GPS SYSTEMS VS SEXTANTS: NOBODY’S RIGHT IF EVERYBODY’S WRONG
By Bill Moore

     When all the information is finally compiled, the next step is to plot the location on a base map.  This is done with the use of a three arm protractor called a tritractor.  To make the tritractor usable for the majority of site areas, the arms of the tritractor must be extended by means of glue, tape or even staples.  Most sites are too large to use the tritractor as it is meant to be used,  hence the extensions.  These extensions render the tritractor useless to any degree of accuracy.

     Another way of plotting, other than the tritractor, is to use a mathematical formula called a three bearing fix.   This is done using the distance between the beach markers and the angles from a boat to the beach markers. This formula gives only the tangent of the angle.  The tangent must then be converted to one angle.  However, because the beach markers do not fall in a true north - south straight line, there are no consistent triangulation’s, which is necessary for this formula.  With approximately 120 beach markers,  none in a straight line, this leads to a different mathematical formula every time a location is plotted.

     Once all calculations are done, if all data entries are correct, in order to plot the information into a computer program, you need to convert from polar to rectangular (X\Y) position.  This requires a whole new set of mathematical formula. Math is based on a constant.  There are no constants with these beach markers, therefore  there are no precise site locations being plotted.

     When there is absolutely no constant (beach markers being moved, rough seas, extended tritractors and human error) there can be no accuracy in any two base maps of the same area.  An example from last season is when three different salvors located and plotted a group of three cannon, all lying in the exact same position and direction.  However, when they were plotted, they appeared to be in three different locations.   These cannon have always been in the same place, have been mapped and have not mysteriously moved - human error, changing beach markers and inaccurate sextant readings have made them  appear to move.  A perfect scenario for artifact plotting is  to be able to replace the artifact where it was found - same position, depth and angle, as it was found, hopefully even decades later.  The only way to accomplish this is to have a constant.  Something that is not to be had at this time for the Florida East Coast Shipwreck Project Sites using the present beach marker system.

     The procedure using the beach markers and sextant was good in its time, but like the astrolabe giving way to the sextant, the sextant must now give way to the Global positioning System (GPS) and ultimately the Differential Global Positioning System (DGPS).  This system is a satellite - based navigation system which provides commercial and recreational users a 24 hour, worldwide navigation coverage.  Rather than using land based points, the GPS uses a constellation made up of 24  satellites which continuously send  radio signals containing precise position and time information for each satellite back to earth.

     Knowing the position of any 3 or 4 of these satellites and calculating various time differences between the transmitted signals, the GPS receiver can determine its present position anywhere on earth.  Due to errors deliberately added by the Department of Defense, GPS can only be relied on to provide accuracy  of approximately 100 meters.  If  a GPS receiver is installed in a known surveyed location, then the error inherent in the GPS measurements can be determined by comparing the measurements from the GPS receiver and a known solution.  These error corrections can then be broadcast in real time to other GPS receivers, improving their accuracy to 1- 10 meters.  This is called Differential Global Positioning System (DGPS).

     The GPS and DGPS systems allow a position to be located using coordinates of latitude and longitude, or Universal Transverse Mercado (UTM).  As it stands now, salvors must use the UTM system in accordance to state regulations.  By using the GPS positioning system, the plotting of artifacts becomes easy.  One calculation from a known Department of Natural Resource (DNR) marker or any other constant surveyed point, is all that is needed.  An artifact location plotted one year can be plotted in the same location years later.  Also, because the positioning and plotting are done through precise computer calculations, no sextant, tritractor or human error is involved.

     In 1995, some GPS and well as DGPS information was compiled. The severity of the summer storms limited the workable days of the salvors and thus inhibited a good test trial of the GPS and DGPS systems.  However, one test conducted by Salvor’s Inc. lab director Bill Moore and  Salvor’s Inc. archaeologist, Dennis Peebles proved the accuracy of the DGPS system.  This was done by taking a Garmen 45 GPS unit equipped with a Differential Corrections Inc. (DCI provides differential corrections by way of FM radio signals) modem and going to DNR marker #189 in Brevard County.  According to DNR marker location printout received from State of Florida Agent Tommy Gore 4/29/93, the lat./long. numbers for DNR marker #189 are: Latitude 27  55’  55.702”,
Longitude   80  29’  19.704”. The readings received from the DGPS system used by the Salvor’s Inc. personnel as of 5/10/95 were: Latitude  27  55’  55.65”
Longitude  80   29’  19.95”.

     The studies thus far show solid indications that the GPS and DGPS systems will provide the accurate information required for the Florida East Coast Shipwreck Projects.  Salvor’s Inc.’s confidence in this method is such that  they already have the electronic components to set  up and work from a differential system as well as the computer capabilities to extrapolate and utilize all necessary information in whatever guise is handed to them.

Click This Line to Review the Results of the GPS Trial Figure 1
Click This Line to Review the Results of the GPS Trial Figure 2

     Though not thoroughly tested within the salvage ranks, our hope is that the State allows the use of GPS and DGPS systems by the Florida East Coast Shipwreck Project instead of the sextant and beach marker system.   It is time to move beyond the shifting of the beach markers, the angles of the sextant, past the inaccuracies of the tritractors and into the technologies that are available.

     With the use of these technologies. the beach marker, sextant, and  base map system has become antiquated.  The time consuming and costly seasonal necessity of  placing, replacing, and maintaining the beach markers is no longer needed, thus saving the State of Florida and  Salvor’s Inc. time and money.  The computer aspect of these technologies will allow us to take all location data and plot it on an x/y scatter graph  using lat./long. or UTM coordinates. (see GPS workschart #1).  This effectively takes the  place of the base map, again, saving the State of Florida and Salvor’s Inc. time and money.

     The monthly activity analysis reports, artifact information and location would be provided by log sheets and a computer disk only,  thus eliminating the majority of time consuming data entry and expensive paper work now done by the
State.  The State of Florida would no longer need to enter data onto the computer, as Salvor’s Inc. will have provided the information in disk form already, again cutting back on time and expense.

      The use of these technologies have already provided Salvor’s Inc.  with accurate data in a more consistent and cost effective manner.  We hope the state will adopt the use of these new technologies so that the archaeological data from the Florida East Coast Shipwreck Project can be  used now and as a model for future maritime archaeological projects.

     Febuary 9,1996
     Jim Dunbar
     Division of Historical Resources
     RA Gray Building
     50 South Bronough
     Tallahassee, FL 32399

     Dear Jim,

     Yesterday, I had a lengthy meeting with Bill Moore and John Brandon, in which we discussed the transition from sextant to DGPS. Many points were brought to my attention which game me second thoughts on my recommendation of Febuary 2, 1996.

One of the main issues was the lack of constant and reliable differential transmissions. I would like to change my reommendations to the following.

1. We request that beach markers be resurveyed and maintained for the 1996 season. However, in order to save the integrity of most of the old data, it would be beneficial for the State to obtain the actual latitude and longitude during the survey.

2. During the 1996 season, DGPS be highly recommended but not required to all subcontractors. DGPS would be in addition to the required sextant.

3. During the 1996 season, Salvors, Inc. and the State of Florida do their best to obtain as many DGPS readings, as possible, on large structural and large non-structural artifacts, such as cannons, anchors, and timbers.

4. During the 1997 season, we start phasing out beach markers by not resurveying them but leaving the remaining ones in place. It would also be required that all subcontractors are equipped with DGPS.

5. Base maps for 1996 should not be required. Alternatively, Salvors, Inc. could provide the state with floppy discs, arranged in the Microsoft Access program, which contain all of the data from the log sheets including sextant readings.

     Please contact me if you have any Questions

     Best regards,
     Taffi Fisher