Speed - GPS or Speedometer
 

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 Sun, 29 Aug 2004 20:52:50 -0400, "JGK"
wrote:

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?

Yes, the speedometer is relative speed to water, GPS is actual speed
over surface of earth.

On Sun, 29 Aug 2004 20:52:50 -0400, "JGK"
wrote:

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?

The GPS is more accurate - the speedo is relative and usually works on
air - much like a pito tube in fact, only in this case, it's water
compressing air in the line. The only real true speedo, is the paddle
type as that works on wheel RPM and doesn't rely on air compression.

Having said that, there shouldn' t be a major difference - more than
10%. Usually, they are pretty close. If you have the air type, take
it off the guage and probe (or motor), blow some air through it and
put the line back. That should bring the two closer together.

You might also want to check if you have the probe type, so be sure
the probe is staying in the water and not moving around a lot - also
want to check the exposed part of the line - I've had a leak in the
line and a probe that moved around.

If it's the paddle type, then look it over and see if there is some
icky sticky stuff creating a problem.

Good luck.

Take care.

Tom

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

"Short Wave Sportfishing" wrote in message
...
On Sun, 29 Aug 2004 20:52:50 -0400, "JGK"
wrote:

Having said that, there shouldn' t be a major difference - more than
10%. Usually, they are pretty close. If you have the air type, take
it off the guage and probe (or motor), blow some air through it and
put the line back. That should bring the two closer together.

Thanks I will give that a try tommorrow.

You might also want to check if you have the probe type, so be sure
the probe is staying in the water and not moving around a lot - also
want to check the exposed part of the line - I've had a leak in the
line and a probe that moved around.

The pick up for my speedometer is in the Outboard. Merc 125 has the buil;t
in pick up tube.

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?

--
Dixon "Short Wave Sportfishing" wrote in
message ...
On Sun, 29 Aug 2004 20:52:50 -0400, "JGK"
wrote:

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?

The GPS is more accurate - the speedo is relative and usually works on
air - much like a pito tube in fact, only in this case, it's water
compressing air in the line. The only real true speedo, is the paddle
type as that works on wheel RPM and doesn't rely on air compression.

I have wondered if the paddle type is flawed in theory. If you placed the
paddle in uninteruped water, such as say alongside or in front of the boat
it should be accurate except for water current. I've been suspicious about
the water the boat is displacing being accelerated along the bottom of the
hull.

dixon

On Mon, 30 Aug 2004 03:44:58 GMT, "dixon"
wrote:
I have wondered if the paddle type is flawed in theory. If you placed the
paddle in uninteruped water, such as say alongside or in front of the boat
it should be accurate except for water current. I've been suspicious about
the water the boat is displacing being accelerated along the bottom of the
hull.
====================================

All of the high quality paddle wheel speed units have a calibration
control where you can compensate for hull effects.

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

JohnC wrote in message . ..
On Sun, 29 Aug 2004 20:52:50 -0400, "JGK"
wrote:

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?

Yes, the speedometer is relative speed to water, GPS is actual speed
over surface of earth.

correct and add to it that boat speedometers are often just speed
estimators ... not very precise ..

Matt

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.

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

"Wayne.B" wrote in message
...
On Mon, 30 Aug 2004 03:44:58 GMT, "dixon"
wrote:
I have wondered if the paddle type is flawed in theory. If you placed the
paddle in uninteruped water, such as say alongside or in front of the
boat
it should be accurate except for water current. I've been suspicious
about
the water the boat is displacing being accelerated along the bottom of
the
hull.
====================================

All of the high quality paddle wheel speed units have a calibration
control where you can compensate for hull effects.

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

Then why do my speedometer on the car, truck, van, dads truck all match w/
the gps and never vary over time. If what you say is true, then at one
point it would match the truck and the next point it would have varied and
not match.
All the stuff you talk about adds maybe a .10 variation. Not 3% to 4% which
would be 2.1 to 2.8 mph off at 70 mph. It is not off far enough to tell.
And by the way - it matches exactly w/ the radar speed indicator on the side
of the highway that the state police put up in construction zones.

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

"Short Wave Sportfishing" wrote in message
...
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

On Tue, 31 Aug 2004 00:34:08 GMT, "Tony Thomas"
wrote:

Then why do my speedometer on the car, truck, van, dads truck all match w/
the gps and never vary over time. If what you say is true, then at one
point it would match the truck and the next point it would have varied and
not match.
All the stuff you talk about adds maybe a .10 variation. Not 3% to 4% which
would be 2.1 to 2.8 mph off at 70 mph. It is not off far enough to tell.
And by the way - it matches exactly w/ the radar speed indicator on the side
of the highway that the state police put up in construction zones.

Believe what you want - those are the numbers.

Later,

Tom



Later,

Tom

"JGK" wrote in message ...
"Short Wave Sportfishing" wrote in message
...
On Sun, 29 Aug 2004 20:52:50 -0400, "JGK"
wrote:

Having said that, there shouldn' t be a major difference - more than
10%. Usually, they are pretty close. If you have the air type, take
it off the guage and probe (or motor), blow some air through it and
put the line back. That should bring the two closer together.

Thanks I will give that a try tommorrow.

You might also want to check if you have the probe type, so be sure
the probe is staying in the water and not moving around a lot - also
want to check the exposed part of the line - I've had a leak in the
line and a probe that moved around.

The pick up for my speedometer is in the Outboard. Merc 125 has the buil;t
in pick up tube.

My neighbor's 135 Merc has the same thing, and that damned thing is
always partially clogging up.

Well let's see, you say your using a "standard speedo nothing special".
Here's a thought, Buy a pair of competition Tow Boat Speedometer's as they
use for exact speeds that are critical for tournament skiing and then
compare the speeds with your gps. You still would have a difference between
boat speed and GPS speed. I agree with all the other posts in the GPS
technical side as well. I went to FAA GPS faqs and the below statement
should help with considering the above mention delay's

GPS FAQ's from: http://gps.faa.gov/FAQ/index.htm

" The receiver uses the time difference between the time of signal reception
and the broadcast time to compute the distance, or range, from the receiver
to the satellite. The receiver must account for propagation delays, or
decreases in the signal's speed caused by the ionosphere and the
troposphere."

Regards,
Ron



"Matt Lang" wrote in message
om...
JohnC wrote in message
. ..
On Sun, 29 Aug 2004 20:52:50 -0400, "JGK"
wrote:

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?

Yes, the speedometer is relative speed to water, GPS is actual speed
over surface of earth.

correct and add to it that boat speedometers are often just speed
estimators ... not very precise ..

Matt