Precise Airborne LiDAR Surveying For Coastal Research And Ge

PRECISE AIRBORNE LIDAR SURVEYING FOR COASTAL RESEARCH AND GEOHAZARDS APPLICATIONS

Roberto Gutierrez, James C. Gibeaut, Rebecca C. Smyth, Tiffany L. Hepner, John R. Andrews

Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas, USA

oskar@mail.utexas.eduChristopher Weed

Center for Space Research, The University of Texas at Austin, Austin, Texas, USA

William Gutelius

Optech, Inc., Toronto, Canada

Mark Mastin

U.S. Geological Survey, Tacoma, Washington, USA

KEYWORDS: LIDAR, laser, ALSM, calibration, natural hazards, shoreline mappingABSTRACT

The monitoring and analysis of many natural hazards requires repeated measurements of a topographic surface whose change reflects somegeologic or hydrologic process. The development of airborne laser surface mapping (ALSM) allows the study of natural hazards over areastens to hundreds of kilometers in extent with a horizontal resolution of 1 meter or less and a vertical accuracy of 0.10-to-0.15m. Changedetection requires that repeated ALSM surveys be precise and accurate. Repeatability is a function of the stability and calibration of theinstrument, the accuracy of GPS aircraft trajectories, the density and completeness of ALSM data coverage, the availability of “ground truth”information, and the accuracy and flexibility of ALSM data classification. Since 1997 The University of Texas at Austin (UT) has mappedvarious portions of the Texas Gulf coast using several small-footprint, scanning ALSM systems developed by Optech, Inc. During summer2000, UT comprehensively mapped the Texas coast from Sabine Pass on the Texas-Louisiana border to the mouth of the Rio Grande River.These data provide a series of Gulf shorelines for estimating beach erosion rates and computing volumetric sand loss. The high-resolutionbeach and dune topography derived from ALSM will help characterize the susceptibility of the coast to hurricane overwash and storm-relatedflooding. In another project UT collaborated with Optech and the U.S. Geological Survey in March 2000 to survey fifteen municipalities inHonduras with ALSM as part of the USAID Hurricane Mitch Recovery program. Digital elevation models produced from these data arebeing used for flood and landslide hazard analysis. During these and other projects, UT began implementing procedures for instrumentcalibration, data classification, and ground GPS surveying that enhance the repeatability of our ALSM surveys.

1 INTRODUCTION

The Bureau of Economic Geology (BEG), a geologic andenvironmental research group within the University of Texas atAustin (UT), is the state agency responsible for providingshoreline information to the Texas legislature and stateregulatory agencies. Because of the requirement for accurateshoreline data, the BEG began a program in airborne lasersurface mapping (ALSM) in collaboration with the UT Centerfor Space Research, and Optech, Inc. This program began with ashoreline survey in December 1997 using an ALSM systemprovided by Optech (Gutierrez et al, 1998). In July 2000, UTacquired an Optech ALTM 1225 instrument, a 25kHz scanninglaser mapping system. In this paper we describe our currentALSM program and how we are implementing geodetictechniques into our operations. We also discuss some resultsfrom our Texas shoreline mapping and a flood-hazard mappingproject in Honduras, C.A.

2 METHODS

NASA began developing ALSM technology in the 1980’s andseveral instruments (RASCAL, SLICER, AOL, LVIS, ATM)were developed for terrain, vegetation, and ice sheet mapping(Rabine et al, 1996; Harding et al, 2000; Krabill et al, 1995;Blair et al, 1999, Krabill et al, 2000). Commercial ALSMsystems became available as the technology matured. Optechdeveloped the ALTM 1020, a compact scanning ALSM systemwith a 5kHz laser repetition pulse rate, in 1995. Increases inlaser power, laser pulse rate, and overall system performancewere incorporated by Optech in subsequent models with the

ALTM 1225 system appearing in 1999. The ALTM 1225 has thefollowing specifications: Operating altitude410-2,000 m AGL Laser pulse rate25 kHz Laser scan anglevariable from 0 to ± 20° from nadir Scanning frequencyvariable, 28 Hz at the 20° scan angle. Beam divergence0.2 milliradian (half angle, 1/e)

The ALTM 1225 does not digitize and record the waveform of thelaser reflection, but records the range and backscatter intensity ofthe first and last laser reflection using a constant-fractiondiscriminator and two Timing Interval Meters (TIM).

ALSM elevation points are computed using three sets of data: laserranges and their associated scan angles, platform position andorientation information, and calibration data and mountingparameters (Wehr and Lohr, 1999). Global Positioning System(GPS) receivers in the aircraft and on the ground provide platformpositioning. The GPS receivers record pseudo-range and phaseinformation for post-processing. Platform orientation informationcomes from an Inertial Measurement Unit (IMU) containing threeorthogonal accelerometers and gyroscopes. An aided-InertialNavigation System (INS) solution for the aircraft’s attitude isestimated from the IMU output and the GPS information.2.1 Calibration

There are no standard instrument calibration procedures, eachequipment manufacturer and ALSM group have developed its owntechniques (Wehr and Lohr, 1999). The instrument calibration forour Optech ALTM 1225 includes the estimation of the scanner roll

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