On Mon, 30 Aug 2004 22:59:06 GMT, "Tony Thomas"
wrote:

My experience has show paddle wheels to be more accurate than the pressure
pickup but they are usually still off by a couple mph.

As to the accuracy of GPS. Look up any specs on the units at garmin.com.
Accuracy:
a.. Position: 15 meters (49 feet) RMS*
* Subject to accuracy degradation to 100m 2DRMS under the United States
Department of Defense-imposed Selective Availability Program.

a.. Velocity: 0.1 knot RMS steady state
Velocity is very accurate as it is a simple time shift of three or more
satallite signals.

FYI: It's only three sats - the others are considered "spares"

And the specs are correct.

However,

GPS specs are figured as being in "free space" - that is without
environmental considerations such as tropospheric distortion, Doppler
shift, phase shifts, loss of differential data, the geometry of the
satellite constellation at any time, how long the receiver hangs on to
a degrading signal, Horzontal/Vertical Dilution of Precision, clock
drift - oh, hell, a whole bunch of factors. Designers try to account
for the variables, but in free space, it's a perfect world.

In the real world, variables can affect GPS calcuations along the
order of 3 to 4% and even higher depending on one or more combinations
of those variables.

The only true way to determine how accurate the GPS is, is to repeat
the same course, at the same speed, a whole bunch of times, and
average the differences.

And I think you will find I'm right.

Later,

Tom

Your gps will be absolutely accurate (within 0.10 mph)
Your boat speedo will be +/- 10% usually but some are worse than that.
The boat speedo is just a hit/miss on being right. At some speed they are
usually right but all other speeds they will be off.

--
Tony
my boats at http://t.thomas.home.mchsi.com

"JGK" wrote in message
. ..
Just curious as to the accuracy between the two.
The speedometer in my boat is a standard speedo nothing special, the
GPS is a Hummingbird Matrix 25 which I just got a week or two ago.
After a few outings I noticed that there is at times a considerable
difference
between the two and other times they seem spot on. Can't quite figure
that
one out. Would a head current account for the boats speedometer to appear
at a higher MPH than the GPS?

On Mon, 30 Aug 2004 03:18:29 GMT, "Tony Thomas"
wrote:

Your gps will be absolutely accurate (within 0.10 mph).

It can be accurate given a correct view of the sky and a constant data
stream, but there are a number of variables in the GPS calculations
that allow for as much as 3 to 4% error. That accounts for the
accuracy of the positional data which can vary from zero to three
meters.

Take care.

Tom

"The beatings will stop when morale improves."
E. Teach, 1717