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My GPS seems to be off be a bit, like a couple of hundred feet to 1,000. Goping out of Clearwater Pass the other day, I was going to a wreck and my GPS took me right to it, or so I thought. We saw a few boats anchored a little ways away and saw more than one bent pole, so they were catching something and I couldn't figure out what they were catching so far off the wreck. After doing several passes, the bottom finder never picked up any structure. So, I went about 50' to their starboard side and then cut to port about 100' in front of their bow and there was the wreck. They were sitting over the stearn of it.
So, we decided to go check another ledge and never could find anything on the bottom finder. After hitting a few other spots, we set the GPS on the numbers for the Clearwater Bell buoy and it took us straight to the inlet. I decided to check the GPS as we went by the buoy and as we went about 50' from it, the directional arrow on the GPS tracked right by it as well....but said it was .3 miles directly off the port side when it was physically 50' from the port side.
Do GPS's get out of calibration? If so, how do you go about verifying the correct co ordinates and getting it calibrated?
Usually actual coordinates are correct but sometimes the map may not be as accurate. I know Hummminbird allows you to make an adjustment but I have never had the need to try this function. In the Humminbird it is called map offset.
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Usually actual coordinates are correct but sometimes the map may not be as accurate. I know Hummminbird allows you to make an adjustment but I have never had the need to try this function. In the Humminbird it is called map offset.
This is a great point. I have not fished for years, but when I did, I used a Micrologic Loran. Why? Because the fishing chart maker used Micrologic. The result was I could run 80 miles offshore and as I got close to the wreck or lumps, I'd throttle down and then cruise right to the spot, right on the money, it was amazing. Once I did this and there was a boat there dropping buoys all over trying to find the wreck, I stopped right on it, but since they were there first, I left for another wreck.
You could check & make sure you're on the correct chart Datum for your area - this may affect the accuracy.
Make sure that you're using the right datum, and, if you've entered the waypoints, make sure that the waypoints are entered in the correct format. The most common error is entering the waypoint as "seconds" rather than "decimal minutes" or vice versa.
I had a Loran coupler go bad once, and it caused some positioning errors. I suppose it's possible that a bad GPS antenna could keep functioning but cause accuracy issues.
Borrow a hand-held GPS, or pull up beside a buddy's boat and compare coordinates.
What GPS?
Does it have software and the proper antenna(s) to allow it to receive DGPS or WAAS corrections? Even without corrections it should be accurate to within about 7 meters ~95% of the time.
Are those corrections enabled on your GPS at the present time?
How old is the chartware?-updated recently in case the nav aid was moved to reflect a shifting channel?
As others mentioned, is the GPS set to use the proper chart datum? (Usually WGS84 but check your chartware ).
Where did the wreck position come from? Converted Loran TD's or Capt Seagull or other chart? Even the paper charts often have wrecks mismarked or PA. If you had been there before and updated the position you should have been able to get much closer.
If the GPS is used, check to see if a prev. owner added user position + or - corrections to lat or lon.
If your GPS was by some chance only receiving a few satellites at the time your GPS's calculated HDOP may have been relatively high, indicating less positioning precision.
What GPS?
Does it have software and the proper antenna(s) to allow it to receive DGPS or WAAS corrections? Even without corrections it should be accurate to within about 7 meters ~95% of the time.
Are those corrections enabled on your GPS at the present time?
How old is the chartware?-updated recently in case the nav aid was moved to reflect a shifting channel?
As others mentioned, is the GPS set to use the proper chart datum? (Usually WGS84 but check your chartware ).
Where did the wreck position come from? Converted Loran TD's or Capt Seagull or other chart? Even the paper charts often have wrecks mismarked or PA. If you had been there before and updated the position you should have been able to get much closer.
If the GPS is used, check to see if a prev. owner added user position + or - corrections to lat or lon.
If your GPS was by some chance only receiving a few satellites at the time your GPS's calculated HDOP may have been relatively high, indicating less positioning precision.
I'll clarify what I can. It is a Garmin 188 w/ a new Garmin GA 30 antenna. I recently put a new t top on the boat and replaced the old antenna with the newer, updated GA 30. I was not using any chartware, just manually entered numbers. This is a unit that was in a boat I recently purchased. Not the best or most up to date, but figured it was good enough until it died (which it may have). The numbers I was using have been used and verified by myself in the past. I also had another guy with me that had his book of numbers along and verified the numbers I had entered against his numbers. The number for the Clearwater Bell buoy is a published number, as it is the navigation aid at the inlet, both of us had the same number for it.
I don't know much about GPS units, besides entering numbers and hitting "go to" and it guides me there. I'm used to the GPS taking me relatively close to the spot(usually 10 ft or so) and then using the bottom finder to find exactly where the ledge or profile is that I'm looking for. Since it is a used unit, how would I check to see if the previous owner added user position + or - to corrections to lat or lon?
Give me the numbers for your fising spots that are in question and I'll go check out the spots for you. That way we'll compare your gps to mine.
If you ever make it over to the gulf side, I would be more than happy to give you some numbers. As for the east coast, I have been diving there for so long that many days I don't even turn on the GPS, I can find the spots by lining up landmarks and then use the bottom finder to get the ledge or profile I want.
GPS coordinates are quite accurate, but charts are not. There is no "offset adjustment" on the 188. While it is an older unit, it should get you within 50' of the coordinates you type in. But there are two sources of error: You can set the unit to talk degrees; minutes, and seconds of lattitde and longitude, or degrees, minutes in decimals. That will throw you off quite a bit. The other gotcha is setting the datum, a piece of work that adjusts the perfect circles of lattitude and longitude to fit the definately not perfect shape of lumpy old earth. Get that wrong and your off a bit too. These are found buried in the menu selections of your GPS, and could easily have been changed while someone was fumble-fingering around, looking for Borneo or something.
A GPS deals with discrete, digital data, and checks it's math continuously. It won't wander off a bit; it will just say something's wrong, and refuse to play any more. It's either as right as it can be, or it's off.
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Each gps unit may show different minutes/seconds, so what minutes/seconds show a wreck on one unit might not be the same numbers for the wreck on another unit. Once you have the correct numbers for your unit then you should be able to repeat going to the wreck each time. Very rarely will chart numbers be exactly right on a channel marker since they are moved fairly often for different reasons.
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The short answer is that GPS Units do not need any calibration.
They get all the information they need from the overhead satellites to get an accurate fix. Having said that there are several things that can degrade the performance of your GPS or cause what appear to be errors.
Some of these include:
Not enough satellites visible to get a good fix.
This can occur if your GPS Antenna is mounted in an area where it is obstructed from getting a good view of the sky and horizon. If you have the GPS in the cockpit of any fiberglass boat this should not be a problem. If you go to the GPS Status screen of your GPS it will tell you how many satellites it can see. Half a dozen is more than enough.
Satellites in view do not allow a good 3D fix.
Just like the old days when trying to find spots by lining up bearings and marks, you need a good spread of satellite locations in the sky to get a good fix. 90% of the time this isn't a problem, but sometimes it can occur. Once again go to your GPS Status Screen and look for a data field called HDOP (Horizintal Dillution of Precision). A HDOP of 1.0 is perfect, and anything up to 1.5 is still ok. But if you are always seeing 2.0 or greater everytime you are going out there is a problem somewhere. Refer bullet point 1.
The datum on your GPS and Chart is not the same.
Where i come from all charts use the WGS84 Datum so I configure my GPS to use the WGS84 datum. All my waypoints I have or are given to me by friends are also in WGS84 so everything is good. However if anyone of these is in a different datum you could experience up to several hundred feet of apparent error. A waypoint is pretty much useless unless you know what the datum is. I suspect the problems you are seeing are related to datum differences between your chart and GPS. As one reader has said if you cant set your GPS to match the chart datum, you can always enter a manual offset in the GPS to compensate. If you can get the exact coordinates for the wreck from a mariners publication somewhere (noting the datum), plug it into you GPS and drive to that waypoint. If you hit the wreck spot on this time you know your problem is the chart datum.
The short answer is that GPS Units do not need any calibration.
They get all the information they need from the overhead satellites to get an accurate fix. Having said that there are several things that can degrade the performance of your GPS or cause what appear to be errors.
Some of these include:
Not enough satellites visible to get a good fix.
This can occur if your GPS Antenna is mounted in an area where it is obstructed from getting a good view of the sky and horizon. If you have the GPS in the cockpit of any fiberglass boat this should not be a problem. If you go to the GPS Status screen of your GPS it will tell you how many satellites it can see. Half a dozen is more than enough.
Satellites in view do not allow a good 3D fix.
Just like the old days when trying to find spots by lining up bearings and marks, you need a good spread of satellite locations in the sky to get a good fix. 90% of the time this isn't a problem, but sometimes it can occur. Once again go to your GPS Status Screen and look for a data field called HDOP (Horizintal Dillution of Precision). A HDOP of 1.0 is perfect, and anything up to 1.5 is still ok. But if you are always seeing 2.0 or greater everytime you are going out there is a problem somewhere. Refer bullet point 1.
The datum on your GPS and Chart is not the same.
Where i come from all charts use the WGS84 Datum so I configure my GPS to use the WGS84 datum. All my waypoints I have or are given to me by friends are also in WGS84 so everything is good. However if anyone of these is in a different datum you could experience up to several hundred feet of apparent error. A waypoint is pretty much useless unless you know what the datum is. I suspect the problems you are seeing are related to datum differences between your chart and GPS. As one reader has said if you cant set your GPS to match the chart datum, you can always enter a manual offset in the GPS to compensate. If you can get the exact coordinates for the wreck from a mariners publication somewhere (noting the datum), plug it into you GPS and drive to that waypoint. If you hit the wreck spot on this time you know your problem is the chart datum.
Good luck with it.
X4 on that - the GPS data stream is constantly adjusted by the ground stations sending corrections to the satellites. That's alway's been part of the system. Accuracy got even better with DGPS (which is what all the modern ones are).
I bet it's set to a different DATUM - I've seen that before when Geocaching and it was putting the guy way off the marks.
when you are actually on the wreck as verified by your fishfinder take note of those coordinates and then use them the next time you return you should then be right on
It sounds like you did not get a manual with the used unit when you got it. I'd highly recommend you download it. The Garmin website has manuals for every (I believe every) GPS they have ever made. For the older units you may have to poke around a little more, but they are there. The download is free.
The manual will show you which of the many menus controls datum selection and will also detail the proper entry of spot formats as the guys are talking about, whether degrees/minutes/seconds, decimal entry or whatever. Might make it easier for you.
Assuming good satellite coverage/signal and right now it is the best it has ever been and by the sounds of things this is an external antenna then there should be absolutely no problem coverage/signal wise.
Now if the distance off the supposed spot is constant then it is a datum issue, if the distance for different spots varies
Quote:
My GPS seems to be off be a bit, like a couple of hundred feet to 1,000.
then is could be a position format issue.
It also could be a combination of both?
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Does a GPS need to be calibrated? 
