Does anyone know if there is a set relationship between the accuracy / DOP/
Differenctial indicated on the GPS page to the accuracy of the compass
direction? ( of the GPS)

ie. Trying to determine how far off the compass rose would be off if WAAS
sat's were being received and there was high accuracy.

Kind of new at this. Any thoughts appreciated.

On Mon, 26 Jan 2004 21:12:28 -0500, "wg992000"
wrote:

Does anyone know if there is a set relationship between the accuracy / DOP/
Differenctial indicated on the GPS page to the accuracy of the compass
direction? ( of the GPS)

ie. Trying to determine how far off the compass rose would be off if WAAS
sat's were being received and there was high accuracy.

Kind of new at this. Any thoughts appreciated.

The compass course given by the gps depends more on how fast you are
going than whether WAAS is on. The magnetic course depends then on
whatever the software has for variation. Variation is a slowly
changing function of location, so won't depend on WAAS at all.

But, since you don't really know what the software does, you would be
better reading true if you are trying to swing your compass with the
GPS. Then use the variation from your chart.




Rodney Myrvaagnes J36 Gjo/a

Entering your freshman dorm for the first time, and seeing
an axe head come through the door on your right.

I'm going to stick my neck out and ask a very newbie type question. Rodney
mentioned reading "True" rather than "Magnetic." I've puzzled for a while
now over why we would use "True" at all. I know... I should know...but I
don't.
The compass is obviously "Magnetic" and it would be used for taking bearings
while under way. That data is then transferred to a paper chart that has a
compass rose with a magnetic measurement in it.
In short, when would one use the "True" measurement on a chart, when
reporting ones position to another, in a electronic charting program or any
other occasion?

Blushing Bob

"SB" wrote in message
...
I'm going to stick my neck out and ask a very newbie type question. Rodney
mentioned reading "True" rather than "Magnetic." I've puzzled for a while
now over why we would use "True" at all. I know... I should know...but I
don't.
The compass is obviously "Magnetic" and it would be used for taking
bearings
while under way. That data is then transferred to a paper chart that has a
compass rose with a magnetic measurement in it.
In short, when would one use the "True" measurement on a chart, when
reporting ones position to another, in a electronic charting program or
any
other occasion?

The GPS, when moving, points to the true geographic north, while a compass
points to the magnetic north. These are not the same. The magnetic north is
a few degrees off, depending on time and where you are. A good chart will
show this magnetic variance in the compass rose printed on the chart.

Meindert

On Tue, 27 Jan 2004 09:05:27 -0800, "SB" wrote:

I'm going to stick my neck out and ask a very newbie type question. Rodney
mentioned reading "True" rather than "Magnetic." I've puzzled for a while
now over why we would use "True" at all. I know... I should know...but I
don't.
The compass is obviously "Magnetic" and it would be used for taking bearings
while under way. That data is then transferred to a paper chart that has a
compass rose with a magnetic measurement in it.
In short, when would one use the "True" measurement on a chart, when
reporting ones position to another, in a electronic charting program or any
other occasion?

Sorry, I tried to make that clear. The 'true' measurement is derived
first from successive Lat-Lon measurements, so it is historical as
others have said. The 'magnetic' depends on a lookup table in memory
if it is done by the gps.

As you will see if you read the compass rose legend on a chart, the
variation changes slowly over the years. Unless you know if your gps
updates its table, and/or when its data were entered, you would be
taking a chance.

Even though the 'true' is historic, it is possible to use it for
swinging. You must motor along an ad hoc range line at constant speed,
the faster the better. A motorboat will be easier., but if the
direction reading doesn't change at all for some time. you should be
ok.

If you can hold the range and the speed over ground, your true
heading will also hold steady and be correct. If conditions prevent
you from holding, it's the wrong time to swing your compass.

HTH






Rodney Myrvaagnes J36 Gjo/a

Entering your freshman dorm for the first time, and seeing
an axe head come through the door on your right.

SB wrote:
I'm going to stick my neck out and ask a very newbie type question.
Rodney mentioned reading "True" rather than "Magnetic." I've puzzled
for a while now over why we would use "True" at all. I know... I
should know...but I don't.
The compass is obviously "Magnetic" and it would be used for taking
bearings while under way. That data is then transferred to a paper
chart that has a compass rose with a magnetic measurement in it.
In short, when would one use the "True" measurement on a chart, when
reporting ones position to another, in a electronic charting program
or any other occasion?

Blushing Bob

Since the position of magnetic north is changing perpetually, it is standard
practice to give all bearings etc, in true notation, and apply the
correction factor appropriate to the time and location of the navigator.

The compass rose on charts, where magnetic north has moved almost as soon as
the chart is published and sold, is for guidance only.

GPS bearings and calculated courses are corrected for magnetic north as at
the time and place of the fix shown on the screen, but the user has the
option to display in true notation if he/she so pleases. I personally set my
GPS to true readout, but others may prefer to set theirs to magnetic.


Remove "nospam" from return address.

GPS has no compass heading information, actually. This is why your
GPS doesn't know which way to point the "heading up" display on the
chart plotter when the boat is stopped, unless you have a compass
sensor input.

The heading information on a GPS-only system depends on measuring the
change in the lat/long over several measuring periods before it
displays heading information. If the boat merely rotates around its
anchor, this information will not be available or will be totally
inaccurate.

GPS receivers only provide 3 pieces of information. Latitude,
Longitude and Altitude....the 3 dimensions in space. All other
information (speed, heading, COG, VMG, etc.) are all derived by the
computer monitoring the changes in these 3 dimensions over
time.....many seconds. That's why it takes so long for it to change
heading or speed or VMG or other derived outputs when you make that
turn or come about.



On Mon, 26 Jan 2004 21:12:28 -0500, "wg992000"
wrote:

Does anyone know if there is a set relationship between the accuracy / DOP/
Differenctial indicated on the GPS page to the accuracy of the compass
direction? ( of the GPS)

ie. Trying to determine how far off the compass rose would be off if WAAS
sat's were being received and there was high accuracy.

Kind of new at this. Any thoughts appreciated.



Larry W4CSC

Is it just me or did the US and UK just capture 1/3
of the world's sweetest oil supply? What idiot wants to
GIVE IT BACK?!!

Larry W4CSC wrote:

GPS has no compass heading information, actually. This is why your
GPS doesn't know which way to point the "heading up" display on the
chart plotter when the boat is stopped, unless you have a compass
sensor input.

The heading information on a GPS-only system depends on measuring the
change in the lat/long over several measuring periods before it
displays heading information. If the boat merely rotates around its
anchor, this information will not be available or will be totally
inaccurate.

GPS receivers only provide 3 pieces of information. Latitude,
Longitude and Altitude....the 3 dimensions in space. All other
information (speed, heading, COG, VMG, etc.) are all derived by the
computer monitoring the changes in these 3 dimensions over
time.....many seconds. That's why it takes so long for it to change
heading or speed or VMG or other derived outputs when you make that
turn or come about.


You forgot time.

And how long is the "so long" you refer to in indicating a change in
heading or speed? It doesn't seem "so long" on my GPS instruments.
Perhaps you wired them up wrong, eh?


--
Email sent to is never read.

"Larry W4CSC" wrote in message
...
GPS receivers only provide 3 pieces of information. Latitude,
Longitude and Altitude....the 3 dimensions in space. All other
information (speed, heading, COG, VMG, etc.) are all derived by the
computer monitoring the changes in these 3 dimensions over
time.....many seconds. That's why it takes so long for it to change
heading or speed or VMG or other derived outputs when you make that
turn or come about.

I have heard otherwise: A GPS derives it's speed from the doppler shift on
the received carriers. A very slow averaging filter smoothes the output,
which causes the delay in indicated speed. The filter is there to prevent
erroneous speed indication due to atmospheric disurbances and multi-path
signals.

Meindert

On Tue, 27 Jan 2004 12:34:35 +0100, "Meindert Sprang"
wrote:

"Larry W4CSC" wrote in message
...
GPS receivers only provide 3 pieces of information. Latitude,
Longitude and Altitude....the 3 dimensions in space. All other
information (speed, heading, COG, VMG, etc.) are all derived by the
computer monitoring the changes in these 3 dimensions over
time.....many seconds. That's why it takes so long for it to change
heading or speed or VMG or other derived outputs when you make that
turn or come about.

I have heard otherwise: A GPS derives it's speed from the doppler shift on
the received carriers. A very slow averaging filter smoothes the output,
which causes the delay in indicated speed. The filter is there to prevent
erroneous speed indication due to atmospheric disurbances and multi-path
signals.

Meindert


This might be true on some $70K sophisticated survey instrument or
other, but, here at least, our discussion is about GPS operation on a
cheaply-made piece of crap sold to the boating consumer at amazing
markups, not sophisticated electronics. What you're talking about
costs serious money. You won't find that at Waste Marine where price
isn't related to quality.....(c;




Larry W4CSC

Is it just me or did the US and UK just capture 1/3
of the world's sweetest oil supply? What idiot wants to
GIVE IT BACK?!!