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GPS, which one would you believe?

Old 08-15-2011, 04:42 PM
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Default GPS, which one would you believe?

I have 3 gps's.
1 is an old Furuno GP31 that I have used and trusted for 12+ years.
1 is a new Garmin 72H handheld.
The newest is a Standard Horizon CP300 with Cmap.

I punched in a GOTO for a wreck approx 35 miles from my slip.
2 to 1 against the CP300 I come up with a difference of almost 5 miles, bearing being the same. But the CP300 has WAAS (but the 72H does too). The Furuno GP31 definitely does not have WAAS or DGPS. It is the predecessor to the GP36, but it agrees with the handheld and has always been reliable.

Is the difference because I am not moving, sitting in the slip?

When I tested real life for a close by sea buoy with published coordinates that I programmed into all of them, all of them have me hitting the buoy. And all of them are on the money for present location sitting in the slip. I usually set everything for magnetic, rather than true.

Ran through the menu's on all of them. I really see nothing in the set up's that would cause the discrepency. Of course I could have missed something. All 3 in agreement would be better!

Any ideas for such a big difference?
Old 08-15-2011, 04:44 PM
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use the cp300
Old 08-17-2011, 07:49 PM
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Originally Posted by Local Motion View Post
I have 3 gps's.
1 is an old Furuno GP31 that I have used and trusted for 12+ years.
1 is a new Garmin 72H handheld.
The newest is a Standard Horizon CP300 with Cmap.

I punched in a GOTO for a wreck approx 35 miles from my slip.
2 to 1 against the CP300 I come up with a difference of almost 5 miles, bearing being the same. But the CP300 has WAAS (but the 72H does too). The Furuno GP31 definitely does not have WAAS or DGPS. It is the predecessor to the GP36, but it agrees with the handheld and has always been reliable.

Is the difference because I am not moving, sitting in the slip?

When I tested real life for a close by sea buoy with published coordinates that I programmed into all of them, all of them have me hitting the buoy. And all of them are on the money for present location sitting in the slip. I usually set everything for magnetic, rather than true.

Ran through the menu's on all of them. I really see nothing in the set up's that would cause the discrepency. Of course I could have missed something. All 3 in agreement would be better!

Any ideas for such a big difference?
If you entered coordinates for the wreck, I don't have any idea why the difference. But if you put the screen cursor over the wreck symbol on a map, maybe they are different map sources, and have the wreck position incorrect. If none of my thoughts seem logical, it's because I'm basically illiterate on tech stuff.... I just know which buttons to push.
Old 08-17-2011, 08:05 PM
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It's not because your boat is not moving. That only affects the heading.

I'm thinking the 1st most likely reason for the disagreement is that your S/H CP300 is set up with a different format for inputiing minutes and or seconds than the Garmin or Furuno . As in DDD-MM.MM vs. DDD-MM-SS or other format so the same numbers represent very different positions.

2nd would be an error in inputing specific numbers if the format does happen to be exactly the same.

Distant 3rd.... The chartplotter datum chosen may be incorrect for the chart on the S/H . Usually WGS 84 is used in the US for modern charting I believe.
Old 08-18-2011, 05:22 AM
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[QUOTE=Local Motion;3957242] Is the difference because I am not moving, sitting in the slip?[QUOTE]

I would say yes. Do you get the same results while drifting 1 to 2 mph? GPS cannot get a good fix on your position unless you are moving, it uses a time and distance method.
Old 08-18-2011, 07:35 AM
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Originally Posted by polarred21 View Post
I would say yes. Do you get the same results while drifting 1 to 2 mph? GPS cannot get a good fix on your position unless you are moving, it uses a time and distance method.
polarred21, your statement is correct if you are referring to obtaining a heading(actually course-over-ground) and speed reading.

However, the GPS system does give perfectly accurate location information while not moving.

And, Local Motion, I would check the coordinates for the waypoint in the Standard. The fact that all three agree on a waypoint you entered manually implies they are tracking together.
Old 08-18-2011, 08:06 AM
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Originally Posted by colecaz View Post
polarred21, your statement is correct if you are referring to obtaining a heading(actually course-over-ground) and speed reading.

However, the GPS system does give perfectly accurate location information while not moving.

And, Local Motion, I would check the coordinates for the waypoint in the Standard. The fact that all three agree on a waypoint you entered manually implies they are tracking together.
+1 on this explanation....GPS is point accurate. What a moving target offers is the opportunity to filter or smooth errors in tracking software.....it's actually the software that is improving the accuracy in that case. GPS is GPS.
Old 08-18-2011, 12:33 PM
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Originally Posted by capecuddy View Post
+1 on this explanation....GPS is point accurate. What a moving target offers is the opportunity to filter or smooth errors in tracking software.....it's actually the software that is improving the accuracy in that case. GPS is GPS.
I stand corrected . Looking back at the notes from the class I took last year "must be moving to know heading and direction, otherwise you will only know fix and time".
Old 08-19-2011, 12:37 AM
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Originally Posted by capecuddy View Post
.....it's actually the software that is improving the accuracy in that case. GPS is GPS.
So how does the tracking software improve the accuracy ;?
Old 08-19-2011, 05:20 AM
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to answer the question - i would believe all 3 .

5 miles is a lot.
5 miles is also about the difference between 35 nautical miles and 35 american miles .

cheers
Old 08-19-2011, 07:43 AM
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Does your current(dockside) position read the same and in the same format on all three devices?
Old 08-24-2011, 11:43 PM
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Ok, I checked all set ups on all 3 machines and saw nothing out of order.

A 5 mile difference for a destination I figured out of line, which is why I posted, even knowing 1 was not using the best technology for accuracy, that being the GP31.

All 3 machines were on the money for my present location for sitting in the slip, which is what prompted my concern for the GP31 being 5 miles off for my destination, 35+ miles away.

When the day for the trip came and I got out there a few miles and underway, accuracy/agreement was within approximately 1 mile, which I figured very acceptable for the Furuno GP31 not having WAAS capability.

The reason for the original discrepency, I cannot explain.

Thanks for the repsonses.
Old 08-25-2011, 03:12 AM
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hi local motion.

the reason for the descrepancy is your machines are set to different units. nautical miles (NM) are a little bit longer than miles (M). over a distance of 35 miles the difference is about 5 miles.

cheers.
Old 08-26-2011, 12:20 AM
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Originally Posted by puppy View Post
hi local motion.

the reason for the descrepancy is your machines are set to different units. nautical miles (NM) are a little bit longer than miles (M). over a distance of 35 miles the difference is about 5 miles.

cheers.
Yep that's close to the money. Nothing much to do with GPS acuracy and ignoring the possible format mismatch basically would appear units configuration issue.

All things being equal with the cooridnates once at the mark the DTG would all be zero (0)

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