GPS Anomaly

ccriders · #1 (**permalink**) 09-15-2011

I have a Garmin GPSMAP 76C. Yesterday I was showing a novice friend some of its features and noticed that I had two recorded tracks from my last outings. What amazed me is that one of the tracks goes over the seawall and across a large concrete and earthen "T" head pier then across the regular planked pier and into my slip. The other track goes around these impediments (as did my boat) and into the slip.
So how does the garmin display a track I did not take?
John

jackdale · #2 (**permalink**) 09-15-2011

As a start, check to ensure that your datum is set to WGS 84.

LinekinBayCD · #3 (**permalink**) 09-15-2011

Quote:

Originally Posted by ccriders

I have a Garmin GPSMAP 76C. Yesterday I was showing a novice friend some of its features and noticed that I had two recorded tracks from my last outings. What amazed me is that one of the tracks goes over the seawall and across a large concrete and earthen "T" head pier then across the regular planked pier and into my slip. The other track goes around these impediments (as did my boat) and into the slip.
So how does the garmin display a track I did not take?
John

It may have lost satelite coverage for a brief period and plotted a bad track point. The track plots merley connect the dots from point to point. The data points are not constantly plotted. Depending on what setting you have it set to, the data points are plotted at different intervals. If the track is a straight line accross the seawall my guess is that there is one bad data point. If there are multiple data points accross the seawall and it meanders rather than being a straight line I don't have a guess.

If you use Garmin's MapSource program you can upload the track and see the date and time of each data point in the track which might help you figure out what happened.

Faster · #4 (**permalink**) 09-15-2011

I have had similar episodes, most recently with a newly acquired SH 180i gps plotter using CmapMAX.

In Pender Harbour it clearly showed me ashore while tied to the gas dock.. also, nearby, showed me crossing a point of land when I was 100 feet off... This past summer the errors seemed to be isolated primarily in that particular area.

HeartsContent · #5 (**permalink**) 09-15-2011

The Garmin charts that come with the units are not accurate. My 561s is often inaccurate and I have sailed thru islands and it shows a pass where there is land.

I am not sure if going to the upgrade chart packages fixes this. Things shift and charts need to update.

My feeling is an iPad with Navionics or a laptop would be the most accurate and updated solution.

dnf777 · #6 (**permalink**) 09-15-2011

I read that the controlling agency was considering narrowing the bandwidth of the open satellite transmissions that are available to consumer GPS units at the urging of the FCC. Other telemetry and data links are starving for freqs. This would noticeably decrease the accuracy and precision of GPS for non-military use. I don't know if this has (or will) actually happen.

puddinlegs · #7 (**permalink**) 09-16-2011

The distance between points that determines your recorded tracks are up to you to set. They can be set by either time or distance on most Garmin toys. More of course is more accurate, but fills memory more quickly. I've sailed across particular neighborhoods near our marina several times without even hitting bottom!

ccriders · #8 (**permalink**) 09-16-2011

I learned the wrong datum trick when I first got the unit.
Didn't realized I could set the distance between data points for creating a track. Will check that.
The chart information for this particulat location is very accurate so its not that.
The only reason this concerned me is that conceivably the GPS could be showing me in one location on the rocks when I'm a few hundred feet away in the fairway. Of course vice versa would be much worse.
Thanks,
John

RichH · #9 (**permalink**) 09-16-2011

If you are near large military/naval bases, especially those with nuclear weapons, the GPS will/may be constantly 'dithered'. My experience is that GPS in the Groton CT, Norfolk VA and St. Marys River FL, etc. are notoriously 'off' and 'unstable'.

puddinlegs · #10 (**permalink**) 09-16-2011

Quote:

Originally Posted by ccriders

I learned the wrong datum trick when I first got the unit.
Didn't realized I could set the distance between data points for creating a track. Will check that.
The chart information for this particulat location is very accurate so its not that.
The only reason this concerned me is that conceivably the GPS could be showing me in one location on the rocks when I'm a few hundred feet away in the fairway. Of course vice versa would be much worse.
Thanks,
John

The track shows you where you've been, not where you're going. The past is measured how you wish. The future is wherever you steer your boat.