Are GPS:S that accurate

[G-UNIT]

I have a garmen e-tech,
I'll run the UL-1 and get a reading of 49.6 mph one time and the next time without changing
anything, it wil read 42 mph,
for the most part the same water conditionds.
the gps might read accueacie of 10 feet and some times 3-5 feet,
is this a difference in possible speed meaurments?
batterys are the same with the same volts and mah.
sorry about the spelling.
what do you guys think about this

[Peregrine]

I have not used one in a boat, but I have used Garmens for years in my cars and I have watched the speed indication on the GPS and compared to the speedometer many times and I have never seen it vary by more than 1 MPH. My unit is accurate to about 3 feet I believe and at 40-50 MPH there is more than enough distance covered for it to get a reading more accurate then the 7 MPH you are seeing. So, I would first test it in your car and see how it does as using cruise control should keep a level speed close enough to check it.
If it checks out, I would have to assume that something is changing the speed of the boat runs as it showed. it is very hard to visually tell 42 from 47 by the naked eye at the distance you are running at. I can't guess what could cause the change though.

[G-UNIT]

thanks for the quick response, I will put the gps on the dash of my car and see/ check the readings with the speedo and see if it's accurate.

[m4a1usr]

I have been telling folks for years how undependable GPS is for accuracy based on my own knowledge and experience. It is a useful tool. It has its place in our hobby for making changes to our boats like when we swap props, install a new motor, batteries, the list is long. But to use it as a bragging tool or use it for comparison in proven race setups as a better choice its not going to hold water. The problem with the GPS units we use in our boats is they are not designed for our intended purpose. Our boats change the X,Y,Z axis immediately. In very short time frames. The software algorythims that do the time Vs distance calculations can have very large errors in them due to all sort of reasons. Losing sat lock, acceleration variences, etc. Differential receivers is how those that need a GPS unit to be dependable in measuring accurate speed. Our hand helds are not differential, they use WAAS. A good system for location. Not speed. Here is an comment right off the Garmin GPS web site.

"Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Garmin® GPS receivers are accurate to within 15 meters on average."

They have seven sources listed on the Garmin web site of signal errors that contribute to GPS inaccuracy. And our hand held GPS units will not tell you during use if any one or any combination of them has influenced your reading. All you get a displayed value to either inflate your ego or convince you it has to be correct.

John