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GPS Shift????
 
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Guest
Link to this postPosted: Thu Apr 23, 2009 12:22 pm    Post subject: Note GPS Shift????

I have noticed with my Garmin, 76(C?) that sometimes I am standing on land and it shows me in the water, I never really cared because when I am sailing I know the difference between land and water however when I used the gps to navigate the short cut catamaran passage from Leverick bay to the dogs I could only do it at or near high tide as the clearance was about 1ft in parts. now that I am thinking about it and after looking at the chart i am thinking it is possible that I was shifted about 20-30 ft to the right IE towards shore. Is this possible?
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walker
Link to this postPosted: Thu Apr 23, 2009 1:08 pm    

The position displayed on your GPS is correct - within a few feet. However, the maps that are in it are not correct!

See my explanation at http://www.nwmangum.com/ChartNav.html .


Last edited by walker on Sat Apr 25, 2009 8:37 am; edited 1 time in total
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Guest
Link to this postPosted: Thu Apr 23, 2009 1:45 pm    

Does GPS suffer from drift the way that charts do? I know they are not magnetic however I can not explain how my track that exactly followed your posted points was so shallow yet when we did it the year before following the Cuan law it was not nearly so shallow. admittedly we did not keep the gps track from that trip to compare so no way to know if we went the same way but the assumption is there. or maybe there is a set up problem with the GPS?
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walker
Link to this postPosted: Thu Apr 23, 2009 2:05 pm    

GPS does not drift.
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Guest
Link to this postPosted: Thu Apr 23, 2009 5:55 pm    

Also the map datum must be matched correctly to the chart being used for reference when drawing the "track". GPS's should do this automatically, but when you use a laptop + GPS, all bets are off. Plus as Walker said, the charts aren't always very accurate in terms of absolute positioning of everything. Relative positions and bearings, yes, but not absolute in terms of exact coordinates.
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DawnB
Link to this postPosted: Sat Apr 25, 2009 7:25 am    

Walker wrote:
The position displayed on your GPS is correct - within a few feet. However, the maps that are in it are not correct!

See my explanation at http://www.nwmangum.com/ChartNav.html


Walker, that link doesn't work?

My Mapquest chart looks great until I am at a certain distance, and then it looks like my tracks go right into the islands, and not the water as I know I was.. ~smiles~

Yet on my GPS, it doesn't show that way.

Any way of fixing it? I wanted to take a screen shot and make it into a t-shirt of where I went on my last trip, but it looks like i hit land about 30% of the time!
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walker
Link to this postPosted: Sat Apr 25, 2009 8:38 am    

All fixed - http://www.nwmangum.com/ChartNav.html
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