If you really want to find ship wrecks with a GPS read this

[Black Duck]

If you want to find wrecks then read this! Not everyone knows how this works. We have found experienced boat captains that don’t have a clue about much of what we shall explain in this section.

This is from the book Shipwrecks of the Dominican Republic and
A Guide to Shipwreck Identification Through Recovered Artifacts
by Black Duck
and
Robert Splash Klein

A common scenario: You are given a set of GPS coordinates from a friend for a hot fishing spot or a possible wreck site. You take the coordinates, plug them into your GPS and head out to the spot, but when you get there – you can’t find it. You know you are right on the spot, your GPS says so, but there is nothing there except flat sandy bottom. What went wrong?
It is not uncommon for two or three people to put the same coordinates into their respective GPS’s and hit “GO TO” as they head to their destination. As they draw near, they find that they are all going to different places, some as much as two miles apart. What causes that? Well the answer is pretty simple, but you would be surprised at how many people using a GPS today don’t know what is explained below.
Here are the two very basic things that MUST be checked if you are to be successful with a GPS:
1. POSITION
2. MAP DATUM
POSITION. In your GPS there is something called “Position”, usually found in the setup menu. You will find the following three options among others:
• Lat/Lon hddd°mm.mmm (h=hemisphere, ddd=degrees of latitude or longitude, mm.mmm=minutes of latitude or longitude expressed to 1/1,000th of a minute)
• Lat/Lon hddd°mm’ss.s” (h=hemisphere, ddd=degrees of latitude or longitude, mm=minutes of latitude or longitude, ss.s=seconds of latitude or longitude expressed to 1/10th of a second)
• Lat/Lon hddd.ddddd° (h=hemisphere, ddd.ddddd=degrees of latitude or longitude expressed to 1/10,000th of a degree)
It is critical that you know the Position format of the coordinates someone gives you. If they give you one format and your GPS is set to another format, it will be confusing to try to
enter the coordinates, and if you do enter them in some fashion, they will be WRONG. ALWAYS make sure you are talking “apples to apples”.
Map Datum. Map Datum refers to various versions of geodetic datum that have been established over the years. A geodetic datum is the tool used to define the shape and size of the earth, as well as the reference point for the various coordinate systems used in mapping the earth. Throughout time, hundreds of different datums have been used - each one changing with the earth views of the times.
Modern GPS technology supports many Map Datums and allows switching back and forth between them. The critical thing for the GPS user to know is WHAT MAP DATUM WAS USED FOR THE COORDINATES he has been given. If someone recorded coordinates on a GPS set to one Map Datum and you enter those coordinates in your GPS, which is set to a different Map Datum, you could be off by as much as a mile or more.
Datums commonly in use today are: WGS 84, WGS 72, NAD27, NAD83, just to name a few. Most GPS units and chart/map software have well over 100 Map Datum selections. The most common settings in use today are: WGS 84 Datum and Position set to hddd° mm.mmm


To be successful with a GPS:
1. Always have your GPS set to whatever the Position and Map Datum were set at when the sites where recorded.
2. If you are getting GPS coordinates off paper charts/maps, LOOK ON THE CHART OR MAP FOR THE MAP DATUM, and make sure your GPS is set to the same Map datum.
3. When using Google Earth or Garmin Mapsource, or other mapping software, make sure the software settings for Position and Map Datum match those of your GPS. Here’s what you will see in the Google Earth setup:
4. When all else fails, READ THE MANUAL? Black Duck says the manuals don’t tell you what you need to know, and he’s right. I looked in the Garmin manual and it says, “To access the Units Setup page: From the Setup Menu, select Units. Position Format—sets the coordinate system in which a location is shown. Map Datum—sets the description for geographic location for, mapping, and navigation and is not an actual map in the unit.” Well, I must agree with Black Duck, that just doesn’t tell you all you need to know; thus the need for our book.
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[comfortably numb]

You can also use the GPS converter.
http://boulter.com/gps/

 

[Black Duck]

Good point Comfortable diver

If they understand map datum

Thanks for the reply

 

[inletsurf]

Good points! All of this assumes its a known wreck with a known GPS number to begin with. IMO, the most common error in finding wrecks when fishing is that someone acquires a GPS number of a wreck that was converted from an old loran number. I prefer loran numbers so I can do my own loran conversions, pretty much because everyone elses' sucks. Secondly, learning effective search patterns and optimizing your bottomfinder plays a huge part in looking for wrecks.

 

[White Feather]

Great information! Thanks a bunch Black Duck. I had always thought that most NOAA charts were North American Datum 1983 (WGS84) and never thought about other options. All of my GPS systems, including my handheld are all WGS84. I get almost all my coordinates from NOAA charts and figure the final locations myself, but as you pointed out, I have found some acquired locations were off by a good bit. The information you have pointed out is cause for closer attention with other source coordinates. It could be why one set of numbers put me in the wrong place. The numbers were probably on another Datum setting. I'll have to recheck the setting for the numbers and try again. It's an eye opening shock when something so simple has eluded you without even a thought of changing Datums. Sure makes me feel silly. I thought I was on top of my coordinates. I guess I have got to crack the books again. The mind is going.....

 

[Black Duck]

Hi Guys thanks for the comments

I found this out while looking for underwater caves in the Dominican Republic we had three people with three different areas but with the same # but the digest were wrong anyway hope it helps

 

[divewrecks]

Hi Guys thanks for the comments

I found this out while looking for underwater caves in the Dominican Republic we had three people with three different areas but with the same # but the digest were wrong anyway hope it helps

Great post Black Duck! Thanks

Stan

 

[signumops]

The WGS84 datum is a generalized datum designed for general fit world-wide. Therefore it is the principal datum for GPS. NAD83 and NAD27 are geoid fits for the U.S. (North American Datum 1927 and North American Datum 1983). NAD is usually applied to State Plane Coordinate Systems (SPCS) or UTM (Universal Transverse Mercator). Each State in the lower 48 maps itself in a SPCS coordinate system zone. Florida is one of the very few that has 3 zones using 2 different projections, because it is long and wide at the top and long and thin from north to south, occupying 2 different UTM zones. The zone for the treasure coast, and indeed all the way through Key West is zone 901. Almost all SPCS data for most states is now cast in NAD83, U.S. Survey feet. NAD27 used the meter exclusively. In Florida, there are a set of "setback" markers placed by the the D.E.P. which are recorded at www.labins.org. They are registered and mapped in UTM NAD83 coordinates and many shoreline surveys are mapped to them, so, to make use of those surveys using your typical GPS unit, out of the box, you have to convert the UTM to WGS84 Geographic coordinates first, before setting up your own datum points for whatever purpose.

Most other countries use other datums for local mapping, not NAD83 or NAD27. You will find that when you acquire aerial photography from the various states, it will usually be fixed in SPCS and will require recasting to WGS84 before you can make it usable with your GPS unit. The states themselves are usually the ones to pay for the highest resolution, and need the best ground-to-grid fit they can get for legal property descriptions. This means they set things up where X = Y in distance as close as possible. The WGS84 datum, on the other hand was designed by the DOD, so they want it to work everywhere, generally.

Datums are not universal because the earth is not the same ellipsoidal distance from its center in all places at any given latitude. That is the reason why you must know what Datum your data is cast in before you start using it for mapping.

As a rule, you can pinpoint a feature on the ground in Google Earth and determine its WGS84 Geographic coordinates within 100 feet or so, which meet the thousandth's place on a decimal minute of distance. Then compare that with the fix you have on the chart data you are using to see what the difference might be using that chart's datum. To use these types of data, you have to host them in a computer where they can be placed in a real world geographic coordinate grid with the pixel width and height adjusted (geo-referened).

As for NOAA charts showing your GPS position in conflict with their position... buyer beware.

 

[Black Duck]

Signumops

thanks for that reply

But the point I am trying to make is that if you do not understand your GPS and what a datum position is ( not just your map datum) that you are putting in your unit will not find your target

 

[capt dom]

excellent posting!

 

[Black Duck]

Thank you Captain Dom

 

[veryoldthunter]

I can totally agree with Black Duck. I worked on a shipwreck site that had been worked for over 10 years. We wanted to go back and re-work some of the earlier areas where goodies had been found, so went to the very extensive logs and pulled off Coordinates. The first ones we tried ended up on dry land. After much research, we found the previous salvors had used at least three different datum settings on their GPS units, and two different position settings. When looking at some of the old charts they had been using, we found them to also have two different map datums, so even locations marked on them were suspect. Very enlightening. My rule is: Always document what map datum and Position format you are using in your log so someone in the future can figure it out. AND it would blow your mind how many people that use a GPS every day have no clue what a map datum is. I've known two captains with 100 ton license that had no idea what I was talking about when I started talking about map datums. Good posting Black Duck.

 

[signumops]

As for the importance of Datum and 'Position' readouts, here's an example of interpretation from one datum to another for the exact purpose of locating the 'goodie' X spot. This sample includes bearings to landmarks, the registration of NAD27 vs. NAD83, and, the qualification of a handheld GPS against 1st order NGS monumentation. This is also a sample of the DNR (now DEP) dune setback Datum as a grid registration point for a true transit survey of a magnetometer study (just because its not tied to GPS does not mean it can not be used).



First, know the deviation of your intstrument, if there is any. In this illustration, a Garmin 12XL, using WGS84, is reading a decimal minute notation in its 'Position' as Latitude 28 degrees, 38.573 minutes North by 80 degrees, 44.834 minutes West. It is lying next to a National Geodetic Survey monument. This is the "Sweet" monument located on a camera pad in the Canaveral National Seashore.

Using a link from www.labins.org, you can find the NGS documentation for the "Sweet" monument as seen below...



The NGS Data Sheet gives you the position of the "Sweet" monument using NAD83 (NOT WGS84) sexigesimal notation positions featuring a decimal second. Sexigesimal is 'degree', 'minute', and 'second', but a second of distance can only get you within 100 feet of a given point. This is why they have used the decimal second notation. FYI, the NAVD 88 is an elevation measurement indicating that "Sweet" is about 15 feet above sea-level (it is on top of an elevated camera pad used to photograph rocket launches from the KSC).

To make use of this benchmark, you have to convert from NAD83 sexigesimal to WGS84 decimal minute. I use GEO4SPCS to do this in the following illustration.



As you can see, there is some difference in the decimal second reading between the NGS fix and my GPS fix. Put your faith in the NGS fix and remember that they use much better equipment than you would have on hand, to determine monument position... far better than a typical GPS unit available to the public. In this case, the Garmin 12XL is differntial capable, but it is not in differential mode in this instance.

Now for the meat and potatoes. What you see in the next illustration is a part of a magnetometer study performed on the Cabin Wreck near Sebastian Inlet. This study was given to me by someone we all know, but, I am not permitted to divulge his name. Whatever the case, the study was done way before the advent of GPS, so the chart had to be registered to land-based monumentation and positions were shot in using transits at locations on shore. Notice the reference to the State Markers and in the lower right, there is a symbol for "R 14" which is a DNR setback monument. There are also lines for lats and longs on the chart which you can not see completely here that are in UTM coordinates. The exact position of R 14 is stored at www.labins.org, so you can get that fix, convert it to WGS84 Geographic coordiantes. Also, the State markers are long gone, so, for a datum point you have the lat-long grid in UTM and the setback monument in UTM.



The last illustration deals with 3 waypoints which were furnished by Tommy Gore. These were fixes computed from monuments on shore in 1966. The monuments were the chimmenys on Kip Wagner's cabin and another cabin (Howard's) to the south. Problem is that Kip's cabin has been picked up twice and moved back to escape the tide, and Howard's cabin has been demolished. Therefore we have two Datum points that no longer exist. To find the three "Point's of Interest"(the Waypoints), I had to obtain aerials (1967) and rectify them using landmarks within the photos that still exist today on known positions. This involved translating from NAD27 meters to NAD83 Survey Feet in SPCS Zone 901, then registering the vectorized 1985 magnetometer data on top of that as seen in this last illustration.

 

[billfishbelize]

Black duck, signumops, This old sailor is truly humbled by your knowledge! Thank You. Bill

 

[Black Duck]

Signumops
Very impressive

Good work

We have a new book out also look under 2nd edition new book guys you will enjoy

 

[signumops]

Thanks Black Duck. I got your first edition and liked it. Sent it to Bobadilla along with a Spanish edition of Claudio's "Galleons And Sunken Treasure". Where is the 2nd edition... still on lulu? I want a copy!

I'm thinking about running a refresher course for TH'ers on GPS (any type) combined with PC programs for dig plotting and automating the Florida State "Daily Field Note & Activity Log". Probably a freebie (beer not included). I've suffered enough trying to set up sensible excavation targets based upon the historic GPS positions/notes of other dig boats. I used to think that the errors were deliberate... now I think they are just pure error most of the time. We should all be just as handy with our GPS gear as we are with our dive gear.

 

[Bum Luck]

Most of the confusion is a result of the evolution of technology.

Simple triangulation that was the best system for decades was transformed by RDF (radio direction finding), short range micro wave, LORAN, and finally GPS, itself the product of internal evolution as it utilized the phase shift of both frequencies instead of just the single one the military had envisioned. The accuracy product we surveyors use is on the order of a couple of millimeters.

Human nature is always on the move to maximize the utility of what it can get, but it has pretty well shook out by now to WGS84, NAD, and county coordinate systems, all readily convertible. My equipment transforms it with a push of a button.

Good post by Signumops!

 

[White Feather]

Wow, how will I ever find my way back to my slip? Before this GPS brain overload I had no problems going back to spots and landing within a few yards of the first anchoring. Now I'll never find my way back. To much info!

Getting on the same spot I dove three or four years ago was pretty easy until now. I was happy to figure I just screwed up putting my numbers in the system when I could not land in the right spot and was happy that I was dyslexic and most likely switched numbers. Knowing I am now illiterate in the use of my GPS that has informed me on how to sail to islands all over the Caribbean and gulf may keep me in port for years. NOT! I guess it's like the man said; "A good navigator never really knows where he is going until he gets there and even then he's not sure." now I'm not sure where any of my dive sites are. That's okay, maybe I'll dive yours by mistake. Mine are worked out anyhow.

 

[signumops]

You know, how ashamed of myself I must be for hijacking this thread and makin with all the big talk about GPS. I forget this is really a board for the hobby of treasure hunting. I take it all back. GPS is a waste of time if you really love the thrill of the hunt. It doesn't matter if you dig the same place you dug yesterday: it doesn't really matter if you find anything... its the hunt that counts. Truely, ignorance is bliss.

 

[Goldminer]

signumops has no reason for shame. Consider the following:
We marked a small (1-3 gamma) hit on the magnetometer into our computers GPS system. This was in 80ft. of water. We returned many days later, using the boats GPS system, dropped a buoy, and put divers down with metal detectors. Within 3ft. of the buoyed weight, a small object with measurements of 8"x 1-1/2"x 3/4" was located and retrieved. A thorough search of the surrounding area, using a circle search pattern yielded no other ferrous metal objects. This process has been repeated many times with similar results. Go GPS !!!