Results are presented from topographic surveys of the Assateague Island National Seashore using an airborne scanning laser altimeter and kinematic Global Positioning System [GPS) technology. The instrument used was the Airborne Topographic Mapper (ATM), developed by the NASA Arctic Ice Mapping [AIM) group from the Goddard Space Flight Center's Wallops Flight Facility. In November, 1995, and again in May, 1996, these topographic surveys were flown as a functionality check prior to conducting missions to measure the elevation of extensive sections of the Greenland Ice Sheet as part of NASA's Global Climate Change program. Differences between overlapping portions of both surveys are compared for quality control. An independent assessment of the accumcy of the ATM survey is provided by comparison to surface surveys which were conducted using standard techniques. The goal of these projects is to make these measurements to an accuracy of 2 10 cm. Differences between the fall 1995 and 1996 surveys provides an assessment of net changes in the beach morphology over an annual cycle. Introduction Beaches are one of the most dynamic geologic (sedimentary) features on Earth. Fluxes in beach morphology occur over a wide spectrum of time scales ranging from periods of hours associated with diurnal tides and storm events to years and decades in response to longer term erosional trends. On a geological scale, beaches follow gross changes in sea level during periods of glaciation and glacial retreat. However, anthropogenic activities, especially during the past century, have created a situation where erosion of beaches has severe economic consequences. Thirty of the nation's 50 states have coastlines on the Atlantic or Pacific Oceans, the Gulf of Mexico, or the Great Lakes. These 30 states contain approximately 85 percent of the nation's population, and about half of this population resides within the coastal zone (Leatherman and Dean, 1991). The U.S. coastline is more than 20,000 kilometers in length. Remote sensing offers the only possibility for producW.B. Krabill and C.W. Wright are with the National Aeronautics 1 and Space Administration, Laboratory for Hydrospheric Processes, Wallops Flight Facility, Wallops Island, VA 23337 (krabill@osb.wff.nasa.gov). R.N. Swift, E.B. Frederick, S.S. Manizade, and J.K. Yungel are 1 with EGM: Services, Wallops Island, VA 23337. C.E Martin and J.G. Sonntag are with EG&G Services, Gaithersburg, MD 20878. M. Duffy and W. Hulslander are with the National Park Service, Assateague Island National Seashore, Berlin, MD 21811. J.C. Brock was with the NOAA Coastal Services Center, Charleston, SC 29407. He is presently with the USGS Center for Coastal Geology, St. Petersburg, FL 33701. ing a time series of elevation surveys of sufficient density to permit these valuable resources to be monitored. Airborne scanning laser altimetry currently offers a strong potential to provide such accurate, detailed, and comprehensive surveys. Annual surveys could be repeated to facilitate an understanding of long-term erosional trends or gauge effects of dredging, beach replenishment, and erosion control structures such as groins. Regional surveys could be conducted following the passage of major storms such as hurricanes and northeasters to quantify the resulting erosion/deposition and permit rapid identification of beach areas which are at risk due to the removal of sand from protective dunes. Survey input could be used by the National Flood hsurance Program (NFIP) of the Federal Emergency Management Agency to make decisions on property coverage regulations. NFIP currently offers flood insurance protection to -1,200 coastal communities amounting to 1.4 million policies and over $120 billion in coverage. The survey of beaches along the entire expanse of the U.S. coast could be accomplished with a modest number of airborne scanning laser altimeters. A comprehensive review of available commercial airborne scanning laser altimeters were given by Flood and Gutelius (1997), and academic researchers have used the technology to study changes in beaches caused by tropical storms and hurricanes (Carter and Shrestha, 1997). Additionally, the federal government operates several such sensors both for research and operational applications (Krabill et al., 1995; Lillycrop eta]., 1996). General Discussion As a demonstration of the application of airborne remote sensing for beach monitoring, the northern portion of Assateague Island has been topographically mapped using an airborne scanning laser altimeter combined with kinematic Global Positioning System (GPS) technology. The site, shown in Figure 1, was initially surveyed with the NASA Airborne Topographic Mapper (ATM) in November, 1995, to evaluate the sensor for use in the Arctic Ice Mapping (AIM] Project (Krabill et al., 19961, a NASA Earth Science program that monitors changes in the height of the large ice sheet that covers most of Greenland. The ATM group, based at Goddard Space Flight Center's Wallops Flight Facility (WFF), has been gathering baseline elevation measurements in surveys regionally distributed over the Greenland ice sheet in annual field deployments between 1993 and 1998. Assateague Island was selected as a test site because of its proximity to WFF and because the beach sand has a Photogrammetric Engineering & Remote Sensing Vol. 66, No. 1, January 2000, pp. 65-71. 0099-1 112/00/6601-00OS3.00/0 tl 2000 Anlerican Society for Photogrammetry

A scanning airborne topographic lidar was evaluated for its ability to quantify beach topography and changes during the Sandy Duck experiment in 1997 along the North Carolina coast. Elevation estimates, acquired with NASA's Airborne Topographic Mapper (ATM), were compared to elevations measured with three types of ground-based measurements-1) differential GPS equipped all-terrain vehicle (ATV) that surveyed a 3-km reach of beach from the shoreline to the dune, 2) GPS antenna mounted on a stadia rod used to intensely survey a different 100 m reach of beach, and 3) a second GPS-equipped ATV that surveyed a 70-km-long transect along the coast. Over 40,000 individual intercomparisons between ATM and ground surveys were calculated. RMS vertical differences associated with the ATM when compared to ground measurements ranged from 13 to 19 em. Considering all of the intercomparisons together, RMS similar or equal to15 cm. This RMS error represents a total error for individual elevation estimates including uncertainties associated with random and mean errors. The latter was the largest source of error and was attributed to drift in differential GPS.The 15 cm vertical accuracy of the ATM is adequate to resolve beach-change signals typical of the impact of storms. For example, ATM surveys of Assateague Island (spanning the border of MD and VA) prior to and immediately following a severe northeaster showed vertical beach changes in places greater than 2 m, much greater than expected errors associated with the ATM. A major asset of airborne lidar is the high spatial data density. Measurements of elevation are acquired every few m(2) over regional scales of hundreds of kilometers. Hence, many scales of beach morphology and change can be resolved, from beach cusps tens of meters in wavelength to entire coastal cells comprising tens to hundreds of kilometers of coast. Topographic lidars similar to the ATM are becoming increasingly available from commercial vendors and should, in the future, be widely used in beach surveying.

Airborne laser altimetry has been used during the past decade to measure the surface elevation of the Greenland ice sheet. These measurements have been made using a scanning laser on a NASA P-3 aircraft which was positioned by differential GPS and flown approximately 500 m above the surface. Flights have been made over major portions of the ice sheet and reflown 5 years later in order to obtain estimates of the rate of overall change of surface elevation. The accuracy with which differential elevations can be made depends upon (a) the GPS positioning accuracy, (b) the instrument calibration accuracy, (c) the stability of the laser and, (d) the accuracy of the aircraft inertial navigation system's estimation of aircraft attitude. Overall, the accuracy of an elevation change estimate is computed to be 8.5 cm over small areas and 7.1 cm when averaged over tens of kilometers as is needed for estimating ice volume changes. This effort supports±1.4 cm/year resolution for long period surface elevation changes from data acquired which are separated by 5 years. Results of inflight data analyses are consistent with these accuracy estimates.

Results are presented from topographic surveys of the Assateague Island National Seashore using an airborne scanning laser altimeter and kinematic Global Positioning System (GPS) technology. The instrument used was the Airborne Topographic Mapper (ATM), developed by the NASA Arctic Ice Mapping (AIM) group from the Goddard Space Flight Center's Wallops Flight Facility. In November, 1995, and again in May, 1996, these topographic surveys were flown as a functionality check prior to conducting missions to measure the elevation of extensive sections of the Greenland Ice Sheet as part of NASA's Global Climate Change program. Differences between overlapping portions of both surveys are compared for quality control. An independent assessment of the accuracy of the ATM survey is provided by comparison to surface surveys which were conducted using standard techniques. The goal of these projects is to make these measurements to an accuracy of +/- 10 cm. Differences between the fall 1995 and 1996 surveys provides an assessment of net changes in the beach morphology over an annual cycle.

Aircraft laser‐altimeter surveys in 1993 and 1998 over Kangerdlugssuaq Glacier in east Greenland reveal thinning, over the 5‐year interim, of several meters for all surveyed areas within 70 km of the seaward ice front, rising to 50 meters in the final 5 km. Such rapid thinning is best explained by increased discharge velocities and associated creep thinning, most probably caused by enhanced lubrication of the glacier bed. The calving ice front over the past decade has occupied approximately the same location as in 1966. Velocity estimates for 1995/96 are about the same as those for 1966 and 1988, but significantly less than for 1999, suggesting that major thinning began after 1995.

During September 1991, April 1992 and June/July 1993, a NASA P-3 aircraft, equipped with a scanning laser altimeter, flew numerous transects of the Greenland ice sheet. The aeroplane location was measured precisely using differential Global Positioning System (GPS) surveying techniques, allowing all altimetry data to be converted into measurements of ice-surface elevation relative to the Earth ellipsoid. Results from flight data indicate that ice-surface elevations can be reliably measured to an accuracy of similar to 20 cm (and possibly to similar to 10 cm) over baselines of more than seven hundred kilometres.

Range measurements made by satellite radar altimeters experience a bias toward the troughs of ocean waves. A series of aircraft flights during February–April 1989 measured this electromagnetic (EM) bias at three radar frequencies and the UV under a variety of wind and wave conditions, and provided the first airborne open‐ocean measurements at the 13.6‐GHz and 5.3‐GHz operating frequencies of the NASA altimeter on the TOPEX/Poseidon satellite. The data suggest that the mean EM bias decreases linearly with increasing radar frequency between 5.3 and 36 GHz, according to the expression: EM bias (% of significant wave height) = (3.0–0.0617 F)(1±0.5), where F is in gigahertz. EM bias is fairly constant over a mesoscale region on a given day but can fluctuate significantly from one day to another. It shows a strong increase at all radar frequencies with increasing wind speed, although other sea state conditions, such as the wind direction relative to the wave direction, are also factors.

Initial base-line field test performance results of the National Aeronautics and Space Administration's airborne oceanographic lidar (AOL) in the bathymetry mode are presented. Flight tests over the Atlantic Ocean yielded water depth measurements to 10 m. Water depths to 4.6 m were measured in the more turbid Chesapeake Bay. Water-truth measurements of depth and beam attenuation coefficients by boat were taken at the same time as the aircraft overflights to aid in determining the system's operational performance. Beam attenuation coefficient a and depth d product alphad was established early in the program as the performance criterion index. A performance product of 6 was determined to be the goal. This performance goal was successfully met or exceeded in the large number of field tests executed. Included are selected data from nadir-angle tests conducted at 0 degrees , 5 degrees , 10 degrees , and 15 degrees . Field-of-view data chosen from the 2-, 5-, 10-, and 20-mrad tests are also presented. Depth measurements obtained to altitudes of 456 m are given for additional comparison. This laser bathymetry system represents a significant improvement over prior models in that (1) the complete surface-to-bottom pulse waveform is digitally recorded on magnetic tape at a rate of 400 pulse waveforms/sec, and (2) wide-swath mapping data may be routinely acquired using the 30 degrees full-angle conical scanner. Space does not allow all the 5,000,000 laser soundings to be included. Qualified interested users may obtain complete data sets for their own in-depth analysis.

The development of the seam efficiency test to measure the sewability of a fabric is described. It is shown that this test, which involves the establishment of a ratio between the strength of the original fabric and the strength of the seamed fabric, is more realistic, reproducible, and sensitive than the previous test, which rated a fabric for sewability on the basis of the number of fabric yarns cut per inch during the sewing operation.