Expedition Navigation Software 
1 of 1- DuncanR
- Expert
Offline - From: Up and down East Coast Aus
- Registered: 9/16/2015
- Posts: 470
GPS Accuracy
All, a question for you.
My current ship has two GPS. It is easy to swap between them. The below Expedition image shows the ships "track" while parked in the marina. The SW portion of the track is GPS 2, the newer of the pair, the NE track is GPS 1. The NW portion, the spaghetti track, is GPS 1 registering when swapped for GPS 2. The entire track is about 10 minutes on each GPS, 20-ish in total.
Note the scale Ive drawn, the two GPS have a systematic location difference of perhaps 200 metres.
So, questions:
1. Is this kind of variance (both pattern and inter-GPS variation) normal? Sailing in places with underwater obstacles and a 200m error doesnt fill me with confidence:
2. How would one determine the accuracy or otherwise of each GPS? I note that I have not been bringing HDOP through into my Expedition logfiles, but HDOP may or may not show the problem
Regards
- Joe Goulet
- Expert
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- From: Fort Lauderdale, FL USA
- Registered: 9/16/2015
- Posts: 83
Re: GPS Accuracy
Hey Duncan,
I am probably belaboring the obvious here but have you checked to insure that both units are set to the same datum? I bring this up because I understand that you have navved on boats that have histories in some widely separated venues where different GPS datums may have served best depending on the datums of the charts of the various areas. You may have "inherited" a datum setting that does not match the datum of the charts you are using. The fix should be an an easy (stop laughing) selection somewhere in the settings of the GPS.
Last edited by Joe Goulet (4/19/2017 7:19 pm)
- sheldon.haynie
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- From: San Francisco Bay
- Registered: 1/13/2016
- Posts: 133
Re: GPS Accuracy
Is your bright blue line a ruler of 0.01 nm.or 18.5 m?
The mostly square corners of the track suggest a bad resolution, with your GPS's snapping to perhaps 1 meter grid.
My log files show much higher resolution, and seem to be reproducible/matching chart to pretty good numbers.
If I leave my plotter on for a week in the slip, I do get a Brownian pattern that can have excursions of 100 m
Semiconductor Development Engineer, Retired Viticulturalist, Occasional sailor
"Lioness" 1962 Hinckley Bermuda 40 Custom Yawl
- DuncanR
- Expert
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- From: Up and down East Coast Aus
- Registered: 9/16/2015
- Posts: 470
Re: GPS Accuracy
Apologies, typo in the distance, 20m not 200m.
keep them cards n letters comin in folks!
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Thread Starter- David
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- Registered: 9/23/2015
- Posts: 15
Re: GPS Accuracy
That drift is way bigger than I'm used to seeing. I use two GPS on my good ship, one is a USB puck the other the on-board Raymarine unit. Something defs amiss in your plot. If you want to borrow my puck for a third opinion let me know!
D
- DuncanR
- Expert
Offline
- From: Up and down East Coast Aus
- Registered: 9/16/2015
- Posts: 470
Re: GPS Accuracy
Dave, good thought. Plug a USB puck into a laptop and run comparative data. I take it position recording via 8.csv files is feasible?
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Thread Starter- DuncanR
- Expert
Offline
- From: Up and down East Coast Aus
- Registered: 9/16/2015
- Posts: 470
Re: GPS Accuracy
Make that * not 8 in the above. Im afraid my old HP EliteBook is getting a bit like me .... crusty and sticky ....
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Thread Starter