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If the GPS says 85mph, how fast am I going?

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Old 12-16-2005, 08:23 PM
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Default Re: If the GPS says 85mph, how fast am I going?

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Originally Posted by Edward R. Cozzi
I'm sorry, but you are incorrect.

The GPS is measuring the speed AFTER all the conditions you mentioned are factored in. Aren't you reading the posts?

Who are you, anyway? Do YOU know Dr. Von Braun and DR. Sagan like WE do? What are your credentials?
Sorry, no credentials here, just plenty of experience on how GPS responds on a moving body of water...
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Old 12-16-2005, 09:18 PM
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Talking Re: If the GPS says 85mph, how fast am I going?

Thanks Troutly...I was getting frustrated.
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Old 12-16-2005, 09:29 PM
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Default Re: If the GPS says 85mph, how fast am I going?

I wonder if this had anything to do with norts fart thread?
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Old 12-16-2005, 09:47 PM
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Default Re: If the GPS says 85mph, how fast am I going?

Call me dumb, but I STILL don't understand the difference of GPS reading on land or water. Even after the numerous repeated attempts to explain.

Seems to me since the GPS gets it's data from satelites triangulations (big word) only thing it is reading is the speed of the moving object (boat) in reference to at least 3 satelites.
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Old 12-16-2005, 10:11 PM
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Default Re: If the GPS says 85mph, how fast am I going?

No difference? I thought all the talk was there is a difference between in your boat and in your car The time to travel from point A to point B is your speed, regardless of what the surface is underneath you. Correct?
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Old 12-16-2005, 10:12 PM
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Default Re: If the GPS says 85mph, how fast am I going?

...just read this whole thread. I'll be back after a while, I'm going to go outside and bang my head against the brick wall.

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Old 12-16-2005, 10:18 PM
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Default Re: If the GPS says 85mph, how fast am I going?

Quote:
Originally Posted by Troutly
Harper.....you are missing the point

The point is that his GPS is reading 85 MPH. So, it makes no difference how fast or slow the water is moving. His true speed over ground (which is stationary), is 85 MPH.

The radar analogy from shore is the same. The radar is stationary, so it is only clocking the true speed of the boat in relation to the stationary radar....so in essence it would be clocking the boats ground speed. It would make no difference how fast the water itself is moving.

Now, place the radar unit in the moving boat and you could theoretically see the reading change depending on going with or against the current (or waves). This is because the radar's low doppler uses the 'ground' to determine the speed of the vehicle. If the 'ground' is moving, then this could effect the accuracy of the reading.

If Nort had said his water pickup speedometer was showing 85 MPH and he was running for or against a current, then his actual ground speed could also be different then the speed shown on the speedo. This is because as the boat passes through the water, it is as if the speedo assumes that water is stationary in the speed it displays, so if the boat is moving for or against the current it can increase or decrease the displayed speed as compared to it's true speed over ground (kind of like an airplane's airspeed versus ground speed).

Here end'th the lesson

I understand where you're coming from, so I haven't missed any point. You may have just misunderstood my point??

Let me ask you this:

If at WOT Nort achieves an 85 mph GPS reading going down stream with a 4 mph current... What would it do to Nort's GPS reading at WOT going upstream against a 4 mph current?
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Old 12-16-2005, 10:30 PM
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Default Re: If the GPS says 85mph, how fast am I going?

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Originally Posted by US1 Fountain
Call me dumb, but I STILL don't understand the difference of GPS reading on land or water. Even after the numerous repeated attempts to explain.

Seems to me since the GPS gets it's data from satelites triangulations (big word) only thing it is reading is the speed of the moving object (boat) in reference to at least 3 satelites.
There really is no difference. It comes down to how much more effort (throttle or rpm's) is needed to achieve a certain speed going up stream vs. going down stream.

A GPS reading will be higher at a specified rpm going down stream vs. going upstream at the same specified rpm.

That's the only point I'm trying to make. Sorry to have confused anyone...
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Old 12-16-2005, 10:57 PM
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Default Re: If the GPS says 85mph, how fast am I going?

Quote:
Originally Posted by Harper220
I understand where you're coming from, so I haven't missed any point. You may have just misunderstood my point??

Let me ask you this:

If at WOT Nort achieves an 85 mph GPS reading going down stream with a 4 mph current... What would it do to Nort's GPS reading at WOT going upstream against a 4 mph current?
The boats doing 81mph through the water. Down stream the GPS will read 85mph. Up stream the GPS reading will be 77mph. You guys probably know this but some of your answers were starting to scare me. This answer is for flat water and no wind. I hope this makes since to somebody.
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Old 12-16-2005, 11:13 PM
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Default Re: If the GPS says 85mph, how fast am I going?

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Originally Posted by FREEDOM US1
The boats doing 81mph through the water. Down stream the GPS will read 85mph. Up stream the GPS reading will be 77mph. You guys probably know this but some of your answers were starting to scare me. This answer is for flat water and no wind. I hope this makes since to somebody.
AUGHHHHHHHHHH! I can't take it no mo. I can see speedo having different readings because the pickup is in the water.

I'll never understand.
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