Indicated Speed VS. GPS Speed
02-27-02, 10:14 PM
#1
Haven't we ALL heard this
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Indicated Speed VS. GPS Speed
When I mean GPS speed that is a Global Posistion Satalite device
I got one for a graduation gift (Garmin 295) and have taken it on several road trips. I have noticed that about 40mph the stock speedo is rather off. I don't remember any speeds but I know what my needle points to is much slower than what the GPS says. I trust the GPS because it has my posistion down to 15 feet.
Noticed the same trend in my 86 and 91. Maybe you guys can take me and my GPS along for some top speed runs
James
I got one for a graduation gift (Garmin 295) and have taken it on several road trips. I have noticed that about 40mph the stock speedo is rather off. I don't remember any speeds but I know what my needle points to is much slower than what the GPS says. I trust the GPS because it has my posistion down to 15 feet.Noticed the same trend in my 86 and 91. Maybe you guys can take me and my GPS along for some top speed runs
James
02-27-02, 10:43 PM
#2
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I work with aircraft and GPS all the time. Your speedo is going to be off but not that much from what your speed is. What does your GPS say when you are at 40mph. normally you are only going to 2- 4 mph faster than what your speedo is saying. And dont always trust your GPS they sometimes are wrong, well at least my in my plane sometimes is.
02-28-02, 07:18 AM
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Isn't gps only accurate to 3 meters or so per reading? So if your talking 9 feet in any direction at the time of location reading, couldn't that throw off the speed from the GPS unit?
02-28-02, 09:36 AM
#6
Lives on the Forum
Don't let GPS readings fool you - they are not the absolute authority on speed readings.  Read the instructions, it will tell you +/- percentage errors.  Most "consumer" grade GPS units are only good to about a 100 feet.
-Ted
-Ted
02-28-02, 10:23 AM
#7
Ex fd *****
The Positional errors quoted are for single reading absolute position,(ie +- 100 ft) when used for differential positions during a single session (ie distance traveled from point a to point b while on continuously) they are very accurate - down to 1 foot/mile and better
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02-28-02, 12:31 PM
#9
Ex fd *****
Yes your ABSOLUTE position error is 15' but the RELATIVE distance Between two positions taken during the SAME session will be veru accurate ie when you take a position reading your true position will be somewhere in a 15'r circle around that point, and the same will be true when you take a second position, BUT the ERROR VECTOR for the two possitions will be aprox the same for two positions taken during a single session - meaning the Distance computed between the two positions are very accurate. even if the LOCATIONS are not very PRECISE
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