High Precision Near Shore Seafloor Positioning
Authors: Briggs Steven M. 
Published: October 2008

The need to be able to establish high precision seafloor positions has necessitated the development of new equipment to be used with triangulation techniques to determine precise surface positions and transfer those positions to the seafloor. The equipment developed includes a rigid calibrated Spar with an instrument package in its anchor base. The instrument package measures tilt in both degree of inclination and direction of inclination. The surface position is established using three first order triangulation stations and one-second theodolites, and is then transferred to the seafloor.

This procedure was tested in seventy to eighty foot water depths approximately one and one-half miles from shore. The results showed that, relative to the triangulation stations, the seafloor positions were established to within twenty-seven hundredths of a foot. The operations of this test required a very high degree of communication so that all the shore sightings and data from the instrument package could be synchronized.

Introduction

Obtaining precise seafloor positions has always been recognized as a requirement for many operations but has almost always been hindered by the problem of transferring data from the surface to the seafloor. Our firm, working in conjuntion with Naval Facilities Engineering Command and Civil Engineering Laboratory, was requested to provide the highest degree of accuracy possible to provide the surface position of "A Survey Spar System", 1 and assist in the design and performance of these techniques.

Objective

The request was to position two seafloor points in approximately seventy feet of water and approximately one and one-half miles from shore to within plus or minus six inches with relation to National Geodetic Survey First Order Shore Stations. The Spar1 was to be constructed by Civil Engineering Laboratory and would be attached by stakes to the seafloor and allowed to pivot about a universal joint. At the base of the Spar there is an instrument package containing an inclinometer and a compass to detect both direction and degree of tilt.

Procedure Design

Normally, a survey of this type performed on land would require very little planning and would be accommodated by conventional triangulation methods as defined by United States Coast and Geodetic (now National Geodetic) Survey. However, due to the fact that the Spar would be a moving object capable of approximately 15 to 20 feet of swing, the procedure had to be modified to sane degree. In addition, it was decided that we should use instead two triangulation shore stations and that a third station would be utilized, primarily as a check against the first two. All three shore stations would be manned by angulation teams equipped with one-second theodolites. In addition to the three angulation parties, there would be one supervisor on shore capable of moving from station to station in case any problems were encountered. Also on board the vessel would be the eighth member of the team who would be a computer to compute the triangle intersections and provide Naval Facilities Engineering Command personnel with the surface position.