| Beach Management Decision Support Using LIDAR
By William C. Eiser, David L. Eslinger and Hanna Goss Introduction
A shoreline survey method that involves an airplane- mounted laser system is helping state coastal resource managers save both time and money. Already being successfully utilized by South Carolina managers to establish beachfront jurisdictional lines, LIDAR technology may also have numerous uses for managers around the country, ranging from mitigating the impact of severe storms to determining the effectiveness of beach renourishment projects. The goal of the federal partners developing these LIDAR tools and protocols is to turn the tested methodology over to the commercial sector, which will give coastal communities the opportunity to employ this cost-effective method of beach monitoring as frequently as required. Benefits
The South Carolina Department of Health and Environmental Control Office of Ocean and Coastal Resource Management (OCRM) successfully used LIDAR, or Light Detection and Ranging, data to revise its beachfront jurisdictional line. South Carolina law requires that OCRM revise its beachfront lines every 10 years by measuring the crest of the primary dunes. All construction on the beach has to be set back 20 feet from the jurisdictional line, and the state has authority over everything in front of the setback line. By utilizing the new technology, OCRM saved money on staff travel expenses and the amount of staff hours spent in the field. As a result, the state's dune crest mapping project was finished ahead of schedule and under budget.
In the past, OCRM used stereo pairs of beachfront aerial photographs to determine the jurisdictional line. While this method provided good vertical views of the sand dunes, the horizontal accuracy of the method was questionable. In 1998, when OCRM began revising the line, the agency was able to take advantage of advances in technology and began using Global Positioning System (GPS) equipment. Although this method was accurate, it was a relatively slow manner of mapping the entire coast of the state because staff were required to walk the dune crest with a GPS backpack to get survey measurements. With the GPS unit, two staff members took all day to survey about three miles of coastline. Using the LIDAR data, one staff person was able to map about 10 miles of beach a day while working at a computer.
LIDAR beach mapping utilizes GPS accuracy and incorporates laser technology to provide dense topographic measurements in a matter of days instead of weeks. By using the aircraft-mounted lasers and GPS satellite receivers, horizontally accurate ground elevation measurements are taken at a rate of 2000 to 5000 pulses per second with a vertical precision of 15 centimeters. Once an area is flown, the millions of individual ground elevation measurements are processed and made available through a web page. While this information is valuable for researchers, many coastal resource managers need the data in an easy-to-use format that will assist them with decision making. Using South Carolina as a test state, the federal partners developed a series of LIDAR-based topographic maps that were distributed to managers in a CD-ROM format.
Coastal managers around the nation may have other uses for the highly detailed LIDAR data, include identifying the impact of severe storms, which could save property, and determining the effects of erosion control devices, such as groins and jetties. Resource managers could use annual LIDAR surveys to monitor long-term erosion trends, and to determine the effectiveness of beach renourishment projects and impacts of erosion on wildlife habitats, such as turtle nesting sites. Decision Delivery Approach
The federal partners bringing LIDAR technology to the states include the National Aeronautics and Space Administration (NASA) Wallops Flight Facility, the National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center, and the U.S. Geological Survey (USGS) Center for Coastal Geology and Regional Marine Studies. NASA originally developed this technology to map polar ice sheets. NOAA and USGS scientists are working to convert data gathered with the NASA-developed technology into viable products that will enable coastal managers and researchers to measure and document coastal topography and movement.
One such product for delivering the beach mapping information is a CD-ROM titled, South Carolina's Coast: A Remote Sensing Perspective. This two-volume CD-ROM set contains land cover and shoreline data, the software needed to bring this information to a desktop computer, and information about South Carolina's coastal management program. Geographic information system (GIS) tutorials on the CD explain how to access and manipulate the data. Examples are given to show how the data can be used to address coastal resources issues, as well as how the tools can be used to aid in data analysis. The CD-ROM provides different audiences with multiple ways to access the data. For instance, there is a web browser for viewing tutorials and background information about remote sensing, GIS, and the data; a data browser for those who don't have GIS software; and project files with LIDAR data for GIS users.
The South Carolina CD-ROM, which is accessible via the Internet at http://www.csc.noaa.gov/products/sccoasts/sccover.html, is one of several packaged geospatial projects for decision support developed by the NOAA Coastal Services Center. Other Center CD-ROMs for coastal managers include the Columbia River Basin in Washington and Oregon, San Francisco Bay, and New Technologies for Coastal Mapping.
Baseline LIDAR topography has been collected for beaches on Cape Cod and Long Island, and from Delaware to Georgia on the East Coast; southern and central California, and central Oregon to central Washington on the West Coast; Florida, Alabama, Mississippi, and Louisiana on the Gulf Coast; and Ohio and Pennsylvania on the Great Lakes Coast. In an effort to determine the effects of recent storms and weather phenomena, scientists have taken post-El Nino readings on the West Coast, and post-nor'easter readings along the coasts of Virginia, Maryland, and Delaware.
Much of the baseline survey data is available to coastal managers and researchers via the Internet at http://www.csc.noaa.gov/crs/beachmap/ATM_download.html. The data set includes over 14 gigabytes of LIDAR points stored online. Although a web-based LIDAR Data Retrieval Tool (LDART) was created to allow easy access to the data set, many coastal resource managers have neither the time nor the resources to manipulate the data into usable GIS formats. To make remote-sensing data readily accessible to coastal managers and desktop GIS users, such products are being developed as the South Carolina CD-ROM, that integrates satellite-derived land cover data, and changes analysis data and LIDAR mapping data with vector data in a multiplatform environment. GIS Architecture, Database Design, and Access
The South Carolina CD-ROM features a variety of tools developed by the NOAA Coastal Services Center that provide greater access to the data. BeachMapper is a software application that can be used to access LIDAR data for South Carolina, LIDAR Loader is an extension created to load LIDAR data into Environmental Systems Research Institute's (ESRI®) ArcView™, and LIDAR Data Handler Extension is an extension created to give users specific tools to manipulate LIDAR data. Data are accessible over the Internet via LDART, which uses a MapObjects™ interface to allow a user to pick a state, date, area of interest, and projection. LDART queries are processed to retrieve the requested data from a database organized by collection date. Retrieved files are placed on a File Transfer Protocol (FTP) server, and an e-mail notification-containing Federal Geographic Data Committee (FGDC) compliant metadata-is sent to the user.
To use the data once it has been downloaded, it must be viewed using an appropriate software package. LIDAR data can be viewed in a GIS as a point-, grid-, or contour data set.
In a grid format, each cell is assigned a value based on surrounding elevation points. This gives the data an even and continuous distribution, resulting in a smooth surface when displayed. The grid- or cell-based format also provides a useful platform for analysis. Grids can be compared to show spatial, temporal, and volumetric change.
A grid or digital elevation model (DEM) display allows for a high-resolution, color-enhanced viewing of the LIDAR data [Figure 1]. Once the LIDAR data are in a grid, other products such as a contour map can be created [Figure 2]. Contours provide an effective way of showing gradient change. The closer the contours are to each other, the steeper the elevation or beachface.
Different applications require LIDAR data to be displayed in particular forms. For example, grid data can provide detailed information about volumetric and surface changes on a beach. This is used in measuring sediment movement after a renourishment project. Remote Sensing Steps
Before LIDAR data are delivered to coastal resource managers, several processing steps must first occur. To collect the data points, project scientists use a NASA pulsed-laser ranging system called the Airborne Topographic Mapper (ATM), which is mounted onboard a NOAA DeHavilland Twin Otter aircraft. The ATM emits laser beams at 2000 to 5000 Hertz per second, directed at the Earth's surface through a port opening in the bottom of the aircraft's fuselage. The system measures how much time it takes for the laser to travel from the aircraft to the ground and back. The aircraft travels over the beach at approximately 60 meters per second, while surveying from the low water line approximately 300 meters inland.
Each laser pulse has a time stamp associated with it that NASA uses to determine where the aircraft was in reference to the ground. This tells scientists the exact location of the laser, plane, and ground points. NASA delivers this raw elevation file to the NOAA Coastal Services Center, where the data is filtered and validated. The end product is accurate, with geographically registered longitude, latitude, and elevation (x, y, z) positions for every data point. These "x, y, z" data points allow the generation of a DEM of the ground surface.
Viewing the data in its native "x, y, z" format allows a user to understand the spatial dispersion of the data horizontally, and each individual point is a discrete geographic entity.
The LIDAR transceiver is rigidly fastened to the aircraft and does not move. However, a scan mirror assembly is mounted beneath the transceiver. A 45-degree folding mirror reflects the laser pulses onto a moving mirror, which then directs the laser pulses to the Earth. The reflected laser light from the ground follows the reverse optical path and is directed into a small Cassegrainian telescope. The moving mirror produces a conical sampling pattern beneath the aircraft over a 30-degree wide swath, thus permitting the collection of topographic information over a strip approximately 300 meters in width from the nominal 600-meter data collection altitude.
Flights are planned to maximize the number of elevation points collected at the lowest tide, for the largest possible area . The aircraft flight path is always parallel to the beach. Four passes are flown over each section of the beach. Two of these passes are flown so that the center of the swath is over the sand/water interface. The other two passes are flown over the center of the sand/development interface.
Flights generally last four hours. Weather conditions must be monitored. The flights cannot be performed during rain or fog, since water vapor in the air could cause the laser beams to scatter and give a false reading. Additionally, the plane cannot fly during high winds, since the returned laser pulse will not be correctly recorded.
Following the initial collection of data in the field, the data may undergo up to four levels of quality assurance/quality control (QA/QC). Level 1 is the generation of raw elevation files. Level 2 is outlier removal, which involves plotting the Level 1 data by latitude and elevation to define upper and lower elevation limits for valid measurements. Level 3 is an internal consistency check, where overlapping elevation measurements are compared for verification. Level 4 is field verification. At this time, LIDAR data distributed through LDART are currently only processed to Level 2. The federal partners plan to add all-new data sets only after the completion of Level 3 QA/QC processing. All existing data sets within the LDART database will be subsequently processed to Level 3.
It is also hoped that users of ATM data will perform their own verification of the absolute data accuracy. Before utilizing the LIDAR-based topographic maps, South Carolina managers wanted to ensure the accuracy of the laser mapping system. OCRM compared the LIDAR data to the GPS data staff had already collected on beaches in two counties. The baselines derived from the contour data from the laser mapping system and the GPS were virtually identical [Figure 3]. Conclusion
As baseline LIDAR surveys are taken of the nation's coastline, the laser technology is being perfected for beach mapping, and improved methods for processing the information are being developed. Protocols are being established for the processing of the data to ensure a nationally standardized database. Once the technology and data processing systems are established and have been adequately tested, the NOAA Coastal Services Center intends to work through the private sector to provide LIDAR data to coastal managers and communities more quickly. Managers in South Carolina have already determined that contracting for LIDAR data once a year would replace the need to take annual wading-depth beach profiles of the state's coastline, which costs the agency about $50,000. Additional data provided by LIDAR would help South Carolina managers re-create the original natural dune line after a hurricane, and help mitigate the impact of after-storm flooding.
LIDAR beach mapping has the potential of revolutionizing the decision-making process for state coastal resource managers. South Carolina's success in using the new technology to establish its jurisdictional line will be the impetus for other states to utilize such data in new and innovative ways. The technology's ability to provide large amounts of highly accurate data more quickly and cost effectively than other methods, will help managers prepare for coastal hazards and respond with precision after a hazardous event. States will be able to better determine the effectiveness of erosion control devices and monitor long-term environmental trends. The federal government's ability to bring coastal applications of this cutting-edge technology to the private sector will foster economic growth, and ensure that coastal communities around the country will benefit from its utilization. Laser beach mapping is a successful example of remote sensing data being used to support the decision-making process of the nation's coastal resource managers. For questions regarding the state's use of LIDAR, contact is: William C. Eiser, Oceanographer South Carolina Department of Health and Environmental Control Office of Ocean and Coastal Resource Management; 1362 McMillan Avenue, Suite 400; Charleston, SC 29405; Tel: (843) 744-5838 E-mail: [email protected] For questions regarding the National Oceanic and Atmospheric Administration : (NOAA) Coastal Services Center and its LIDAR program, contact either David L. Eslinger, Oceanographer; NOAA Coastal Services Center; 2234 South Hobson Avenue; Charleston, SC 29405 ; Tel:(843) 740-1270; E-mail: [email protected] or Hanna Goss, Technical Writer; NOAA Coastal Services Center; 2234 South Hobson Avenue; Charleston, SC 29405-2413; Tel: (843) 740-1332; E-mail: [email protected] Back |
