AIRBORNE LASER TERRAIN MAPPING (ALTM)
(also known as airborne LIDAR, which stands for Light Detection And Ranging)
Shoreline erosion and flooding are coastal hazards that underscore the need for accurate topographic information along coastal areas. When combined with ground surveys, ALTM can rapidly collect topographic data that are detailed and accurate enough to measure dune and beach topography and to detect change. We are conducting repeated airborne laser altimeter surveys, shoreline change detection, and land-cover classification.
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Figure 1. The Optech ALTM-1020 system (we are now using an Optech ALTM 1225 system) combines a pulsed, solid-state laser, an inertial measuring unit (IMU), and a geodetic GPS receiver in a compact and modular configuration. The laser determines the distance to the target, the IMU (accelerometers and gyroscopes) monitors the aircraft attitude, and the GPS receiver provides aircraft position data. Rotating optics in the instrument's sensor head scans the laser across the ground, illuminating a swath under the aircraft. |
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Figure 2. The OPTECH ALTM is installed in a Cessna 206 (other aircraft can be used). The laser sensor head is floor mounted over the camera port and the equipment rack and Ashtech Z-12 GPS receiver are placed in the cargo space behind the ALTM operator's seat. Trimble 4000SSI GPS receivers collect data on the ground. |
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Figure 3. On 8 November 1997 we flew 34 flightlines over Bolivar Peninsula and Galveston Island. We set the ALTM-1020 at a laser pulse rate of 2000 Hz and a scan angle of ±20o from nadir. The aircraft speed was 46 meters per second. The resulting ground swaths had a width of 340-540 m with 30% overlap. The ALTM-1020 can operate at laser pulse rates up to 5000Hz, but because of technical problems we could not acquire accurate data at rates above 2000 Hz. Nonetheless, we were able to map more than 163 km2 in one day ( 40 km2 per hour) with a horizontal data spacing of 2 to 6 m. In 1998 we returned for additional and repeat surveys in the area. |
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Figure 4. Shaded relief of southwest end of Bolivar Peninsula. Topographic data acquired by ALTM. The Intracoastal Waterway (ICW) with barge traffic is visible along the upper left-hand edge of the image. A large rectangular pile of dredge spoil forms a topographic high to the northeast of Port Bolivar. Identifiable geomorphic features include the shoreline, beach and foredunes, recurved spits, beach ridges, and small tidal creeks. The recurved spits and tidal flats immediately northeast of the North Jetty are the current site of peninsular accretion. In the center of the peninsula are a series of straight ridge and swale features that represent accretion along the peninsula prior to the construction of the North Jetty. |
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Figure 5. Three-dimensional contoured image of a 650-m-long portion of the Galveston seawall. The swath width is approximately 340 m. The image shows shorefront buildings, several piers and a groin extending from the seawall, and a gently sloping beach in front of the seawall. The chaotic contours in front of the beach represent the surf zone. Cars parked and driving along the top of the seawall are visible as small, closed contours. |
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Figure 6. The shore-normal beach profile A-A' traverses an undeveloped portion of the peninsula and extends from the waterline across the beach, foredunes, and the barrier flat (see fig. 4 for location). The day after the ALTM mapping flight we conducted a rapid-static GPS survey to measure the topography along this beach profile, fig. 4. These ALTM elevation points were adjusted upward by 0.184 m, the mean difference between the GPS road survey and the ALTM in Table 1. We estimated the sand volume for beach and foredunes of the GPS ground and ALTM profiles. The ALTM profile overestimated the sand volume in the beach and dunes by 40% in comparison to the GPS profile. This volume error is due to dense vegetation in the dune and interdune portion of the profile. Vegetation coverage on seaward side of the dunes is typically 20-50%, and the dominant plant species are relatively short. Vegetation coverage increases to 80-90% on the backside of the dunes and the interdune area. As a result ALTM elevation errors are as large as +0.4 m on the backside of dunes and in the interdune areas. |
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Figure 7. We merged the 1997 ALTM data, approximately 14.4 million data points, from all flightlines to create a digital elevation model with a 5 x 5 m horizontal resolution. Major cultural features in the DEM include the Intracoastal Waterway (ICW) and two towns, Port Bolivar and Crystal Beach. The history of the peninsula is apparent in the topographic variations mapped by ALTM. A large, recurved spit is visible in the middle of the peninsula, cutting across the ICW and a composite washover fan. This spit and fan are relicts from the early, migratory phase of the peninsula's development. Ridge and swale topography running through the center of the peninsula represents a more recent accretionary phase. The highest natural feature in the DEM is a 3-m-high beach ridge running along the center of the peninsula from Port Bolivar through Crystal Beach. |
ALTM ACCURACY
To help evaluate the ALTM data we conducted GPS ground surveys along 50 km of roads on Bolivar Peninsula. Table 1 describes the mean elevation difference (de) between the road elevation determined by ground GPS and by ALTM. Table 1 also shows the associated standard deviations (s), the number of elevation pairs, and the approximate aircraft altitude for the ALTM data. This comparison between ALTM and ground GPS surveys indicates that the ALTM elevation errors contain an elevation bias (mean de) and a noise component (s). The ALTM elevation bias includes the laser instrument calibration error, the atmospheric refraction of the laser path between the aircraft and the ground, and the GPS height errors due to tropospheric delay. The noise component in the ALTM data is the sum of random errors in laser range measurements, in the GPS aircraft positioning, and in the measurements of the scan angle or aircraft attitude. We interpret the associated values of 0.12 and 0.15 m as representing the RMS error of the entire ALTM-1020 system including the supporting geodetic GPS equipment.
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The 30% overlap between flightlines allowed us to compare the elevations measured by successive ALTM mapping swaths. We examined those portions of flightlines 1,2,11,12,13, and 14 that mapped the peninsula's barrier flat, a region with few trees or buildings and very low relief. We computed the mean elevation difference (de) between these adjacent flightlines and the standard deviation (s) of these elevation differences. Table 2 describes the de and s for overlapping flightlines 1-2, 11-12, 12-13, and 13-14. The de represents the variation in ALTM elevation bias from flightline to flightline. In the overlap area between adjacent flightlines, measurement errors in the laser geometry are exaggerated. Therefore the de between the overlapping edges of two adjacent mapping swaths is a measure of the accuracy of the IMU and the optical scanning system. The s in Table 2 indicate that the IMU and optical scanning components in the ALTM system have a combined accuracy of about 0.025o to 0.05o.
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Rollover
Pass, Bolivar Peninsula
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Figure 8. ALTM surveying is proving very useful for beach and dune mapping. The above image displays data acquired with one pass of the aircraft flying at 500 m above ground level. The scalloped landward (at top) boundary of the point distributions is caused by aircraft roll. The number of laser returns is greatly diminished over water. The shaded relief view shows detail in the beach and the contour map, with one contour highlighted, illustrates the potential to map shorelines using a contour line. |
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Rollover
Pass, Bolivar Peninsula
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Height(HAE)
Difference Between 1997 and 1998 ALTM DEMs
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Figure 9. The above images show the level of detail obtainable for topographic change analysis. The 1997 DEM was subtracted from the 1998 DEM. Note that the ocean level was higher during the 1998 flight than during the 1997 flight. ALTM surveys may also be useful for studying coastal circulation and wave refraction phenomena. |
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Enlarged
View of HAE Differences: 1998-1997
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Figure 10. Left: Beach house straddling the beach scarp in November 1997 has been moved inland by August 1998 to prevent destruction. Other houses are outlined in apparent HAE differences, an artifact created by the poorer ALTM coverage in 1997. Figure 10. Right: Increase in beach elevation caused by sand accretion against the downdrift side of the Rollover Pass jetty. Elevation differences also caused by variations in the vehicles parked on the jetty. |
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Figure 11. ALTM data combined with imagery is proving useful for delineating environments on barrier islands. This oblique view (left) of the southwestern end of Bolivar Peninsula was created by draping a color infra red (IR) digital orthophoto on a 5 m x 5 m grid of the ALTM data. The vertical aerial photograph was acquired in February, 1995, and was georeferenced using GPS ground control points under the Texas Orthophoto Program. The scanned image has a 2.5 m resolution. In the color IR photo, vegetation appears as various shades of red. Barren areas, such as roads, houses, and the beach, are gray or white, and water is blue to black. Vertical exaggeration is about 25x. The view is to the north with the Gulf of Mexico on the right. Housing subdivisions, trees, dunes, tidal creeks, ridge and swale topography with dark water in the swales, and a coastal marsh and tidal flat area at the top of the scene are evident. The right image is an enlargement of the beach and shows well the vegetated foredune and coppice mounds and houses. |
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For more information, please contact Jeff Paine