Range-azimuth

Table 4

Accuracy of Common Positioning Systems (from USACE

EM 1110-2-1003)

Positioning System

Estimated Accuracy, Meters RMS

Range-azimuth

0.5 to 3

LORAN-C (low-frequency)

50 to 2,000

Microwave (high-frequency)

1 to 4

GPS

50 to 100

DGPS

0.1 to 1.0

buoy placement), and the exact dredged material placement location is sub-

ject only to the tug or dredge captain's discretion of buoy offset distance.

Placement offset from the buoy depends on local weather and safety con-

cerns. Specific guidance varies from site to site, but the New England

Division has found success with specifying placement within 25 to 50 m

of buoy location depending on weather/sea conditions. Experience has

shown that this type of placement tends to concentrate material at one

point or in a transect along the direction of travel of the tug and barge.

This factor should be taken into consideration in buoy placement or in

placement specifications for tug operators.

Range-azimuth

Range-azimuth positioning is a traditional surveying technique where a

shore-based station (transit, theodolite, or total station) is used to determine

an angular azimuth to the vessel of interest. This azimuth is then coupled

with an electronically determined distance obtained from an electronic

distance measurement (EDM) device (microwave EPS, laser EDM, or in-

frared EDM) at the same location. Range-azimuth positioning is very ac-

curate, but because of the shore station requirement, it is applicable only

at sites where dredged material placement is relatively close to shore

(USACE EM 1110-2-1003). Range-azimuth positioning has been used by

the Seattle District for several capping projects, e.g., the Duwamish Water

project in 1984 (Truitt 1986b) and the Denny Way project (Sumeri 1989).

Electronic positioning systems (EPS)

Generally, the higher the frequency is of EPS, the more accurate the po-

sitioning. LORAN-C is a low-frequency, time-differencing hyperbolic

phase/pulse system that triangulates vessel position based on relative dis-

tances from shore-based stations. Because LORAN-C is a low-frequency

system, it has a low accuracy and is the least desirable for vessel position-

ing. For hydrographic surveys, LORAN-C is only suitable for Class 3

surveys (reconnaissance level), and absolute accuracy without onsite cali-

Chapter 5 Equipment and Placement Techniques