GPS surveyed time-invariant sea floor depths for safe navigation

Computing ship’s two clearances while underway

A. Clearance for ship’s keel – When the keel clearance (BD) is required, the mariner first surveys the geodetic position (f, ?, h) of the ship’s mainmast using GPS. Then, using the general arrangement drawings for the particular vessel, and combining with the ellipsoidal depth or height of the sea floor, the clearance can be computed (when ellipsoid is below sea floor) as

BD = BC – DC = [BF – CF] – DC

where BC is surveyed while underway and DC is taken from the database. Similarly the freeboard distance to the Plimsoll marks can be readily known.

B. Clearance for ship’s mainmast – The clearance for the mainmast GF can be computed as

GF = CG – CF

where CF is surveyed with GPS while underway and CG is taken from the database.

Ellipsoid depths and soundings accuracy

In open ocean areas, the accuracy of an ellipsoidal height (h) with GPS will be about ± 5 meters and thus the sea floor location with this order of accuracy will be suffi cient for most practical purposes. In shallow waters, along coastlines, and inside harbors, specially designed Differential GPS (DGPS) surveys can measure ellipsoidal heights in the range of ± 5-10 cm. The accuracy for keel clearance BD will be as per presently used acoustic soundings. To match the DGPS accuracy, it will be worthwhile that acoustic sounding techniques, procedures, and algorithms are examined again and updated as necessary.

Special surveying for high accuracy

In congested areas, the accuracy of ellipsoidal heights and depths can be increased by surveying in DGPS mode. The following additional surveying procedures will help to improve the collected data for safer navigation, particularly to the traditional mariner:

1. In harbors, buoys with small DGPS receivers can monitor AC as it fl uctuates with the tide, to provide real-time data to the ship’s captain. Similarly the ship’s draft AD can be monitored with a keel-placed pressure sensor.

2. In cases where the sea fl oor heights vary signifi cantly from one time to another, it will require regular DGPS surveys to keep the mariner informed of shoaling and other shifts in the channels.

3. In case of small ships, DGPS surveys for roll, pitch, and yaw will improve the accuracy of the ellipsoidal heights of the keel and its clearance with the sea floor.

Conclusions

GPS and DGPS surveys provide the accuracy necessary for the ellipsoidal depths and ship operations, whereby all measurements are referenced to the WGS 84 Ellipsoid. Thus, an invariant zero reference surface (or a vertical datum) will eliminate the necessity of measuring tides and ship’s draft, settlement(s), and squat during bathymetric surveying. In addition, this approach will replace any the time- variant tidal surfaces. The ellipsoid as the zero reference surface also allows the mariner, while underway, to determine keel and overhead obstruction clearance independent of the stage of the tide, and the draft of the ship and freeboard. As is traditional however, the prudent seaman will seek independent verifi cation of all the available nautical information.

Copyright. We take the copyright for this geodetic approach in this manuscript.

The authors would like to acknowledge that the paper was first published in GPS Word Sep 2005.