Different GPS #'s

Post by **Sir reel** » July 24th, 2003, 11:05 am

OK Boys and girls.... its been a bit slowl (IMO) recently here on the forum so Iâ€™m going to take this opportunity to talk about a fishâ€™n issue and try and express some frustration and maybe give someone a chance to teach me a thing or two. I'm not sure if this area is where this belongs so Tom, feel free to move it.

It looks to me like there are different GPS numbers for what seems like should be the same location. I donâ€™t expect anyone to divulge any secrets here, but Iâ€™d sure like some help on what seems to be widely held as public information. I also donâ€™t mind spending time and gas on the water but I feel like Iâ€™ve done that and Iâ€™m tired of lookâ€™n for those needles. (I canâ€™t even find the hay stack sometimes).

Last weekend I traveled out to what I had in my GPS as the St. Marks reef. We fished around a bit and it was slow so I decided to go on out a bit further to what I had as coordinates for the 2 Wakulla regional reefs. I had been there before and always come away with the feeling that I hadnâ€™t really found the right bottom. When I got back home I took my Garmin inside and started checking #â€™s and comparing them with the hardcopy information that I also carry with me. I found some different numbers and put them in and brought up the map to see how close they were to each other. Most of the new locations were anywhere from a couple hundred feet to 1/4 mile from the same location per #â€™s from another source. Now Iâ€™m not the best searcher on the water, but my bread crumb trails leave little doubt that I covered the area pretty well. It may be that Iâ€™m not reading the bottom correctly but I also know that unless I get within 30-40 ft or so (depending on depth)I probably wonâ€™t see it . About this time was when I decided to do a little comparison. I have put the table below together to document my point. With a couple of exceptions, I think most folks would agree that the numbers sure donâ€™t seem to be very close . I didnâ€™t want to create any bigger table but I have 2 more sources, one from a seminar I attended a few years ago and another set of numbers from a chart titled â€œreefs of the big bend gulfâ€

Post by **jonguthrie** » July 24th, 2003, 5:30 pm

To keep this going here are some of my observations:

Many published spots were marked before the most recent GPS tecnology was available (WASS, Differental or disabled selective availibility) or were coverted Loran time differences which can be 1/4 mile off.

On a lesser scale different boats will mark a spot differently based on how horizontal the face of the transducer is.

Smaller spots can be moved by storms or buried by sand.

You need a good unit adjusted properly with bottom lock and zoom, preferably with color to pick up low relief structure.

The best thing is to dive the spot when you mark it then you know. If you are not a diver look for a dive WEB site that verifies the numbers. There are a number of sites that do that.

EddieJoe · Post by **EddieJoe** » July 24th, 2003, 8:51 pm

Jon is right on. Note on the FWC web site lists that the original equipment used to capture the number is indicated (e.g., LORAN, GPS, GPS w/differential). If it was in LORAN, the L/L is usually a mathematical calculation from a computer program, which are not precise. Also, a number in GPS can be 30 meters off, differential brings it down to 3-4.

So, if an original number was in LORAN you will have to go to it with a LORAN unit, or do some hunting if you use the L/L converted number. Even LORAN-LORAN numbers are off from one machine to another, just not as much as GPS/LORAN conversions.

Usually, if I use someone else's number or a published number I figure on a search. When I find the place I like with my bottom machine, that becomes the "new" number for the site, and I store it on my GPS/differential machine. Its the real "numbers" game.

EJ

Post by **tin can** » July 24th, 2003, 9:40 pm

Eddiejoe is on the mark. Most of the older public sites were located with LORAN. One would think that a spot on the earth's surface could be marked by many methods. If you do some research, you will find that LORAN is repeatable ( on itself), but not accurate. The radio signals are deflected or obstructed by fixed objects. As long as those fixed objects (trees, buildings, hills.....) don't move or go away, the LORAN signal is repeatable. Most LORAN to GPS conversions don't account for warpage of the LORAN signal. That's why conversion from LORAN to GPS doesn't seem to work. There is a piece of software that accounts for most fixed objects, in most LORAN coverage areas. But it's expensive, and not fool proof.

If a spot was marked with GPS (no differential) before Selective Availability was turned off, you might never find it again. I conducted my own research with a Garmin GPS12, and a Garmin GPS 38 before Selective Availibility was turned off. I captured 20 waypoints, all the same location, with both units side by side, over a 3 day period. I exported the numbers, and imported them into an Excel spreadsheet, and did an X/Y scatter graph. It took a 1500 foot radius circle to cover all the points. That's a 3000 food diameter circle. That indicated to me that if the point were originally captured without differential, and before Selective availability was turned off. The actual spot could be as much as 1500 feet, in any direction, from those numbers.

All of this brings to mind that WE (I include myself) DEPEND WAY TOO MUCH ON THESE LITTLE ELECTRONIC BOXES. The bottom line is, if you're 15 miles out there ad your GPS/LORAN quits, and the fog rolls in, you better have a compass heading to get you home.

SR, I have some points that were captured before SA was turned off. One day I'll show you a plot of some good fishing spots.....................on dry land.

Did I muddy the water enough?

Post by AJ » July 24th, 2003, 11:18 pm

You might want to go to marinefisheries.org. Thier numbers are pretty close but I almost always have to circle around a time or two, then edit the number once I'm on it.
Like EJ said, it's a numbers game

Post by **Chalk** » July 25th, 2003, 7:04 am

The biggest difference I found was the use of coordinate systems inside the GPS realm. Alot of the governement sites use NAD-27 and most Fishing type instruments use the WGS-84(NAD-83)...So, its important to know the coordinate system being used. Once you know that you can use a conversion program to switch them.....For example...For Terraserver coordinates, I use http://www.jeeep.com/details/coord/index.html to convert the NAD-27 to NAD-83......Its early and I might have it bassackwards, but thats what I do and I can go straight to __________ creek and catch some redfish

Post by **Sir reel** » July 25th, 2003, 8:45 am

Well.... if any of the vistors and regular readers to this site thought we were populated by a bunch of ole smelly beer drinking smart A##'s who don't do anything but thrash the water into foam trying to catch fish.

..... they sure didn't make it to this class!!

Thanks! I appreciate the trouble and length you all went to to explain much of what is going on with the differences. This site as a source of creditable information and a willingness to share it is still the best.

Thanks Guys. (I'll keep running in circles and mark'n spots

)

(ps if anyone else has thoughts please feel free to share them)

Post by **tin can** » July 26th, 2003, 5:55 am

Chalk brought up a good point. Most GPS units now allow you to switch between coordinate systems. You can enter those NAD27 coordinates in your unit, tell the unit to switch to NAD27, and go to the point. However, what chalk is doing is the way to go. Mhen you use multiple coordinate systems, YOU have to keep track of what points are on what coordinate system.

Post by **Chalk** » July 27th, 2003, 8:58 am

All right here is the down a dirty, do not report my azz for the picture either.

In my poboy Magellan GPS310, I have the mouth of Econfina marked as below:

30Â° 02.11N for Latitude
083Â° 55.82W for Longitude

Note: The 0 for Longitude, in the http://www.jeep page it considers a 0 a negative (-). This applies to the http://www.terrafly.com also. Omitting the negative sign or zero could put you somewhere in Australia.

The coordinates above are NAD 83 MinDec format. To navigate terrafly you have to use NAD83 DegDec, which looks like this:

30.035166 for Latitude
-83.930333 for Longitude

All the above are NAD83, the difference is the format inside NAD83, MinDec to DegDec.

The cross hair marks the spot.

Â«-- WGS 84 (NAD 83) --Â»
The WGS-84 (NAD-83) datum is mainly used by the Global Positioning System (GPS). Up-to-date USGS maps use NAD-83 (which is the same as WGS-84). Even though GPS's use WGS-84, they can translate that data to most any map datum. If you're using a map, make sure your GPS is using the same datum.
Latitude Longitude
DMS 0Â°2'6.6" N 83Â°55'49.19" W
MinDec 30Â°2.11 -83Â°55.82
DegDec 30.035166 -83.930333
Universal Transverse Mercator (UTM)
Zone Easting (meters) Northing (meters)
17 217413 3326301
All the above are in the NAD83 coordinate system. NAD27 looks similar, but very different. It could mean the difference between the channel and a hazard.
Hope you guys appreciate thisâ€¦I spent a few days chasâ€™n this rabbit last year

I have used this to create a Word document with every creek west of the Econfina and a few east....I have a picture and the coordinates of each creek. I might let you take a peek at it one day