Do chart plotters "lie" ?

[Jaxfishgyd]

I installed a Garman GPSmap 182C and a Garman 2010 GPSmap chart plotter in 2004 at the same time ... I WANTED dual units if for no other reason, safety....
But this morning I was playing with them (I turn all my electronics on every 2 weeks or so if not away from the dock) and started looking at the different functions...
I noticed the 182 showed we've traveled 1348 miles total at an ave. speed of 7 kts... OK, that sounds right... but it also showed our TOP SPEED to of been 44 kts..... Now that's funny.....
So then I checked the 2010 info... It showed a total of 1354 miles traveled, but also showed an extra hour of use... Again, this is realistic as I always leave that unit on when we're anchored out for the anchor alarm & I would not turn the 182 unit on every time before we pulled in the anchor( and both units only record hours used when the engines are running)... And this unit showed an ave. speed of 7.1 kts...
But then it showed my top speed during this time as 246 mph.....
I don't think even a sailboat can do that speed...
But the engine hour meters also agree with the GPS's hours shown........

 

[Avenger]

I've had GPSs misread speeds when they weren't getting good satellite signal. I had a handheld in a car once tell me I was doing 245 MPH.

 

[bobk]

The high speed thing also happened to me on a 2010. In my case, it was due to a faulty power supply. It would shut down for a few minutes, then start and when it restarted, it registered a large distance traveled in just the few seconds that the machine was repowered.

Bobk

 

[Freeebird]

Yep, they lie. Sooner or later they'll show you running 200MPH+... on dry land.

 

[jim rosenthal]

The GPS in my truck does the same thing. Unless you believe a Toyota Tacoma (the four cylinder version) can do 180 mph.

 

[MikeP]

I don't think they mean to, they just don't know any better.

I have had my chartplotter, with the boat tied in the slip, claim the boat had just moved several hundred yards at speeds up to 70 Knots. I took a picture of that - my plan was to post it here to show how fast a well-tuned 53MY could go! But I never got around to posting it. That was several years ago and I have no idea where in the ether that pic now resides.

But yes, chart plotters lie!

 

[rsmith]

There used to be a time sampling setting on the fix intervals. Back before the SA was turned off fix intervals less than 5 min apart would yield direction and speed errors. Loran C was really bad for this when the TD's were less than perpendicular.

 

[Dan Mapes]

When we did the Erie Canal I could swear we were in the water doing about 8 knots but it consistently told us we were moving across dry land on a parallel course.

 

[bobk]

When we did the Erie Canal I could swear we were in the water doing about 8 knots but it consistently told us we were moving across dry land on a parallel course.

That's probably a charting problem. It happens with the AICW charts in places like near Myrtle Beach. Old longitude errors that were never corrected.

Bobk

 

[Jaxfishgyd]

Was on the www.marinetraffic.com last night and AIS showed a boat doing 77 mph.... So I clicked on it and it WAS doing 77 mph!!! seems it was on a trailer on I-95 just south of W. Palm Beach headed north...