otnmbrd wrote in message
ink.net...
Jeff Morris wrote:
Eisboch wrote:


I have been told that factory GPS and most high-end retrofit systems
for
cars also use a speed sensor input from the car transmission that is
separate from the GPS signal. It is used primarily to keep position
calculations going when the satellite signal is lost (tunnel) but it
also
helps improve the accuracy of the position readings.


That is the claim for my wife's Toyota nav.

I don't think ours uses a "lock to road" feature - watching the marker
as we drive we were frequently 10 to 20 feet on either side of the road.
Also, it seems to take us "off road" 50 feet going down the driveway.



This is a similar problem on marine chart plotters when doing close-in
work.
Aside from the typical "lag" time, you will frequently find yourself not
exactly where you want to be and/or are. G Gonna be awhile before GPS
takes over completely, but as processors get faster......

otn

Yeah. It's a son-of-a-gun when you think you know where you are in an
unmarked channel and suddenly, as the GPS gets a download update, you find
yourself having moved 30 feet sideways.

Eisboch

We tried a WAAS enabled Garmin and were able to successfully navigate to
each
of the Taps at our Sailing Clubs bar. We entered waypoints and were
successful
in navigating back to each tap. Unfortunately after several demonstrations,
our
ability to navigate back to the appropriate tap started to
diminish...However,
I'm reasonable sure it had nothing to do with the position displayed on the
GPS. :-)

--
Cheers,
Jeffrey Nelson
Muir Caileag
C&C 30
"Jeff Morris" wrote in message
...
Try this for an experiment:

Cover up your windshield and drive entirely by trying to follow the line
on you car's GPS system. That's what you doing on a boat in the fog.

Also, consider that the streets have been well mapped in the last few
years by survey companies that drive around in cars and mark each
intersection. Many nautical charts are based on century old data. Much
of the issue is the map/chart quality, not the accuracy of the GPS itself.

Actually, the boat gps is just as accurate, perhaps even better since
there is no shielding from high buildings. Its just that on a boat you
may be relying 100% on the GPS as the primary, and perhaps only, source of
position. In a car, you glance occasionally at the map plotter and would
never notice if the GPS sometimes said you were on someone's front lawn.

One can easily imagine a situation on a boat where failure of the GPS
would cause you to drop anchor and wait until the weather clears; I doubt
you would do the same in a car!


BTW, my wife got a new car with a nav system just last night. It
currently in the driveway reporting two different street addresses
(depending on how you ask), both wrong. Both are actually nonexistent
addresses - one of them would be a hundred yards past the other end of a
dead end street. The actually position on the map is well reported,
however.


anchorlt wrote:
I keep reading about GPS for boats and how innacurate it can be.

Why is the same not innacurate in cars? My car GPS is so accurate it
tells me I am on the white line at a
traffic signal stop light or in my driveway.

Why not the same for boats?

Hilarious!

But as we know (and has been mentioned at length in another thread), one
should not rely on a single method of navigation. I presume the GPS in this
case was supplemented by celestial (if an outdoor bar), or dead reckoning???

--Alan G.

"DARat" wrote in message
...
We tried a WAAS enabled Garmin and were able to successfully navigate to
each
of the Taps at our Sailing Clubs bar. We entered waypoints and were
successful
in navigating back to each tap. Unfortunately after several
demonstrations, our
ability to navigate back to the appropriate tap started to
diminish...However,
I'm reasonable sure it had nothing to do with the position displayed on
the
GPS. :-)

--
Cheers,
Jeffrey Nelson
Muir Caileag
C&C 30
"Jeff Morris" wrote in message
...
Try this for an experiment:

Cover up your windshield and drive entirely by trying to follow the line
on you car's GPS system. That's what you doing on a boat in the fog.

Also, consider that the streets have been well mapped in the last few
years by survey companies that drive around in cars and mark each
intersection. Many nautical charts are based on century old data. Much
of the issue is the map/chart quality, not the accuracy of the GPS
itself.

Actually, the boat gps is just as accurate, perhaps even better since
there is no shielding from high buildings. Its just that on a boat you
may be relying 100% on the GPS as the primary, and perhaps only, source
of position. In a car, you glance occasionally at the map plotter and
would never notice if the GPS sometimes said you were on someone's front
lawn.

One can easily imagine a situation on a boat where failure of the GPS
would cause you to drop anchor and wait until the weather clears; I doubt
you would do the same in a car!


BTW, my wife got a new car with a nav system just last night. It
currently in the driveway reporting two different street addresses
(depending on how you ask), both wrong. Both are actually nonexistent
addresses - one of them would be a hundred yards past the other end of a
dead end street. The actually position on the map is well reported,
however.


anchorlt wrote:
I keep reading about GPS for boats and how innacurate it can be.

Why is the same not innacurate in cars? My car GPS is so accurate it
tells me I am on the white line at a
traffic signal stop light or in my driveway.

Why not the same for boats?

"anchorlt" wrote in message
om...
I keep reading about GPS for boats and how innacurate it can be.

Why is the same not innacurate in cars?

My car GPS is so accurate it tells me I am on the white line at a
traffic signal stop light or in my driveway.

Why not the same for boats?

I'm a land surveyor (and boat owner) and I've been working with GPS for
about 11 years now. Accuracy of GPS depends on several factors, but here's
some general info...

In the early days of GPS, SA (Selective Availability) was active. SA was a
method by which the Defense Department (DoD) purposely degraded the quality
of the signal and data to presumably prevent a potential enemy from using
the system for targeting purposes against US. The accuracy of an autonomous
(stand alone, no corrections, post-processing, etc...) GPS position during
that period was about +/- 100 m horizontally, about 150 m vertically at
two-sigma (95% of the time).

During that period, the marine community (and others) obviously had a need
for better accuracy than that. This is when the USCG began installing the
GPS beacons. These beacons consisted of a GPS receiver being placed over a
precisely surveyed point. The receiver would compute its autonomous position
based upon the satellite signals. The resulting GPS position would be
compared to the known position, and a correction computed. That computed
correction data was then broadcast over the airwaves, and was freely
accessible to anyone who had a "beacon receiver". Some GPS receivers had
beacon capabilities built-in, other set-ups used a seperate beacon receiver
linked to the GPS unit via a NEMA or RTCM interface. With beacon corrections
applied, the accuracy of the GPS was on the order of 10 to 15 m
horizontally.

Towards the end of the Clinton era, the White House ordered the DoD to come
up with some other method of safeguarding us from hostile use of GPS. There
were too many developing commercial uses for GPS to limit the accuracy,
vehicle navigation being one of them. So in May 1998 (I believe), SA was
turned off. The result was that just about any commercially available GPS
receiver could now determine positions to about 10 - 15 m without the aid of
the beacons.

During this time, the FAA began developing WAAS (Wide Area Augmentation
System) as the aviation community's answer to GPS accuracy. WAAS makes use
of corrections computed from a network of ground-based GPS receivers all
over the country. The correction data is computed for various regions (the
corrections will vary over distance and location) and uploaded to various
geo-stationary satellites which each have an "area" of coverage on the face
of the earth. From these satellites, the corrections are broadcast over
their respective coverage areas. The WAAS correction is broadcast on the
same frequency as the GPS L1 signal, somewhere around 1.2 GHz. Since it's on
the same frequency as that used by most consumer GPS receivers (there are
currently two GPS frequencies, with plans to add three more), GPS
manufacturers simply had to add the capability to use the data. Both the FAA
and the NGS (National Geodetic Survey) have performed extensive testing on
the accuracy of WAAS enabled GPS. The results have shown accuracies of
better than 5 m horizontally, at 95%. My personal experience with a Garmin
GPS76S handheld has shown typical accuracies of 3-5 m (I do high accuracy
geodetic control, so have lots of chances to compare). Not all GPS units are
WAAS capable, so if you're in the market, make sure it says so. Also, as a
CYA, the FAA has not officially sanctioned WAAS for use in mission-critical
applications, where lives and property are at stake. The system currently
broadcasts a "health" bit in the data stream saying the system is not
functional. Most WAAS capable receivers have an option to use the data even
though the health bit has been set. You may need to change the default
settings on the receiver.

So, the accuracy of the GPS, be it a vehicle, vessel or aircraft is
essentially the same. You will most likely see better performance on a boat
since you should typically have an un-obstructed view of the sky. As others
have mentioned, the most likely cause of any discrepancies you might see
when watching the map display is the accuracy of the base map data. Most of
the mapping and charting that has been done in the US was originally based
upon the North American Datum of 1927 (NAD27). Recent mapping may have been
done on NAD83, a more recent datum. The GPS is based upon the World Geodetic
System of 1984 (WGS84). Confused yet? WGS84 and NAD83 are very nearly
identical (To the geodesists, yes, I know they are not identical...) and for
most purposes they may be treated as the same. NAD27 is a different story.
If you plot a NAD83/WGS84 position on a NAD27 map or chart, it will be in
error somewhere on the order of 30 m in the northing and 100 m in the
easting (Lat and Lon). The exact error is dependent upon your latitude. The
numbers cited are good at around 45 degrees north.

Therefore, it is critical that for navigation, both your GPS and the
maps/charts are on the same datum. Most GPS receivers can be set to work on
just about any datum you want, but it needs to be the same as your maps.

Because most of the mapping was NAD27, and the GPS uses WGS84, many mapping
packages have been "transformed" from the old system to the new.
Unfortunately, there is no exact mathematical translation from NAD27 to
NAD83/WGS84. Some map makers have done a better job of transforming their
products than others. Some have been transformed using the best models
available from the NGS and others, but some have merely been
"rubber-banded". It's advisable to do your homework and make sure that both
your base maps and GPS are capable of working in the same datum.

Sorry for the length, but hope it helps...

Tom




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Yeah, definitely dead reckoning. :-)

--
Cheers,
Jeffrey Nelson
Muir Caileag
C&C 30
"Alan Gomes" wrote in message
news:C4apd.80995$V41.318@attbi_s52...
Hilarious!

But as we know (and has been mentioned at length in another thread), one
should not rely on a single method of navigation. I presume the GPS in
this case was supplemented by celestial (if an outdoor bar), or dead
reckoning???

--Alan G.

"DARat" wrote in message
...
We tried a WAAS enabled Garmin and were able to successfully navigate to
each
of the Taps at our Sailing Clubs bar. We entered waypoints and were
successful
in navigating back to each tap. Unfortunately after several
demonstrations, our
ability to navigate back to the appropriate tap started to
diminish...However,
I'm reasonable sure it had nothing to do with the position displayed on
the
GPS. :-)

--
Cheers,
Jeffrey Nelson
Muir Caileag
C&C 30
"Jeff Morris" wrote in message
...
Try this for an experiment:

Cover up your windshield and drive entirely by trying to follow the line
on you car's GPS system. That's what you doing on a boat in the fog.

Also, consider that the streets have been well mapped in the last few
years by survey companies that drive around in cars and mark each
intersection. Many nautical charts are based on century old data. Much
of the issue is the map/chart quality, not the accuracy of the GPS
itself.

Actually, the boat gps is just as accurate, perhaps even better since
there is no shielding from high buildings. Its just that on a boat you
may be relying 100% on the GPS as the primary, and perhaps only, source
of position. In a car, you glance occasionally at the map plotter and
would never notice if the GPS sometimes said you were on someone's front
lawn.

One can easily imagine a situation on a boat where failure of the GPS
would cause you to drop anchor and wait until the weather clears; I
doubt you would do the same in a car!


BTW, my wife got a new car with a nav system just last night. It
currently in the driveway reporting two different street addresses
(depending on how you ask), both wrong. Both are actually nonexistent
addresses - one of them would be a hundred yards past the other end of a
dead end street. The actually position on the map is well reported,
however.


anchorlt wrote:
I keep reading about GPS for boats and how innacurate it can be.

Why is the same not innacurate in cars? My car GPS is so accurate it
tells me I am on the white line at a
traffic signal stop light or in my driveway.

Why not the same for boats?

Tom

this may well be one of the best posts I have reac on this group!

Thanks for sharing info so clearly.

Jimmy


Tom Cade wrote:
"anchorlt" wrote in message
om...

I keep reading about GPS for boats and how innacurate it can be.

Why is the same not innacurate in cars?

My car GPS is so accurate it tells me I am on the white line at a
traffic signal stop light or in my driveway.

Why not the same for boats?


I'm a land surveyor (and boat owner) and I've been working with GPS for
about 11 years now. Accuracy of GPS depends on several factors, but here's
some general info...

In the early days of GPS, SA (Selective Availability) was active. SA was a
method by which the Defense Department (DoD) purposely degraded the quality
of the signal and data to presumably prevent a potential enemy from using
the system for targeting purposes against US. The accuracy of an autonomous
(stand alone, no corrections, post-processing, etc...) GPS position during
that period was about +/- 100 m horizontally, about 150 m vertically at
two-sigma (95% of the time).

During that period, the marine community (and others) obviously had a need
for better accuracy than that. This is when the USCG began installing the
GPS beacons. These beacons consisted of a GPS receiver being placed over a
precisely surveyed point. The receiver would compute its autonomous position
based upon the satellite signals. The resulting GPS position would be
compared to the known position, and a correction computed. That computed
correction data was then broadcast over the airwaves, and was freely
accessible to anyone who had a "beacon receiver". Some GPS receivers had
beacon capabilities built-in, other set-ups used a seperate beacon receiver
linked to the GPS unit via a NEMA or RTCM interface. With beacon corrections
applied, the accuracy of the GPS was on the order of 10 to 15 m
horizontally.

Towards the end of the Clinton era, the White House ordered the DoD to come
up with some other method of safeguarding us from hostile use of GPS. There
were too many developing commercial uses for GPS to limit the accuracy,
vehicle navigation being one of them. So in May 1998 (I believe), SA was
turned off. The result was that just about any commercially available GPS
receiver could now determine positions to about 10 - 15 m without the aid of
the beacons.

During this time, the FAA began developing WAAS (Wide Area Augmentation
System) as the aviation community's answer to GPS accuracy. WAAS makes use
of corrections computed from a network of ground-based GPS receivers all
over the country. The correction data is computed for various regions (the
corrections will vary over distance and location) and uploaded to various
geo-stationary satellites which each have an "area" of coverage on the face
of the earth. From these satellites, the corrections are broadcast over
their respective coverage areas. The WAAS correction is broadcast on the
same frequency as the GPS L1 signal, somewhere around 1.2 GHz. Since it's on
the same frequency as that used by most consumer GPS receivers (there are
currently two GPS frequencies, with plans to add three more), GPS
manufacturers simply had to add the capability to use the data. Both the FAA
and the NGS (National Geodetic Survey) have performed extensive testing on
the accuracy of WAAS enabled GPS. The results have shown accuracies of
better than 5 m horizontally, at 95%. My personal experience with a Garmin
GPS76S handheld has shown typical accuracies of 3-5 m (I do high accuracy
geodetic control, so have lots of chances to compare). Not all GPS units are
WAAS capable, so if you're in the market, make sure it says so. Also, as a
CYA, the FAA has not officially sanctioned WAAS for use in mission-critical
applications, where lives and property are at stake. The system currently
broadcasts a "health" bit in the data stream saying the system is not
functional. Most WAAS capable receivers have an option to use the data even
though the health bit has been set. You may need to change the default
settings on the receiver.

So, the accuracy of the GPS, be it a vehicle, vessel or aircraft is
essentially the same. You will most likely see better performance on a boat
since you should typically have an un-obstructed view of the sky. As others
have mentioned, the most likely cause of any discrepancies you might see
when watching the map display is the accuracy of the base map data. Most of
the mapping and charting that has been done in the US was originally based
upon the North American Datum of 1927 (NAD27). Recent mapping may have been
done on NAD83, a more recent datum. The GPS is based upon the World Geodetic
System of 1984 (WGS84). Confused yet? WGS84 and NAD83 are very nearly
identical (To the geodesists, yes, I know they are not identical...) and for
most purposes they may be treated as the same. NAD27 is a different story.
If you plot a NAD83/WGS84 position on a NAD27 map or chart, it will be in
error somewhere on the order of 30 m in the northing and 100 m in the
easting (Lat and Lon). The exact error is dependent upon your latitude. The
numbers cited are good at around 45 degrees north.

Therefore, it is critical that for navigation, both your GPS and the
maps/charts are on the same datum. Most GPS receivers can be set to work on
just about any datum you want, but it needs to be the same as your maps.

Because most of the mapping was NAD27, and the GPS uses WGS84, many mapping
packages have been "transformed" from the old system to the new.
Unfortunately, there is no exact mathematical translation from NAD27 to
NAD83/WGS84. Some map makers have done a better job of transforming their
products than others. Some have been transformed using the best models
available from the NGS and others, but some have merely been
"rubber-banded". It's advisable to do your homework and make sure that both
your base maps and GPS are capable of working in the same datum.

Sorry for the length, but hope it helps...

Tom




x-- 100 Proof News - http://www.100ProofNews.com
x-- 3,500+ Binary NewsGroups, and over 90,000 other groups
x-- Access to over 1 Terabyte per Day - $8.95/Month
x-- UNLIMITED DOWNLOAD