Fig. 4. LIDAR survey of Merrimack River Inlet ebb tidal delta.

Fitzgerald-etal-CEJ-03
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laser pulse from a plane, one infrared and one green. Two return signals are recorded

-- the first, the infrared signal from the water's surface and the second, the green

from the seafloor (USACE 2001). Water depth is determined by the time lapse

between the two signals. The operational depth of the SHOALS system is limited

by water clarity and substrate type. Generally this is not more than 4060 m or 2

3 times the Secchi depth (Irish and Lillycrop, 1999, Parson, Lillycrop, and McClung,

1999, USACE 2001). The SHOALS system also has the capability to map subaerial

areas. The SHOALS system conforms to USACE Class 1 and International Hydro-

graphic Organization Standards (Riley, 1995). The vertical accuracy of the SHOALS

system is 15 cm and the vertical accuracy is 3 m with differential GPS and 1 m

with Kinematic GPS (Irish and Lillycrop, 1999, USACE 2001).

Fig. 4. LIDAR survey of Merrimack River Inlet ebb tidal delta. Note the large sandwaves that are

migrating seaward in the main ebb channel (from FitzGerald et al., 2002).