GPS surveyed time-invariant sea floor depths for safe navigation

For safe sailing, two of the most important requirements are to be able to determine the clearance between 1) the sea floor and the keel of the ship and 2) the mainmast and bridges or overhead cables. If these clearances can be measured with respect to a time-invariant zero reference, mariners would have them whenever required. In shallow waters and in harbors, it would enable the captain of a ship to find the clearances accurately and with confidence to avoid grounding or striking overhead obstructions. Combining GPS surveys and acoustic soundings, highly accurate ellipsoid depths of the sea floor can be established in new areas or to fill in gaps independent of the stage of the tide. These time-invariant depths can be stored in a bathymetric database for future use. Similarly, from previously surveyed ellipsoidal heights of the bridges and cables, overhead clearances can be determined. While underway with GPS positioning, this approach will enable the mariner to recover the already established sea floor depths and overhead heights in shallow waters and/or entering any harbor, and compute the ship´s clearance for safe navigation without regard to the time-variant tides and the ship´s draft.