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Old 27 February 2012, 14:15   #1
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Google Earth GPS accuracy

When you move your cursor across Google Earth you get a lat-long reading at the bottom of the page that seems very sensitive. Has anyone determined how accurate this is in relation to chart plotter data?
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Old 27 February 2012, 14:34   #2
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Need to be careful what format you're reading the position i.e. hddd°mm.mmm or hddd.ddddd° etc. You could be looking at decimals of a minute or seconds etc.

I do find it pretty accurate though, on the Garmin Homeport software theres an option to overlay your waypoints and tracks etc onto google earth, if I do this on all my marks for the Gutters you can see my path through the rocks and transits over landmark buildings etc, mostly pretty much spot on
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Old 27 February 2012, 15:01   #3
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Google Earth uses WGS 84 as its datum coordinate system and this is the same datum reference that modern chart plotters default to - so they are both compatible.

Did you know that 000.00 on a GPS is 102.5m to the east of the Prime Meridan at Greenwich due to WGS 84
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Old 27 February 2012, 15:26   #4
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Originally Posted by Ribochet View Post
Google Earth uses WGS 84 as its datum coordinate system and this is the same datum reference that modern chart plotters default to - so they are both compatible.

Did you know that 000.00 on a GPS is 102.5m to the east of the Prime Meridan at Greenwich due to WGS 84
That's because the GPS system is centered on the center of the continental united states and this 100m shift is actually part of the revised estimate of the shape of the earth incorporated into WGS84. Its not a sphere as the equator and prime meridian might suggest. (more like an egg)

Most charts were made or have been revised with WGS84 in mind. But some older charts and or in remote areas aren't drawn to the same precision as GPS would suggest. In other words your position is exact, the rock you ran into was merely approximated on the chart.
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Old 28 February 2012, 04:10   #5
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there is also the accuracy of your reciever at any point in time to remember about......

I have noticed on google maps "out in the wilds of Northern Scotland" there are chunks they still haven't managed to get a clear day for a decent photo. if you look closely at the join, not all the roads etc line up perfectly, so always treat with a small pinch of salt. I read somewhere that it's best to assume a worst case accuracy of around 30m radius, although that might have been Streetmap on the 1:50K sheets.
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Old 29 February 2012, 16:31   #6
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That's because the GPS system is centered on the center of the continental united states
I think it's actually centred on the Earth's centre of mass, but I do appreciate that you Americans may think these two are the same thing

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Old 29 February 2012, 16:48   #7
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I think it's actually centred on the Earth's centre of mass, but I do appreciate that you Americans may think these two are the same thing

0,0 is the center of mass but you aren't presumed to view the world from the inside out

I thought WGS84 was supposed to "expire" but haven't heard of any replacements.
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Old 01 March 2012, 04:09   #8
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0,0,0 unless we lost a dimension somewhere en route?

And the planet isn't a perfect sphere annyway, so it's all a bit hypothetical. As long as the electronic systems all sing the same tune you know you're roughly in the right place.

Always beware of credible rubbish....

(can we have a running aground / hitting a rock smiley?)
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