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  • UFL
  • Airborne Laser Swath Mapping: Applications to Shoreline Mapping

    A Special GeoImaging Feature Submission by W E Carter and R L Shrestha, Department of Civil Engineering, University of Florida & S P Leatherman, Laboratory for Coastal Research and International Hurricane Center, Florida International University

    Advances in laser ranging instrumentation, inertial navigation units, phase difference kinematic GPS positioning and the speed and storage capacity of personal computers have resulted in the development of a compact, lightweight, and energy efficient airborne laser swath mapping (ALSM) system that can be operated from a light twin engine, or even single engine, airplane. Flying at 150 to 200 kilometers per hour and an altitude of 500 to 1000 meters, with a scan angle of 10 to 20 degrees and a laser repetition rate 5000 to 25000 pulses per second, it is possible to cover an area several hundred meters in width and hundreds of kilometers in length with 10 to 15 cm diameter laser footprints spaced by a few decimeters to a few meters, in hours. Airborne laser swath mapping is particularly well suited to mapping linear topographic features, and for the first time offers an economically viable method to map a wide variety of scientifically interesting features including shorelines and beaches.


    University of Florida (UF) and Florida International University (FIU) researchers have performed a number of projects during the past two years to test and demonstrate the capabilities of ALSM techniques, to improve the data reduction and analysis procedures and to explore the most useful way to provide the results to users. The projects have included mapping of sandy beaches subject to erosion and tropical storm damage and environmentally sensitive marshlands. Early results of these projects indicate that the system can yield heights accurate to 5 to 10 centimeters, and detect linear topographic features with height differences of 2 to 5 centimeters. The UF and the International Hurricane Center, Florida International University, purchased an ALSM system in 1999, and researchers interested in using ALSM in their research are invited to contact the authors.

    INTRODUCTION

    The basic concepts of airborne laser swath mapping are simple. A pulsed laser ranging system is mounted in an aircraft equipped with a precise kinematic Global Positioning System (GPS) receiver and an inertial navigation system. Solid state lasers are now available that can produce



    thousands of pulses per second, each pulse having a duration of a few nanoseconds (10-9 seconds). Light travels approximately 30 centimeters in one nanosecond. By accurately timing the round trip travel time of the light pulses from the aircraft to the ground (water, foliage, buildings or other surface features) it is possible to determine the range with a precision of one centimeter or better. Using a rotating mirror inside the laser transmitter, the laser pulses can be made to sweep through an angle, tracing out a line on the ground. By reversing the direction of rotation at a selected angular interval, the laser pulses can be made to scan back and forth along a line. When such a laser ranging system is mounted in a aircraft with the scan line perpendicular to the direction of flight, it produces a saw tooth pattern of ranges within a strip centered directly along the flight path (Figure 1). The width of the strip or "swath" covered by the ranges, and the spacing between measurement points, depends on the scan angle of the laser ranging system and the airplane height. Using a light twin or single engine aircraft, typical operating parameters are flying speeds of 200 to 250 kilometers per hour (55 to 70 meters per second), flying heights of 300 to 1000 meters, scan angles up to 20 degrees, and pulse rates of 2000 to 5000 pulses per second. These parameters can be selected to yield a measurement point every few meters, with a footprint of 10 to 15 centimeters, providing enough information to create a digital terrain model (DTM) adequate for most applications, including the mapping of storm damage to beaches, in a single pass.

    After a flight the precise position of the aircraft at the exact epoch of each range measurement is computed relative to nearby GPS ground stations using phase differenced kinematic Global Positioning System (DGPS) techniques. The laser ranging vectors are added to the aircraft positions to derive three dimensional X,Y,Z coordinates of each ground point. As long as the airplane is within 10 to 20 kilometers of a ground base station it is possible to determine the position of the airplane within about 3 to 5 centimeters using single frequency GPS observations and the broadcast ephemeris. However, as the distance from the aircraft to the nearest ground GPS station increases the error in the aircraft coordinates will increase, and it becomes important to use a precise ephemeris and dual frequency observations to remove errors caused by the ionosphere. The X,Y,Z coordinates of the ground points can be processed using a number of commercially available software packages to produce a Digital Terrain Model (DTM) and many other products such as shaded relief maps, contour maps, cross sections, surface profiles, and to compute engineering quantities such as land areas and changes in volumes associated with civil engineering works or storm erosion.



    RECENT PROJECTS

    Project LASER

    In early October 1995 hurricane Opal struck the Gulf Coast along the panhandle of Florida, doing extensive damage to upland structures and to the beaches and dunes. Researchers at the University of Florida organized a project to map the area hit by hurricane Opal using airborne laser swath mapping techniques1,2,3,4. This multi-agency effort was named project LASER and the field data collection portion of project LASER was conducted during the period October 15, 16 and 17, 1996. A light twin engine aircraft, operated by the Florida Department of Transportation (FDOT) for airborne photogrammetry and already equipped with a dual frequency GPS antenna, camera port, and mountings that could easily accommodate the sensor package and electronics rack of the Optech Inc. ALTM 1020 laser ranging system (Figure 2), was used for the data collection. Table 1. summarizes the specifications of the ALTM 1020 system. The entire stretch of beach from Mexico Beach, Florida, to the west tip of Perdido Key, Alabama, more than 250 kilometers in length, was mapped in both directions in just over two hours. The nominal operating parameters were: flying height of 350 meters, scan rate of 25 cycles per second, scan angle of plus and minus 15 degrees, laser pulse rate of 5000 pps.

    The raw GPS, inertial navigation, and laser ranging data collected during the flight were stored on 8 mm cassette tapes. After completion of each flight the data from the GPS reference ground stations were brought together with the data from the aircraft, and Optech personnel used proprietary computer programs to combine the GPS, inertial navigation, and laser ranging data to compute the coordinates of the surface points. The results were ASCII files containing lists of UTM Eastings, Northings, ellipsoid heights, and times for all the successful range measurements collected during the observing window. The percentage of successful range measurements, as a function to the number of laser pulses transmitted was typically more than 90 percent over the high reflectivity beach areas, but dropped quickly over water, reaching fewer than 10 percent when the point on the water surface was more than 6 to 8 degrees off nadir.

    Figure 3 shows an area of beach and near shore water at the western end of Perdido Key, Alabama, with the locations of the surface points from which reflections of sufficient strength were received to obtain range measurements. The coverage is very dense over the relatively diffusely reflective white sand beaches and dunes. Over the near shore water many of the returns were insufficient to measure the range because the reflection is more directional from the surface of the water, and often does not reflect in the direction of the aircraft. Figure 4 is a shaded relief map of the same area shown in Figure 3, with a number of features labeled. Figure 5 is the same area as Figure 4, but the results are presented as a false color contour map. In both Figures 4 and 5 waves propagating past the tip of the land (past a jetty) are quite apparent. Figure 6 shows a cross section taken perpendicular to the wave fronts showing the heights and spatial separation of the waves.

    Table 1. ALTM 1020 Specifications
    Airborne Module Operating altitude: 330 - 1000 m nominal
    Range accuracy: 15 cm single-shot
    Range resolution: 1 cm
    Scan angle: Variable from 0 to 20
    Swath width: Variable from 0 to 0.68 x altitude
    Angle accuracy: 0.05
    Angle resolution: 0.01
    Scan frequency: Variable; depends on scan angle; e.g., 30 Hz for 20 scan, 50 Hz for 10 scan
    Roll and Pitch Accuracy: 0.04
    Heading Accuracy: 0.05
    Supported GPS Receivers: Sercel NR103T, Trimble 4000SSE, or Astech Z12
    Laser wavelength: 1047 nm
    Laser repetition rate: 100 Hz to 5kHz
    Beam divergence: 0.25 mrad
    Laser classification Class IV laser product (FDA CFR 21)
    Eyesafe range: 308 m (single shot)
    Power requirements: 28 VDC @ 15 A
    Operating temp.: 10 - 35C
    Humidity: 0 - 95% non-condensing
    Sensor: Fits all existing camera mounts, or can be directly mounted to the floor
    Dimensions: 290 x 250 x 430 mm
    Weight: 11.4 kg (25 lbs)
    Control rack: 1 stackable vibration-isolated transportable case
    Dimensions: 60 x 60 x 65 cm, excluding GPS
    Weight: 45.4 kg (100 lbs) including shipping covers and cables
    Video Output: NTSC or PAL (annotated video out)
    Data Storage: 8 mm digital data tape
    Ground based module Software: ALTM 1020 GBPP
    GPS receiver: Sercel NR103T, Trimble 4000SSE, or Astech Z12
    Personal computer:

    (min. requirements)

    Pentium laptop, 1 Gb hard disk

    8 Mbyte RAM, SVGA graphics

    Exabyte 8505 tape drive, SCSI adapter






































    Waccasassa Marsh

    During November 1997, airborne laser swath mapping was used to map portions of Waccasassa Marsh, near Cedar Key, Florida. Previous projects had demonstrated that the technology worked well in bare beach areas and sparsely vegetated dunes, but it was questionable if the laser pulses would penetrate the dense vegetation in the marsh.

    The Waccasassa flights were made with a Cessna 206 single engine aircraft. The aircraft was equipped with a real time DGPS receiver and navigation aid that enabled the pilot to fly preplanned patterns to within a few tens of meters. The nominal flying height was 375 meters with an air speed of approximately 50 meters per second. The laser ranging system operated at 3000 pulses per second and the scan pattern was a sawtooth with 25 cycles per second, with a swath width of 180 meters. The raw mapping data were processed by Optech Inc. personnel using proprietary software to produce north, east and up coordinates relative to the WGS 84 UTM coordinate reference frame. The data delivered included files with all ranges and files which were processed to "remove" the vegetation. UF personnel processed the data using both in-house software GATORPLEX and commercial software to produce a variety of products, including three dimensional shaded relief maps, contour maps, and selected profiles. An example of the initial results is shown in Figure 7.





    Pinellas County

    The UF, FIU and Pinellas County have a 3 year cooperative research program to explore the application of ALSM to County programs and projects, including a project to check the Federal Emergency Management Agency (FEMA) flood zone maps for the County. During the first phase of this program ALSM data were collected using an Optech Inc. Model ALTM 1020 system, mounted in a Cessna 206 single engine aircraft equipped with a Starlink real time GPS receiver and Light Bar to assist the pilots in flying straight parallel flight lines. Several different flight line layouts were considered but the unusual shape and complex patterns of land and water areas throughout the county made attempts to separate high priority (flood zones) areas from the general coverage difficult, and it was decided to use a simple pattern. The pattern selected was 77 parallel north-south flight lines of varying lengths providing total coverage of land within the County. The combined length of the 77 flight lines totals approximately 3700 kilometers. At a nominal ground speed of 60 meters per second it required more than 20 hours of on-line data collection. At 5000 laser pulses per second, more than 350 million measurements were made, just for the basic coverage, exclusive of special observations for calibration and validation of the ALSM results.

    The general coverage flights were all planned for a nominal altitude of 600 meters, an air speed of 60 meters per second (135 miles per hour), a scan angle of plus and minus 20 degrees, a scan frequency of 12 cycles per second, and a laser pulse rate of 5000 pulses per second. The nominal swath width, i.e., the ground coverage from each swath, was 450 meters. The nominal overlap between adjacent swaths was 80 meters. The actual flight lines vary significantly from those planned largely because of wind effects on the aircraft and other lesser errors including errors in the real time GPS navigation of the aircraft. The altitude varies tens of meters because of atmospheric affects on the aircraft and pilot skill even in the best of conditions, and in a few instances the flight lines were intentionally reduced in order to continue operations in areas with clouds at or very near the 600 meter level.

    Two additional sets of flight lines were planned; 1) a pair of lines following immediately along the beach line, and 2) eight transverse flight lines spaced at several mile intervals covering the north-south extent of the County. The purposes of the beach line flights were to obtain continuous coverage along the beach to improve the delineation of the beach up to and a few hundred meter beyond the line of the sea wall, and to provide an optimum data set for comparison with the ground survey beach cross sections to calibrate the laser swath mapping observational data. To achieve the best possible detection of the sea wall the scan angle was reduced to plus and minus 15 degrees with a scan rate of 4 cycles per second and the nominal flying height was reduced to 450 meters. The purpose of the transverse flights was to check the primary north-south trajectories for systematic errors such as day to day offsets or slopes. The laser system parameters were selected to maximize foliage penetration, using a scan angle of plus and minus 15 degrees and a scan rate of 4 cycles per second. Figure 8 shows some early results from the Pinellas County project.

















    ACCURACY OF LASER SWATH MAPPING

    The position of the aircraft obtained by differential kinematic GPS is determined in an earth centered earth fixed (ECEF) X,Y,Z cartesian coordinate system. The origin, scale and orientation of the reference frame depends on the satellite ephemerides and the coordinates of the GPS ground reference stations, which should be consistent with one another. Adding the laser ranging vectors to the aircraft positions yields ECEF X,Y,Z cartesian coordinates for each of the ground points surveyed. However, most users need (or prefer) to work in a reference system such as a national geodetic reference system or a state plane coordinate system, where the coordinates more nearly represent local north, east, and up. Also, most users prefer to use orthometric heights, i.e., heights above the geoid, because they often want to mix or compare the heights with those obtained from conventional (spirit) leveling. To transform the cartesian coordinates obtained from airborne laser swath mapping to a system involving orthometric heights, the undulation of the geoid must be known at each point. In the United States geoid models are produced by the National Geodetic Survey(NGS) , and the models are periodically improved as additional data become available5. The ALSM data used in this paper were converted to orthometric heights using NGS geoid model GEOID96.

    Geoid models often are the least accurate immediately along shore lines, because the geoid changes rapidly in coastal areas and because the offshore gravity data required to develop the geoid model, usually collected by aircraft or ships, is often sparse and of lower accuracy than elsewhere. Errors in the geoid model could cause errors in the orthometric heights of a decimeter or more in the Florida panhandle area. For applications involving only changes in the surface, such as beach erosion, this is not a problem for repeat surveys using only airborne laser swath mapping data, because the geoid can be assumed to remain fixed, and the changes can be accurately determined. To check for systematic errors and estimate the accuracy of the laser mapping, the results were compared to cross sections of the beach obtained by classical spirit leveling (Figure 9). Comparison of the several laser and survey profiles spread widely spread along the beach show the laser results to be accurate to the 5 to 10 cm level.







    Table 2. Specifications of the ALTM 1210 Currently Under development by Optech, Inc.


    Operating altitude: 330 - 2000 m
    Range accuracy: 10 cm single-shot
    Range resolution: 1 cm
    Relative accuracy 2-4 cm @2khz, 5-10 cm @10 khz
    Scan angle: Variable from 0 to 20
    Options Intensity data
    Simultaneous first and last pulse measurements
    Extended altitude (up to 2000m) operation
    Swath width: Variable from 0 to 0.68 x altitude
    Angle accuracy: 0.05
    Angle resolution: 0.01
    Scan frequency: Variable; depends on scan angle; e.g., 30 Hz for 20 scan, 50 Hz for 10 scan
    Roll and Pitch Accuracy: 0.04
    Heading Accuracy: 0.05
    Supported GPS Receivers: Astech Z12 or Trimble 4000SSE
    Laser wavelength: 1047 nm
    Laser repetition rate: 100 Hz to 10kHz
    Beam divergence: 0.30 mrad
    Laser classification Class IV laser product (FDA CFR 21)
    Eyesafe range: 308 m (single shot)
    Power requirements: 28 VDC @ 30 A
    Operating temp.: 10 - 35C
    Humidity: 0 - 95% non-condensing
    Sensor: Fits all existing camera mounts, or can be directly mounted to the floor in a small single engine aircraft such as Cessna 172
    Dimensions: 290 x 250 x 500 mm
    Weight: 11.4 kg (25 lbs)
    Control rack: 1 stackable vibration-isolated transportable case
    Dimensions: 60 x 60 x 75 cm, excluding GPS
    Weight: 50 kg including shipping covers and cables
    Video Output: NTSC or PAL (annotated video out)
    Data Storage: 12 hour capacity (8 mm digital data tape)
    Software: ALTM 1020 GBPP
    GPS receiver: Astech Z12 or Trimble 4000SSE
    Personal computer:

    (min. requirements)

    Pentium laptop, 1 GB hard disk, 8 Mbyte RAM

    SVGA graphics, Exabyte 8505 tape drive, SCSI adapter



    FUTURE OF THE ALSM TECHNOLOGY


    The UF and the International Hurricane Center, Florida International University, are purchasing an ALSM system which is expected to be operational in the first quarter of 1999. Specifications of this new instrumentation are shown in Table 2. UF and FIU have also purchased a Cessna 337 (in-line twin engine) which is being modified to house the ALTM 1012G system. The Universities plan to make this system available to collect data for research at other organizations and institutions on a cost reimbursable basis. Researchers interested in using ALSM are invited to contact the authors.

    CONCLUSIONS


    ALSM technology has been used to accurately map hundreds of kilometers of beaches and near shore water surfaces in a few hours. This new capability opens for the first time the possibility to regularly monitor all the beaches of the nation, to map specific beaches immediately prior to and after major storms to obtain accurate quantitative information about the extent of the damage, to follow the natural recovery of beaches after storms and to monitor the effects of actions such as beach nourishment or other anthropogenic modifications. Laser mapping of near shore waters can be used to determine the direction wave spectra of a coastal area to determine the characteristics of storms that result in the most damage to specific areas. There is no doubt that airborne laser swath mapping technology will continue to improve as higher pulse rate lasers become available, kinematic GPS techniques are refined, and more powerful computers and software are developed. But, the technology has already reached a point where the time and costs are so small relative to the benefits to be derived that it should be placed into operational use immediately.

    REFERENCES

    1. Carter, W.E. and R.L. Shrestha, "Airborne Laser Swath Mapping: Instant Snapshots of Our Changing Beaches," In Proceedings of the Fourth International Conference: Remote Sensing for Marine and Coastal Environments," Environmental Research Institute of Michigan (ERIM), P.O. Box 134001, Ann Arbor, MI 48113-4001, USA, Vol. I, pp. 298 - 307, 1997.

    2. Carter, W. E, R. L. Shrestha, P.Y. Thompson, and R. G. Dean; "Project LASER: Final Report to FDEP," Department of Civil Engineering, University of Florida, Gainesville, FL 32611, pp 27, April 4, 1997.

    3. Carter, W. E, R. L. Shrestha, and P.Y. Thompson; "Project LASER: Final Report to Florida Department of Transportation," Department of Civil Engineering, University of Florida, Gainesville, FL 32611, pp 31, June 16, 1997.

    4 Shrestha, R.L., W.E. Carter, P.Y. Thompson, R.G. Dean and H. Harrell, "Coastal & Highway Mapping by Airborne Laser Swath Mapping Technology," presented and published in The Proceedings of Third International Airborne Remote Sensing Conference and Exhibition, Copenhagen, Denmark, Vol. I, pp. 632 - 639, 1997

    5. Milbert, D.S., Computing GPS-Derived Orthometric Heights with Geoid90 Height Model, Presented at ACSM Fall Meeting, GIS/LIS, Atlanta, GA, 1991.


    Copyright 2000 University of Florida  holds all rights reserved. Reproduction in whole or in part in any form or medium without the express written permission of The University of Florida is prohibited. (Article reprinted with permission of Bill Carter). 


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