Perhaps a silly question

Post by **Wheresthefish** » May 29th, 2005, 4:56 pm

I have a question about GPS coordinates. I went out a couple of weeks ago to find one of the wrecks off Carrabelle. I had the DGPS numbers and was off. To my surprise I spent about 45 minutes looking for the structure but never found it.

Towards the end of the day I was several miles away from that spot but decided to go home by a route that was close to the Wreck that I was looking for. Much to my surprise I saw the depth finder shoot up from 70 foot to 40 foot and then back down. I stopped, circled and located the suncken barge.

I was more than a quarter mile from the original spot.

Do the numbers "drift" as the years go by? If so is that drift predictable?

If not, is there a reason that the numbers (published) were so far off?

I guess its a silly question but there are many more numbers that I want to find but if they are all 1/4 mile off I will be looking for some time to come.

bait boy · Post by **bait boy** » May 29th, 2005, 6:04 pm

A lot of numbers are converted mathematically from the old loran units. I have had the same problem.

Post by **Chalk** » May 29th, 2005, 6:55 pm

As BB said, you also need to ensure what convention of GPS datum your using...

There is WGS-84, NAD-83, and NAD-27....Puching in one convention and being set on another can throw you off...

Post by **Cranfield** » May 30th, 2005, 5:05 am

I suspect violent storms and hurricanes have a part to play in relocating (or creating), underwater structures.

You are fortunate to have almost no tide in the Gulf.
We have average 16-18ft rise and falls.
Sand banks, channels and structures are continually moving and being covered and uncovered , due to the relentless nature of the tides.

Post by **Jumptrout51** » May 30th, 2005, 10:28 am

In part what Cranfield said. I have heard from several Charter Captains that bottom structure and bottom debris has changed considerably since all the hurricanes we had last year.

In fact Greymare Rock moved 100 feet!

pennfish · Post by **pennfish** » May 30th, 2005, 1:59 pm

Try the numbers Tom has listed on this site and build your private inventory from there. Ive found the # are dead on, thanks TOM

Post by **Wheresthefish** » May 30th, 2005, 4:55 pm

Wow, I am impressed. I never had discovered the numbers on this site! Secondly I compared them to my numbers (where I actually located the Barge) and they are dead on!

I wish I would have used these numbers first!

Thanks for you help folks.... I hope to go Friday and try some cobia off Carrabelle (Snapper, Grouper, AJ's and anything else that will bite)

Post by **arcadiainc** » June 2nd, 2005, 4:35 pm

Recently I bought a used handheld Garmin out of the mid west. When I got it I took it outside and let it do its 8-10 minute thing to reorient itself. Off we go to St. Marks fat and happy. Naturally the motor quit late on a Wednesday afternoon. Called towboat and gave my position. Three hours later they found us. Seems that I need to match my GPS with known coordinates or it will not position itself. My GPS was giving Towboat our position about 6 miles from where we were. If you get a used one check it with a known value or pay the price.

Post by **tin can** » June 2nd, 2005, 8:14 pm

arcadiainc, if it was off that far, check to see what datum it's set on. That's the only thing I can think of that would put you off that far.

If you didn't get the book with your used GPS, you can go to the Garmin website and download the book for your unit.