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LASER TERRAIN MAPPING (ALTM)
known as airborne LIDAR, which stands for Light Detection And Ranging)
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.
images to enlarge
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
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.
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.
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.
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.
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
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.
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.
1. Differences in Road Elevation Between GPS Ground Surveys and
over surveyed roads
HAE difference (de)
Table 2. Elevation Difference Between Overlapping ALTM Flightlines
HAE difference (de)
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
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.
Pass, Bolivar Peninsula
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.
Pass, Bolivar Peninsula
Difference Between 1997 and 1998 ALTM DEMs
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.
View of HAE Differences: 1998-1997
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.
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
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.