"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

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.

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.

Larry W4CSC wrote:
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?!!
If you want a good DGPS I would suggest Trimble. Really execllent.

Rick

Thanks for all the input -
It was the swinging the compass input I was after.

Thought process was that maybe with decent sat reception - swinging the
compass (adjustments and compass card) may be easier and more accurate with
the GPS then using traditional methods i.e.. lining up bearing points,
shadows etc with my 13 year old at the wheel and me trying to do this. Have
a 21 coastal boat with compass in 5 degree increments. Sounds like the best
idea is stick with traditional methods or farm it out. In any case thanks
for all the help and thoughts.


"Larry W4CSC" wrote in message
...
Oops....I also forgot TIME. GPS does provide very accurate TIME to
make the calculations in my other post....

Sorry....(blush)

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?!!

"Meindert Sprang" wrote in message
...
"Larry W4CSC" wrote in message
...
On Tue, 27 Jan 2004 12:34:35 +0100, "Meindert Sprang"
wrote:

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;

Well, the datasheet of my "cheaply-made piece of crap" GPS module, costing
a
whopping $70, clearly states the presence of doppler shift data in the raw
datastream I can extract from that module. The used chipset is a very
common
one in low end GPS receivers. Do a google search on "gps speed doppler"
and
you'll find this info.

Meindert


Tried a Google search for "gps speed doppler" and got the following message:

Your search - "gps speed doppler" - did not match any documents.

Suggestions:
- Make sure all words are spelled correctly.
- Try different keywords.
- Try more general keywords.
Also, you can try Google Answers for expert help with your search.

"BrianR" wrote in message
...

Tried a Google search for "gps speed doppler" and got the following
message:

Your search - "gps speed doppler" - did not match any documents.

Leave out the quote signs and you'll get 13,300 hits.

Meindert

On Tue, 27 Jan 2004 20:17:32 -0500, "wg992000"
wrote:

Thanks for all the input -
It was the swinging the compass input I was after.

Thought process was that maybe with decent sat reception - swinging the
compass (adjustments and compass card) may be easier and more accurate with
the GPS then using traditional methods i.e.. lining up bearing points,
shadows etc with my 13 year old at the wheel and me trying to do this. Have
a 21 coastal boat with compass in 5 degree increments. Sounds like the best
idea is stick with traditional methods or farm it out. In any case thanks
for all the help and thoughts.

I still like the idea of using the charts and local ranges across
Charleston Harbor......

Wouldn't it be nice if there were some kind of exact crossing ranges
setup for just compass swinging in some out of the commercial shipping
channel place. Two ranges pointing into an area unobstructed,
crossing in the middle. One N-S range and one E-W range with markers
on both ends so you could go either direction and KNOW you were 0 or
90 or 180 or 270.


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?!!

Thanks, I left the quotes in originally as I was looking for the exact
phrase.
Now get the 13,300 hits, much appreciated.


"Meindert Sprang" wrote in message
...
"BrianR" wrote in message
...

Tried a Google search for "gps speed doppler" and got the following
message:

Your search - "gps speed doppler" - did not match any documents.

Leave out the quote signs and you'll get 13,300 hits.

Meindert

"BrianR wrote:

Thanks, I left the quotes in originally as I was looking for
the exact phrase.
Now get the 13,300 hits, much appreciated.

If you go to http://www.alltheweb.com and enter GPS Speed Doppler
without quotes, and use the "all the words" function, you will get
34,344 hits.

I don't know if the extra 21,000+ hits matter to you, but there is
a wealth of information on some of these pages.

I hope this helps!
---
-
Topic-Mimara
Unique in the World!
---


-=-
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