# GPS accuracy?



## fishkiller (Feb 6, 2007)

I have some questions to ask those of you who are more electronics savvy than myself (doesn't take much). Do all GPS units have a built in margin of error? I have an older Garmin unit and the distance it is off is substantial on the background map.
I have done the recommendations from the company and the error is still there, today I was using a Navionics app on my Iphone and there was also a large error looking at the depth #'s on my sonar vs. the #'s on the Navionics chart. Do any GPS units (brands) have a higher degree of accuracy than others or do they all have substantial location issues in regards to the background map? I remember looking at depth #'s on a sonar & looking at a chart depth #'s & having an idea where you were. I would not trust either one of these devices to do that.


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## FISHIN 2 (Jun 22, 2005)

Humminbird 788, I don't think any will be exact, too many variables as far as depth, however, my 788 will bring me to within 3' of a marked gps spot, trust me, those walleyes ain't got a chance in the winter !! Fish the same hole 3 days in a snowstorm, gps has got to be good. Mike


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## Snakecharmer (Apr 9, 2008)

Depth numbers can vary due to the raising and lowering of inland lakes due to rainfall and opening the dam. Use need to adjust maps based on how high or low the later is based on normal pool.


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## PatSea (Oct 10, 2004)

All GPS have error on the location. Best you can do is +- approx. 15 feet. It's the capability of the GPS system, not the GPS unit on your boat.


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## KaGee (Sep 8, 2006)

PatSea said:


> All GPS have error on the location. Best you can do is +- approx. 15 feet. It's the capability of the GPS system, not the GPS unit on your boat.


The older units were not nearly as accurate. At one time the military blocked consumer GPS units so they only received signal from 6 different satellites. That has since changed and I believe the number is 12 satellites which greatly improves accuracy.


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## OldSchool (Oct 18, 2006)

Let’s just say that the answer to that question is “complicated” and also a lot of the perceived accuracy “depends”. The Federal Gov specification for the GPS system is for the accuracy to be within 7 meters (appox 23 feet!). With software augmentations that can be driven down to the 2.5-3 Meter level (8-10ft). A lot of things affect the accuracy including weather and atmospheric conditions so a given GPS device can be, at times, much more accurate than on some other day/time.
For boat mounted Sonar/GPS units the offsets in the location of the transducer and the GPS can introduce several more feet of variation.
Lastly, many of the “cartoon maps” that are loaded into GPS devices are from 30-40 year old survey data from which many of the lake contours are somewhat estimated – it is not unheard of for ones boat to be traveling over what these maps depict as dry land! Updated (premium) lake maps that are based on actual sonar readings are much more accurate but still aren’t reliable enough to trust for boat navigation in shallow waters. 
Bottom line is, these things are a reference tool but they aren’t the gospel truth -- some days it might be spot-on other days you may be off by 10ft (as they say, “individual results may vary”.)


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## chaunc (Apr 11, 2004)

I agree with oldschool. When there is heavy cloud cover, my humminbird may be off by 15 to 20 feet but me having a little bit of experience, i can still work with it.


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## fishkiller (Feb 6, 2007)

Snakecharmer said:


> Depth numbers can vary due to the raising and lowering of inland lakes due to rainfall and opening the dam. Use need to adjust maps based on how high or low the later is based on normal pool.


 Some how I doubt that Lake Erie has varied by 4'.


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## fishkiller (Feb 6, 2007)

What I am talking about has more to do with the background maps & the boats position. I understand the built in error for military purposes & I know the # of Satelites (mine has 12) is also a factor. But the variation between the boats actual position and the base map is what I am trying to describe. On the west side of Mosquito the base map shows me in a cornfield, at Lakeshore Park the same unit shows me on St. Rt. 11 when I am in the water at the ramp. This a fairly large error in two different directions. Fishing at Ashtabula my Navionics chart on my Iphone showed me in 35' fow when I was in 39' on my sonar. They used to say with the Loran-C that you needed to use the same unit to get the exact same location, maybe things haven't changed all that much.


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## Misdirection (Jul 16, 2012)

fishkiller said:


> Some how I doubt that Lake Erie has varied by 4'.


Actually it has varied by about 6' since it has been tracked. But that is over decades, not year to year.

http://www.epa.gov/med/grosseile_site/indicators/waterlevels.html


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## OldSchool (Oct 18, 2006)

Wow, Fishkill, that is crazy bad. These things should at least get you to within a stones-throw of any particular coordinate. Something is totally not right with your Sonar/GPS. First thing I would do is to check to make sure that I hadn't configured any "offsets" into the unit. (Go back to factory defaults?)
If you can borrow a Lakemaster or Navionics map card from a friend, see if that will give you a corrected picture. 
Also create yourself a waypoint in a known spot, jot down the GPS coordinates from your unit and then go to Google Earth and plug those in -- this could tell you whether or not the unit is at least making the correct calculations with the GPS info.
Once you gathered up the facts it is time to call the manufacture's Cust Service number. Good luck.


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## PatSea (Oct 10, 2004)

Fishkiller, you might want to check the horizontal datum your unit is set on. It needs to be set to the same datum as the chart you're using. In most cases, this is WGS84.


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## fishkiller (Feb 6, 2007)

Misdirection said:


> Actually it has varied by about 6' since it has been tracked. But that is over decades, not year to year.
> 
> http://www.epa.gov/med/grosseile_site/indicators/waterlevels.html


I would think that at -6' of mean average depth a very significant portion of low water areas in the lake would be dry.Portions of Sandusky & Maumee Bays & other areas of low water depth, Lakeshore Park ramp only has about 3' of water, has anyone alive ever seen this?


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## Misdirection (Jul 16, 2012)

fishkiller said:


> I would think that at -6' of mean average depth a very significant portion of low water areas in the lake would be dry.Portions of Sandusky & Maumee Bays & other areas of low water depth, Lakeshore Park ramp only has about 3' of water, has anyone alive ever seen this?


My dock neighbor was around for the high end of the mean, he's 84. They had to nail 2x4's onto the posts on his fixed dock in Ashtabula because the existing posts were below his rub line (which are 4' out of the water today) and banging into the sides of his boat.

Keep in mind that the chart doesn't say 6' below today, its the long term high vs long term low had a difference of 6'.


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