for long range, spatial density and speed. There was: the Riegl LMS-Z420i terrestrial laser scanner system. Brennan composed a team to test the scanner integrated with hydrographic surveying components to see if the laser could be added to the workflow and methods of the organization.
“The real benefits I see for [using] lasers is in increasing the safety for the survey vessel as well as simultaneously increasing the number of points and resolution of those near-shore, above-water features that are typically very dangerous for us to map,” Brennan says.
To execute the test, a vessel and personnel were coordinated, the scanner was incorporated with appropriate software, and the platforms and mounts were designed and fabricated. Integrated with the Riegl LMS-Z420i long-range scanner was an Applanix (Trimble, Sunnyvale, Calif.) POS MV 320 GPS/IMU unit. With all aspects in line, installation came next for the pilot run in Norfolk, Va.
To begin seamless integration of the scanner with GPS position and IMU attitude data, firmware upgrades were needed for a time-stamping mechanism to add real-time clock information to each laser range measurement, a significant improvement over traditional methods which only time each scan. The original format of the data was to be generated in a Cartesian geocentric format, then reformatted to the more commonly used UTM coordinates. Riegl further facilitated the development of coordinate transformation software to effectively register the data in the WGS84 format.
Brennan’s team then looked to the waters for a real run with the equipment. It was an exciting project--one that had the potential to transform hydrographic surveying methods.
The Pilot RunOn a cold and blustery morning in February, Brennan’s team converged on the NOAA vessel “Bay Hydrographer” in Norfolk. The team was made up of people and equipment from Riegl’s Austria and USA offices, personnel from Applanix’s Houston and Toronto offices, the crew of the vessel, and Jason Woolard and Jon Sellars, cartographers with the NGS Remote Sensing Division. Woolard and Sellars set up a GPS base station and established a series of control points on the wharf to provide an assessment of the overall system accuracy.
Crew members installed a mounting platform on the vessel for the Applanix GPS/IMU unit and the Riegl scanner. The instruments were placed next to one another so the identification of the instrument origins in the post-processing step would be easily facilitated. Power and data cables for the instruments were strategically placed so operators could work inside the cabin of the vessel. A winter storm had just passed through the area making the ambient temperature a chilly 10 degrees Fahrenheit, so a team member sacrificed his jacket to place around the scanner until it reached acceptable operating temperatures.
The team then started up the Applanix GPS/IMU system and completed a short registration process to make sure the correct heading, roll, pitch, heave and yaw information was reported. The scanner then synchronized with the GPS receiver with real-time UTC (coordinated universal time) clock information so that the laser data in the post-processing stage could be correctly attached to the trajectory created by the Applanix GPS/IMU system.
With all systems go, Bay Hydrographer was on its way. For two hours the combined system made several passes at various urban areas along the Elizabeth River, the naval shipyards and the dock area around downtown Norfolk. The data collection software, Riegl’s RiScan Pro, provided a real-time glimpse of the data as it was acquired and allowed team members to confirm correct data collection. Within just a few hours, the system had collected more than 2 kilometers of data around the Elizabeth River. It was time to head back to the dock and begin the post-processing phase of the test.
The ResultsOnce docked, the team retrieved the trajectory information from the GPS/IMU system, which was further aided by data from the GPS base station and the GPS satellite position information determined by Applanix’s POSPac software.
The Riegl scanner collected the laser information in its own coordinate system as did the Applanix GPS/IMU system. Peter Rieger, technical manager from Riegl GmbH, guided the coordinate transformations so that the laser data, trajectory, GPS antenna locations and vessel could be brought together in a WGS84 real world coordinate system. Later, Rieger displayed the organized point cloud of the various data collections.
Then, the results; excitement from the group was high. Data revealed that their careful preparation, dedicated field work and crucial mobile data collection methods were successful. “The resolution [of the point cloud] is impressive, providing almost photographic quality,” Brennan says. “It was possible to identify cleats, bits and other pier side hardware.”
The results of the pilot indicated that there was sufficient range and detail from the system to be able to significantly add to the speed, accuracy and safety of shoreline verification in challenging areas along the nation’s shoreline. The system holds promise for areas where the shoreline structure is entirely natural, such as much of Alaska, and for items such as rocks and islets that dominate a survey.
In the weeks that followed the tests, other benefits of the laser scanner and GPS/IMU unit integration became apparent. For one, data of the Elizabeth River bed from the integration of the laser scanner, the side scan sonar and photogrammetry provided a more complete and accurate representation of the area surveyed. A synergistic effect exists that correctly registered data from multiple sensors, which in turn enhanced the accuracy of the overall data set. The system holds promise for areas where the shoreline structure is entirely natural, such as much of Alaska, and for items such as rocks and islets that dominate a survey.
The integrated imagery also had additional detailed information showing the tide levels on pier structures and channel scouring due to water currents. The laser data reflecting off the waves assisted registration of other sensor information. The integrated imagery also showed natural and man-made shore structures that aid in navigation. “It is apparent that these technologies will lead to more complete information for ports and harbors, rivers, levees and nautical charting, as well as coastal erosion and the impact of storms and changing sea levels,” Brennan says.
Ensuring Accurate ShorelinesThe addition of the laser scanning technology as tested in February aboard Bay Hydrographer will help to improve the accuracy and completeness of the country’s existing nautical charts. In the future, NGS has projected that long-range laser scanning will afford improved geometric accuracy and spatial resolution over traditional techniques, safer surveying methods, and efficient collection, processing and quality control of data. The Riegl scanner system integrated with the Applanix GPS/IMU facilitates the development of new remote sensing technologies to improve coastal mapping production.
“The vessel-mounted laser will speed production, provide greater safety [due to greater stand-off capability] and increase resolution of shoreline surveys,” Brennan says, “which will lead to more automated means of dealing with cartography.”