Newbie Question: GPS and "North"
Zig Popko
set it to Magnetic North? I understand the difference between the
two, but am interested in the advantages and disadvantages of each
setting. I always use a compass which offsets for the area's
declination.
Sam Wormley
Since you use a compass which offsets for the area's declination, the
answer should be obvious... use True North.
___________________________________________________
Sam Wormley - http://www.cnde.iastate.edu/maps.html
Dave Green
> When do you set your GPS to Geographic North and when do you
>set it to Magnetic North? I understand the difference between the
>two, but am interested in the advantages and disadvantages of each
>setting. I always use a compass which offsets for the area's
>declination.
I always navigate using true readings and offset my compass needle by
the amount of declination. It makes it easier for me to go from the
map to real world directions. I set my GPS to use true as well.
I was taught to always use one or the other and not to interchange, so
as to lessen the possibility of mistakes in navigating.
From your last comment it looks as though you use true readings as
well.
Dave
--
** Please remove the -anon if replying by email **
Tan PS
towards the ground rather than horizontal. It does not change the North
orientation. All compass are designed with a pendulous needle-pivot
arrangement so that the needle stays as horizontal as possible, prevents the
needling touching the dial base; for card types, prevents the card moving
out of the viewing window.
Declination on the other hand needs to be offset, but this can only be done
manually be making adjustments to your readings.
If u are using charts with Lat/Long readings, it will be easier to use True
North. If u are taking sightings with a compass, you will need magnetic
readings.
True North is used in areas where the magnetic reading is subject to large
variations or changing variations within a small area such as places with
mineral deposits or places with large declination, like the poles, resulting
in very weak horizontal component of the magnetic field to position the
needle.
Its is unlikely to be able to have a GPS/Magnetic compass device due to SA
which causes the unit to move when it is stationary and this will mess the
readings.
Tan PS
qys...@mbox2.singnet.com.sg
Zig Popko <SPo...@concentric.net> wrote in message
news:379d171f...@news.concentric.net...
Rhett James Barnes
(Zig Popko) wrote:
> When do you set your GPS to Geographic North and when do you
> set it to Magnetic North? I understand the difference between the
> two, but am interested in the advantages and disadvantages of each
> setting. I always use a compass which offsets for the area's
> declination.
Just my two cents....
I was already working with a compass which had a declination
adjustment, and maps with grid-lines oriented to true North for land
navagation. So it was easy just to set the GPS to true North when I
bought it, and I've never had a problem. I think that it would introduce
more complication to use magnetic North for me (mostly because the maps
are already ruled to true).
Since you already compensate for magnetic declination with your
compass, I would say stick with what you're used to. My understanding is
that all air navagation is based on magnetic North, so this would be the
one example where you would actually need the GPS set up for magnetic
North in order to work with the other cockpit instruments, charts, and
nav-aids (don't know about water navigation). Otherwise, you might want
to think about sticking with what is already comfortable for you...
Just as a side note, I think that my older GPS 45 would tell you the
magnetic correction (if you didn't already have it handy from the map) in
the set up menu. I thought this was a neat use of the GPS, if not kind of
useless for me.
Don't know if that was a help...
-Rhett
PPS. I know the time scale is a little long, but what are we all going to
do when the poles change, and all of our little gadgets no longer can
calculate the magnetic heading correctly? Does anyone know of a GPS unit
that will let you manually enter the precise locations of the poles? (Ok,
ok, I know, but our great, great, great, great, great...grandkids might
have to think about it... I thought that there was evidence that pole
reversal could occur on a fairly short time scale, by geological
standards)
Tan PS
unit do a mathematical correction. Magnetic variations depend on
physical/metal deposits and is not linear, around the far south
Atlantic/Indian Ocean it can be as much as 30-40 degrees.
Magnetic north is determined from isogonal charts, which plots the
variations based on survey measurements. This table is kept in units and
magnetic headings are determined from cross-referenced values.
If the mag poles change rapidly, it would be easier to use to stick with
True North only. If it is slow, as it is now, chart updates over time will
keep cross-ref values current. We won't have to worry about updates, our
units would by unserviceable or stolen by then.
Tan PS
Rhett James Barnes <cryor...@aol.com> wrote in message
news:cryorunner-27...@dt085n38.san.rr.com...
Phil Brady
Sam Wormley <swor...@cnde.iastate.edu> wrote in article
<379D1F97...@cnde.iastate.edu>...
> Zig Popko wrote:
> >
> > When do you set your GPS to Geographic North and when do you
> > set it to Magnetic North? I understand the difference between the
> > two, but am interested in the advantages and disadvantages of each
> > setting. I always use a compass which offsets for the area's
> > declination.
>
> Since you use a compass which offsets for the area's declination, the
> answer should be obvious... use True North.
>
> ___________________________________________________
> Sam Wormley - http://www.cnde.iastate.edu/maps.html
>
Sam,
what about the difference between true and grid north?
My own view is that true north is pretty unimportant and that the issues
surround grid and magnetic. My practice is to set my GPS for grid north
and to always set my compass dial similarly (that's easy if you simply omit
to adjust for declination/variation). When using the compass I always
align the needle with the point on the dial corresponding to the difference
between grid and magnetic north. That translates to pointing it at the
lower left of the N on the dial here in the UK but you will have to decide
your own point or put a dash of paint on the dial.
It's simple, you don't have to remember whether to add or subtract numbers,
and the procedure is the same whether you are taking GPS readings, compass
bearings or protractor bearings directly off the map.
Phil Brady
Dave Martindale
>There's no real need to offset declination as it causes the needle to tilt
>towards the ground rather than horizontal.
You seem to be talking about magnetic inclination here, not declination.
In fact, some manufacturers build different models of their compasses
for different inclination zones - the balance of the needle is adjusted
to keep it level in the centre of each zone. A few compasses use
separate magnet and pointer, allowing the magnet to tilt while the
pointer remains horizontal.
>Declination on the other hand needs to be offset, but this can only be done
>manually be making adjustments to your readings.
Most good compasses have internal mechanical declination adjustment, so
you don't have to make any mental adjustments yourself. The compass
reads directly in true north once the declination has been set correctly.
Dave
Roger C. Green
> "Tan PS" <NOSPAM...@mbox2.singnet.com.sg> writes:
> >There's no real need to offset declination as it causes the needle to tilt
> >towards the ground rather than horizontal.
>
> You seem to be talking about magnetic inclination here, not declination.
> In fact, some manufacturers build different models of their compasses
-----------------------------
The problem is that the terms "declination" and "variation" are used differently
in different parts of the world. This explains the discussion in this thread.
The angle between true N and magnetic N is called variation in some places and
declination in others.
To some, declination means something quite different: viz, the (vertical) angle
between the magnetic field line and the earth's surface.
Remember the golden rule: "Nobody says it has to make sense."
The variation in usage, which we hope would be declining, just depends on your
personal inclination.
Roger
Dave Martindale
>The variation in usage, which we hope would be declining, just depends on your
>personal inclination.
GROAN. But good!
Dave
Paul Edwards
Rhett James Barnes <cryor...@aol.com> wrote in message
news:cryorunner-27...@dt085n38.san.rr.com...
> PPS. I know the time scale is a little long, but what are we all going to
> do when the poles change, and all of our little gadgets no longer can
> calculate the magnetic heading correctly? Does anyone know of a GPS unit
> that will let you manually enter the precise locations of the poles? (Ok,
> ok, I know, but our great, great, great, great, great...grandkids might
> have to think about it... I thought that there was evidence that pole
> reversal could occur on a fairly short time scale, by geological
> standards)
My Garmin 12 has a flashrom, so as long as Garmin are willing to fix BUGS
due to their lack of foresight, I'm in with a shot. BFN. Paul.
Peter Bennett
<rcg...@morgan.ucs.mun.caNOJUNK> wrote:
>The problem is that the terms "declination" and "variation" are used differently
>in different parts of the world. This explains the discussion in this thread.
>The angle between true N and magnetic N is called variation in some places and
>declination in others.
I believe the terminology varies depending on the activity, rather than the
location.
In marine navigation, "variation" is the difference between true and
magnetic north in Canadian, US and British practice. "Declination" appears
to be used more in land navigation.
--
Peter Bennett, VE7CEI
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver-webpages.com/van-ps
Peter Bennett
>Most good compasses have internal mechanical declination adjustment, so
>you don't have to make any mental adjustments yourself. The compass
>reads directly in true north once the declination has been set correctly.
>
> Dave
I've never seen a marine compass (good or cheap) with a variation
adjustment - it must be some peculiarity of compasses used on land...
I have my GPSs set to show magnetic directions, for convenient use with my
magnetic compass.
Steve Hinch
>adjustment - it must be some peculiarity of compasses used on land...
Probably because in marine and air navigation, you frequently travel over great
distances, in which case the declination (or variation, for you sailors), can
change significantly. Rather than having to always remember to reset the
declination adjustment as you travel, it is probably safer to just work with
magnetic bearings. Often in land navigation, you are travelling over
reasonably short distances where it is acceptable to assume the declination is
constant. So you can just set the compass declination and forget it.
Here are some good web pages to give you more detail on the subject than you
might wish to know:
http://www.cam.org/~gouletc/decl_faq.html
http://geomag.usgs.gov/charts.html
http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl
Steve Hinch
Tan PS
Tan PS
Roger C. Green <rcg...@morgan.ucs.mun.caNOJUNK> wrote in message
news:379E2EF1...@morgan.ucs.mun.caNOJUNK...
> Dave Martindale wrote:
> -----------------------------
>
> The problem is that the terms "declination" and "variation" are used
differently
> in different parts of the world. This explains the discussion in this
thread.
> The angle between true N and magnetic N is called variation in some
places and
> declination in others.
>
> To some, declination means something quite different: viz, the (vertical)
angle
> between the magnetic field line and the earth's surface.
>
> Remember the golden rule: "Nobody says it has to make sense."
>
> The variation in usage, which we hope would be declining, just depends on
your
> personal inclination.
>
> Roger
>
Dave Martindale
>I've never seen a marine compass (good or cheap) with a variation
>adjustment - it must be some peculiarity of compasses used on land...
It's a feature of map compasses, which are designed to be used as
protractors for reading a course from a map, as well as measuring
directions. You can do this by laying the edge of the compass along
your desired course, then rotating the compass capsule until the
reference lines on it are oriented vertical, aligning with the UTM
grid on the map, or the longitude lines. At that point, the bearing
scale on the compass reads your course - but it's true course, not
magnetic. For this feature to be useful, the compass needs a
declination adjustment. It's done by rotating the arrow on the
baseplate relative to the 0 degree mark on the scale an amount
equal to the local declination.
It helps that map compasses are designed to be read from above,
not the side. They are also designed to be used by setting a
course, then rotating the whole compass until the needle lines
up with an indicator.
Marine compasses and sighting compasses are often designed to be
read from the side, and have the scale graduations on the rotating
magnetic card immersed in liquid. There's no real way to get into
it to set a declination offset between the magnet and the scale.
You *could* provide an adjustable lubber's line on the case to
give declination compensation - but then you'd have to read it
with your eye offset to one side. Suunto actually has a sighting
compass with two "lubbers line" markings - one for reading magnetic
bearing, the other adjustable for reading true bearing.
But mostly mariners seem accustomed to using magnetic compasses as they
are, working on charts that have magnetic compass roses on them, and
parallel rules to transfer the orientation of magnetic north to the
protractor they use to measure course bearings with. It all works,
but sort of requires a table to work with the chart, parallel, and
protractor (as far as I can see). A GPS set to read magnetic
bearings fits nicely into this system.
The nice thing about map compasses is that they don't need the
magnetic compass rose or the parallel rulers, and the protractor
is built in, so you can calculate course bearings on foot. A GPS
set to read true bearings fits nicely into this system.
Dave
Sam Wormley
>
> PPS. I know the time scale is a little long, but what are we all going to
> do when the poles change, and all of our little gadgets no longer can
> calculate the magnetic heading correctly? Does anyone know of a GPS unit
> that will let you manually enter the precise locations of the poles? (Ok,
> ok, I know, but our great, great, great, great, great...grandkids might
> have to think about it... I thought that there was evidence that pole
> reversal could occur on a fairly short time scale, by geological
> standards)
Knowledge of the coordinates of the Earth's magnetic poles tells you little
about the orientation of the Earth's magnetic field at a specific location.
So the GPS receiver couldn't make use of magnetic pole coordinates.
There are several utilities and links about the Earth's magnetic field located
at: http://www.cnde.iastate.edu/maps.html
Eventually the Sun will cease to fuse Hydrogen into Helium... the core will
contract... Gravitational energy will heat it up to such a point that Helium
fusion to Carbon will begin... With all that new energy being generated, the
Sun's outer layers will swell up to engulf Mercury, Venus.... and the Earth,
vaporizing each. Speaking about wild magnetic fields.... but then that's
another story.
-Sam Wormley
Dave Martindale
>Does the declination quoted by my GPS mean that my compass is
>wrong by 13 degrees East all the time, so I need to add 13 degrees
>West to my figures or else adjust my compass so that that is the case?
>Or does it mean that I need to add 13 degrees East to all my figures,
>or adjust my compass so it does it automatically?
The declination of 13 degrees east means that when the compass is
actually pointed to true north, the magnetic needle points 13 degrees
east of this.
If you have a declination-adjustable compass, you need to turn its
adjusting screw until the north end of the arrow on the baseplate
is at 13 degrees on the heading scale, rather than at zero degrees.
If your compass doesn't have an adjustment for this, you need to
do the correction mentally. Just remember that when the compass
points to magnetic north (zero degrees on the compass), you heading
is really 13 degrees east of true north - i.e. your true heading is
13 degrees. So, to convert true heading to magnetic, subtract 13.
To convert magnetic to true, add 13.
Dave
Paul Edwards
at least temporarily lost my compass manual.
My GPS says that where I am at the moment (Sydney, Australia) the
offset is E013. This is different from where I used to be, so I need to
adjust my compass.
But I've only got a 50% chance of getting it right.
Does the declination quoted by my GPS mean that my compass is
wrong by 13 degrees East all the time, so I need to add 13 degrees
West to my figures or else adjust my compass so that that is the case?
Or does it mean that I need to add 13 degrees East to all my figures,
or adjust my compass so it does it automatically?
Thanks. Paul.
Geoff Lambert
Here in Sydney, I would have thought the present declination was only
about 12.5 degrees east, if its already 13, the change must have
speeded up in recent years.
Anyway, what 13 deg E means is that your magnetic compass points 13
degrees more easterly than the true geographic north.
Here in Sydney, which is in a particulare place in UTM Zone 56 (or is
it 55?), there is also a significant grid convergence, which is the
difference between geoghraphic north and the "vertical" lines on the
local maps. I have a feeling this is about 0.5 degrees.
In any event, a local topo map [Auslig (national) ofrr Central Mapping
Authority (NSW state)] will show both grid convergence and
declination, the latter will be true for a particular date and the
rate of drift will be given. Both are shown diagramaatically in the
margins of the maps, so there can be no confusion about what "East"
means.
Geoff Lambert
Sam Wormley
>
> Can someone please tell me the philosophy on the declination, as I've
> at least temporarily lost my compass manual.
>
> My GPS says that where I am at the moment (Sydney, Australia) the
> offset is E013. This is different from where I used to be, so I need to
> adjust my compass.
>
> But I've only got a 50% chance of getting it right.
>
> Does the declination quoted by my GPS mean that my compass is
> wrong by 13 degrees East all the time, so I need to add 13 degrees
> West to my figures or else adjust my compass so that that is the case?
>
> Or does it mean that I need to add 13 degrees East to all my figures,
>
> Thanks. Paul.
Paul, you could adjust your compass to +13 degrees 50% of the time and -13
degrees the other 50% of the time and average the results. Your average
will be BETTER than your worst case angular error! Sorry, I couldn't
resist. ;-)
-Sam
Dave Martindale
>More likely my GPS only quotes to the nearest degree.
The LCD screen only shows the nearest degree, and only lets you specify
a manual adjustment to the nearest degree. But if you set it to "Auto",
it uses an estimate of the local declination that has more precision
than that. The NMEA data output stream has a field that gives the
local declination being used to the nearest tenth of a degree.
Dave
Dave Martindale
>> If your compass doesn't have an adjustment for this, you need to
>> do the correction mentally. Just remember that when the compass
>> points to magnetic north (zero degrees on the compass), you heading
>> is really 13 degrees east of true north - i.e. your true heading is
>> 13 degrees. So, to convert true heading to magnetic, subtract 13.
>> To convert magnetic to true, add 13.
>Thanks for your reply, which I understood, but (if I did understand
>properly!) I think you've got these last 2 sentences the wrong way
>around. If my magnetic reading is 13 degrees East, I need to
>SUBTRACT 13 to get to true (which is North). BFN. Paul.
No. The magnetic needle always points 13 degrees east of true north.
If you use a compass with no declination adjustment, and the compass
happens to be pointed to magnetic north, the compass bearing will read
zero, and you have to add 13 to get your true bearing of 13.
On the other hand, if you have the compass aimed so the magnetic
bearing is 13, then the compass is aimed 13 degrees clockwise (east)
of magnetic north, and the true bearing of the compass is 26 degrees:
13 degrees for the magnetic/true ofset, and another 13 for the compass/
magnetic north offset, with both being in the same direction. So, again,
you add 13 when converting magnetic to true bearing.
Dave
Paul Edwards
Paul Edwards
Sam Wormley <swor...@cnde.iastate.edu> wrote in message
news:37A1377C...@cnde.iastate.edu...
> > But I've only got a 50% chance of getting it right.
> Paul, you could adjust your compass to +13 degrees 50% of the time and -13
> degrees the other 50% of the time and average the results. Your average
> will be BETTER than your worst case angular error! Sorry, I couldn't
> resist. ;-)
Only if you square it Sam, only if you square it. :-) BFN. Paul.
Paul Edwards
Geoff Lambert <G.La...@unsw.edu.au> wrote in message
news:7nr33p$m46$1...@mirv.unsw.edu.au...
> >My GPS says that where I am at the moment (Sydney, Australia) the
> >offset is E013. This is different from where I used to be, so I need to
> >adjust my compass.
>
> about 12.5 degrees east, if its already 13, the change must have
> speeded up in recent years.
More likely my GPS only quotes to the nearest degree. I must admit,
that is one thing I don't like about my G12 - I can't enter relative
waypoints at a distance more accurate than 100m, and the distance
calculation only shows results down to 10m I think it is (unrelated
complaint). I don't care about the declination not showing the half
degree since on my compass I've have a tough time getting settings to
2 degrees accuracy!
BFN. Paul.
Paul Edwards
> "Paul Edwards" <kerr...@nosppaam.w3.to> writes:
>
> >> 13 degrees. So, to convert true heading to magnetic, subtract 13.
> >> To convert magnetic to true, add 13.
>
> >Thanks for your reply, which I understood, but (if I did understand
> >properly!) I think you've got these last 2 sentences the wrong way
> >around. If my magnetic reading is 13 degrees East, I need to
> No. The magnetic needle always points 13 degrees east of true north.
> If you use a compass with no declination adjustment, and the compass
> happens to be pointed to magnetic north, the compass bearing will read
> zero, and you have to add 13 to get your true bearing of 13.
Ok, thanks, I understand properly now (I swear!). BFN. Paul.
Bobby Davis
help our student pilots remember the conversion method from True to Mag:
" East is least, West is best". Simply stated, you subtract easterly
variation from the true course and add westerly variation to true course
to obtain mag course.
To add more fuel to the fire, you can then apply magnetic deviation
issues to this mess. This helps to explain local aircraft disturbances
that affect the magnetic compasses in the aircraft and then there's dip
error due to acceleration/deceleration...you get the idea. You would
travel halfway to Asia before you figure it all out. Thank God for
triple INS/IRS and GPS nav systems.
Dave Martindale wrote:
>
> "Paul Edwards" <kerr...@nosppaam.w3.to> writes:
>
> >Does the declination quoted by my GPS mean that my compass is
> >wrong by 13 degrees East all the time, so I need to add 13 degrees
> >West to my figures or else adjust my compass so that that is the case?
>
> >Or does it mean that I need to add 13 degrees East to all my figures,
> >or adjust my compass so it does it automatically?
>
> The declination of 13 degrees east means that when the compass is
> actually pointed to true north, the magnetic needle points 13 degrees
> east of this.
>
> If you have a declination-adjustable compass, you need to turn its
> adjusting screw until the north end of the arrow on the baseplate
> is at 13 degrees on the heading scale, rather than at zero degrees.
>
> If your compass doesn't have an adjustment for this, you need to
> do the correction mentally. Just remember that when the compass
> points to magnetic north (zero degrees on the compass), you heading
> is really 13 degrees east of true north - i.e. your true heading is
> 13 degrees. So, to convert true heading to magnetic, subtract 13.
> To convert magnetic to true, add 13.
>
> Dave
Dave Martindale
>Right you are David! In the flying community we have a little saying to
>help our student pilots remember the conversion method from True to Mag:
>" East is least, West is best". Simply stated, you subtract easterly
>variation from the true course and add westerly variation to true course
>to obtain mag course.
Yeah, I learned that too. The trouble is, it only applies in one
direction - converting true headings obtained from a map to magnetic
headings to fly. That's fine for flight planning. But in flight,
you often need to convert magnetic headings (VOR or ADF readings)
into true for plotting on a map - and then you have to invert the
clever saying.
Instead of trying to keep all of this straight in my head, I just
have a mental picture of a compass pointing to true north, reading
true bearings. To convert to magnetic, I know that the magnetic
variation here is 20 degrees east, meaning the magnetic pole appears
to be 20 degrees east of true north - so I just rotate the mental
compass 20 degrees clockwise, to make it point to magnetic north.
The degree scale rotates with the compass body, so now the heading
to any place on the map reads 20 degrees less than it did before.
Converting magnetic to true works the other way: first line up the
mental compass with mag north, then rotate 20 degrees CCW to line up
with true north, and all the numerical bearings increase by 20.
Dave
John Blaby
> magnetic compass.
>
>
> --
> Peter Bennett, VE7CEI
> new newsgroup users info : http://vancouver-webpages.com/nnq
> GPS and NMEA info: http://vancouver-webpages.com/peter
> Vancouver Power Squadron: http://vancouver-webpages.com/van-ps
Hmmm..I am totally in the other direction. I have my GPS set to true. It
makes it easier when using chart bearing lines such as the one on the
Porlier Pass chart. They are always plotted as true. When the GPS is running
my Comnav (autopilot), it doesn't matter as waypoints are being used. If I
run the pilot of my compass, it just uses the compass to maintain the course
I set (as you know). Of course, the only time I normally do that is when I
am in visual with the position I wish to be at.
Even so, I do maintain my compass and make sure all is well. You can't have
too much redundancy :)
John
--
Alt. Email: jbl...@canada.com
Web site: http://members.home.net/jblaby