Compass setting?
Bill
Say I'm in a wilderness area using a set & forget compass, the area has a
magnetic declination of 14 degrees, I adjust my compass accordingly, put the
needle in the gate, line up and I'm looking at true north. What I need to
know is, do I set my GPS III+ to auto mag var. or true north? Or, vice
versa?
Thanks again...
rover
"Bill" <bs...@worldnet.att.net> wrote in message
news:4Cl86.3982$Ep3.2...@bgtnsc04-news.ops.worldnet.att.net...
Depends, are you using a map? Do you want the map and the compass/GPS
bearings to corelate? Then you have to set your declination and not
necessarily to magnetic...
If you are using a GPS, by itself, no map involved etc. choose which ever
you want.. I don't think it will matter much.
See yah later
.
Bill
want to be able to do is make my way to trailess areas with the most
accurate sources possible. I'll go to a waypoint, get the bearing, turn off
GPS, use compass and go. I'll repeat that step often.
"rover" <rove...@hotmail.com> wrote in message
news:6Yl86.71599$b6.3...@news2.mts.net...
Dave Martindale
I don't know what a "set & forget compass" is.
If you have a declination-adjustable compass, and adjust it properly
for the local declination, then it reads bearings relative to true north.
In this case, you should set your GPS to read true bearnings as well.
Then your GPS, your compass, and your maps will all agree with one
another.
Dave
Dave Martindale
>I use 7.5 USGS topos. I do want the map and GPS to be correlated. What I
>want to be able to do is make my way to trailess areas with the most
>accurate sources possible. I'll go to a waypoint, get the bearing, turn off
>GPS, use compass and go. I'll repeat that step often.
At the least, you want GPS and compass to agree. If you set declination
adjustment properly on the compass and set the GPS to "true", you have
that, PLUS they both also agree with your map.
You can also leave the compass uncompensated and set the GPS to "auto mag",
or "user mag" with the correct declination, and again your compass and
GPS will agree - but now they will not match the map. You'll have to
add or subtract the declination when transferring bearings to/from the
map and either compass or GPS.
Thus, if you do have a declination-adjustable compass, set the GPS to true.
The "mag" setting of the GPS is mainly useful if you have a compass that
cannot be adjusted for declination. Actually, declination adjustment is
only common on more expensive map/orienteering compasses. Almost all
marine and aviation compasses, and optical sighting compasses lack any
declination adjustment.
Dave
Sam Wormley
use it to determine true north, let your GPS receiver also read true north.
-Sam
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Bill
screw that adjusts to the necessary declination freeing you from the math...
"Dave Martindale" <da...@cs.ubc.ca> wrote in message
news:93ssr7$bsi$1...@lily.cs.ubc.ca...
Roger Russell
A map is the most important nav tool for most of us.
You have just set your compass so you can use it to orientate your
map (else why bother)
Now do the same with the GPS. Let it display true north.
Then run your entire nav problem in true north coordinates.
If you didn't have the map, I would say use magnetic north and
thereby avoid the problem of setting the wrong correction into the
compass.
On Sun, 14 Jan 2001 13:08:16 -0600, Sam Wormley
<swor...@cnde.iastate.edu> wrote:
>Bill wrote:
>>
>> OK, I know I'm a pain in the ass but I'm gaining ground thanks to you all.
>> Say I'm in a wilderness area using a set & forget compass, the area has a
>> magnetic declination of 14 degrees, I adjust my compass accordingly, put the
>> needle in the gate, line up and I'm looking at true north. What I need to
>> know is, do I set my GPS III+ to auto mag var. or true north? Or, vice
>> versa?
>>
>> Thanks again...
>
>Bill, since you have corrected your compass for declination so you can
>use it to determine true north, let your GPS receiver also read true north.
>
>-Sam
>
BTW the accepted convention in other cultures was to speak of a
compass as pointing south, or seeking south.
--
Roger Russell
If it doesn't work the way the designer intended, it is a bug
If it doesn't work the way you want it too, it is a feature.
for EMail the best .gov is no .gov
Ron Wilson
Bill <bs...@worldnet.att.net> wrote in message
news:4Cl86.3982$Ep3.2...@bgtnsc04-news.ops.worldnet.att.net...
Bill,
Here we go again. The only thing that's a pain in the a_ _ is the confusion
surrounding the correcting of a compass for declination. When you're using
it with a GPS, you really don't need to bother. If you're using a GPS to
navigate to a waypoint, the GPS will calculate the bearing to follow
automatically from wherever you are at any given time. You DO NOT need the
map to calculate a bearing. You only need it to determine the coordinates
of the waypoint you're navigating to. Once the coordinates are entered, the
GPS will give you the bearing in either magnetic or true north. Just set if
for magnetic and the bearing it gives will be the same as your compass
naturally reads. No need to figure out if you add or subtract declination.
Yes I realize that the bearing you are following will not be the same as it
would be, if you actually drew in the course line on a map, but why would
you care??? Why would anyone go to this much trouble, when a GPS will do
the calculation for you? I've lost count of how many times this thread has
come up in the last 3 or 4 years. I can tell you from experience, that
unless you work with correcting your compass for declination frequently,
you'll forget how it works and end up being confused. The likelihood of a
novice making a navigational error in this case is much higher than if he
had just used magnetic bearings on the compass and GPS.
--
Ron Wilson <rwi...@dmv.com>
Dave Martindale
>I can tell you from experience, that
>unless you work with correcting your compass for declination frequently,
>you'll forget how it works and end up being confused.
I disagree with this. It's no trouble to set up right if you think
about it a bit.
If you have a topo map, there's probably a little diagram somewhere on
it with an arrow pointing to magnetic north. True north is sometimes
vertical, but it might be slightly off vertical in which case there
will be an arrow pointing to true north as well. All you want to do is
set up your compass so the 0 degree mark on the bearing scale points
to true north *at the same time* as the magnetic needle points to
magnetic north. That's it. If you set the declination in the wrong
direction, it will be immediately obvious.
If you don't have a map, you can still pretend you do and imagine
you have the little compass diagram in front of you. A declination of
20 degrees east means that magnetic north appears 20 degrees east of
true north - i.e. clockwise. That's all you need to figure out which
side to adjust the compass arrow to.
>The likelihood of a
>novice making a navigational error in this case is much higher than if he
>had just used magnetic bearings on the compass and GPS.
It *is* easier to just make compass and GPS read magnetic, if those are
the only two navigational tools you are going to use.
But if you are going to use a map, and measure bearings from the map
using the compass as a protractor, or transfer sightings made with the
compass to the map again using the compass as a protractor, you have to
convert between true and magnetic bearings. And *that* is easy to get
wrong - do you add when going from true to magnetic, or magnetic to
true. Setting the compass *once* for the correct declination and then
doing everything with true bearings is much more foolproof.
Dave
Hozee
Nice story. But navigating on a magnetic compass give two corrections.
1. Variation
2. Deviation.
So what you are doing is only correct for variation. Deviation can give much
bigger mistakes and tid mistakes are uncertain. Deviation depends on the
influence of iron objects or other magnetic stuf in your vincinity. On iron
vessels you can compensate for deviation.
I hope this makes it more clear what you are doing with the old fashioned
magnetic compass.
Wim H
"Ron Wilson" <rwi...@dmv.com> schreef in bericht
news:t67f4o2...@corp.supernews.com...
Ron Wilson
> "Ron Wilson" <rwi...@dmv.com> writes:
>
> >I can tell you from experience, that
> >unless you work with correcting your compass for declination frequently,
> >you'll forget how it works and end up being confused.
>
> I disagree with this. It's no trouble to set up right if you think
> about it a bit.
>
> If you have a topo map, there's probably a little diagram somewhere on
> it with an arrow pointing to magnetic north. True north is sometimes
> vertical, but it might be slightly off vertical in which case there
> will be an arrow pointing to true north as well. All you want to do is
> set up your compass so the 0 degree mark on the bearing scale points
> to true north *at the same time* as the magnetic needle points to
> magnetic north. That's it. If you set the declination in the wrong
> direction, it will be immediately obvious.
>
> If you don't have a map, you can still pretend you do and imagine
> you have the little compass diagram in front of you. A declination of
> 20 degrees east means that magnetic north appears 20 degrees east of
> true north - i.e. clockwise. That's all you need to figure out which
> side to adjust the compass arrow to.
Dave,
You just took 2 paragraphs to explain how to do it. Perhaps I didn't make
it clear, but my comments above were addressed to the "average" user who
probably only uses his/her GPS to navigate to a waypoint. Since the GPS
does the bearing calculation for you, it just isn't necessary to worry about
making that bearing correspond to a map bearing. I guess I'm advocating the
K.I.S.S method. Sure this process seems simple to you, but you're
experienced at this sort of thing. Most novice GPS users probably don't
even know what declination is, much less how to correct for it. I just feel
it's a lot of trouble for very little gain. Frankly, I don't even set my
compass for declination, because I find I just don't need it that often. If
I were using ONLY a compass and map, then my tune might sound a little
different.
> >The likelihood of a
> >novice making a navigational error in this case is much higher than if he
> >had just used magnetic bearings on the compass and GPS.
>
> It *is* easier to just make compass and GPS read magnetic, if those are
> the only two navigational tools you are going to use.
>
> But if you are going to use a map, and measure bearings from the map
> using the compass as a protractor, or transfer sightings made with the
> compass to the map again using the compass as a protractor, you have to
> convert between true and magnetic bearings. And *that* is easy to get
> wrong - do you add when going from true to magnetic, or magnetic to
> true. Setting the compass *once* for the correct declination and then
> doing everything with true bearings is much more foolproof.
Once again, who actually draws course lines on a map and measures them with
a protractor anymore? I sure don't, even though I know how to do it. I
used to do it all of the time, before a Loran and then a GPS started doing
it for me. As you mention above, for transferring sightings to a map, it
would be easier to have the original sighting measured in true north. I
certainly don't disagree with you on this one, because it can get very
confusing when converting from magnetic to true and vice-versa. But this is
about the only case where I see a clear advantage for setting your compass
for declination and I don't see this being done enough to override the
simplicity of operation gained for the other 99% of the time you're using
the GPS/compass combo.
--
Ron Wilson <rwi...@dmv.com>
Sam Wormley
Also See:
Magnetic Compass
Navigation with Compass
http://www.google.com/search?q=Navigation+with+Compass
Magnetic Compasses
http://www.cnde.iastate.edu/staff/swormley/maps/maps.html#compass
Magnetic Declination
http://www.cnde.iastate.edu/staff/swormley/maps/maps.html#declination
Dave Martindale
>You just took 2 paragraphs to explain how to do it. Perhaps I didn't make
>it clear, but my comments above were addressed to the "average" user who
>probably only uses his/her GPS to navigate to a waypoint. Since the GPS
>does the bearing calculation for you, it just isn't necessary to worry about
>making that bearing correspond to a map bearing.
I see your point. You're arguing that most users don't actually use a
map for navigation anymore. They might look at it as a diagram of
what's around them, they might never look at it but keep it in their
pack "just in case", or they might not even have a map along. In all
of these cases, they aren't making bearing measurements on the map, and
it doesn't matter whether map bearings correspond to GPS and compass
bearings. And you're right.
But consider what happens when the GPS is dropped over a cliff edge or
the batteries die (no spares). Suddenly our traveller has to start
using map and compass for navigation. This is not a good time for them
to have to figure out whether to add or subtract the declination, nor
to expect them to think of using the little arrows on the map border to
get it right.
So, I'm advocating setting up the compass and GPS *as if* they were
really going to use the map. They can take as long as they want to set
the declination adjustment on the compass while they're still warm and
dry. If they still get it backwards, they'll find out soon enough when
trying to use the compass to follow a GPS-provided bearing.
Eventually, they *will* have the declination adjustment set right.
Then, if they lose the GPS, everything is as ready as possible to
transfer to navigating with the map.
I'd argue that it is the inexperienced who are most likely to screw up
handling declination under stress if they've never worried about it
before, so it's particularly valuable to them to set things up in such
a way that they *don't* have to think about it in a crisis.
Of course, one way to handle this is to make it easy to work with
magnetic bearings on the map, and ignore true bearings entirely.
Marine charts seem to be designed for this with a huge compass rose
always somewhere. But for land maps, working with a
declination-adjustable compass and true bearings seems easiest to me.
Dave
Ron Wilson
> "Ron Wilson" <rwi...@dmv.com> writes:
>
> I see your point. You're arguing that most users don't actually use a
> map for navigation anymore. They might look at it as a diagram of
> what's around them, they might never look at it but keep it in their
> pack "just in case", or they might not even have a map along. In all
> of these cases, they aren't making bearing measurements on the map, and
> it doesn't matter whether map bearings correspond to GPS and compass
> bearings. And you're right.
I'm certainly NOT advocating that anyone go out in the woods without a map
and the knowledge to use one. I hope I didn't imply that. Even though I
let the GPS calculate the bearing to a waypoint, I still refer to my paper
map at intervals. I just don't use it in the original calculation of
distance and bearing anymore.
> But consider what happens when the GPS is dropped over a cliff edge or
> the batteries die (no spares). Suddenly our traveller has to start
> using map and compass for navigation. This is not a good time for them
> to have to figure out whether to add or subtract the declination, nor
> to expect them to think of using the little arrows on the map border to
> get it right.
Being one of the GPS Nuts, I hardly ever go out in the woods anymore without
taking 2 GPS units with me. The odds of both of them dying on the same trip
are so staggering, that I don't even worry about it. Of course, I guess
most "normal" people don't go out with a backup GPS unit, so your point is
well taken. The difference between magnetic and true north in my area is
only about 11 degrees, so unless you're on an extremely long hike, you would
still be able to find your way out of just about any situation even without
correcting for it. In some areas, though, I guess you might have more to
worry about.
> So, I'm advocating setting up the compass and GPS *as if* they were
> really going to use the map. They can take as long as they want to set
> the declination adjustment on the compass while they're still warm and
> dry. If they still get it backwards, they'll find out soon enough when
> trying to use the compass to follow a GPS-provided bearing.
> Eventually, they *will* have the declination adjustment set right.
> Then, if they lose the GPS, everything is as ready as possible to
> transfer to navigating with the map.
Again, good points if your GPS should fail.
> I'd argue that it is the inexperienced who are most likely to screw up
> handling declination under stress if they've never worried about it
> before, so it's particularly valuable to them to set things up in such
> a way that they *don't* have to think about it in a crisis.
>
> Of course, one way to handle this is to make it easy to work with
> magnetic bearings on the map, and ignore true bearings entirely.
> Marine charts seem to be designed for this with a huge compass rose
> always somewhere. But for land maps, working with a
> declination-adjustable compass and true bearings seems easiest to me.
Yes, on my boat, I always used the good old compass rose to convert to
magnetic bearings, before Loran and GPS came along. My boat's compass
didn't have a declination adjustment on it. Now that you mention it, this
brings two questions to mind:
1) Why don't most marine compasses have declination adjustments on them?
2) Why don't land maps, such as USGS topo maps have compass roses on them?
--
Ron Wilson <rwi...@dmv.com>
tto...@bio.vu.nl
> 1) Why don't most marine compasses have declination adjustments on them?
Because you tend to travel longer distances by ship and would have to
reset it often do different values? And the person behind the wheel
would tend to be a different one from the navigator, hence making it
better to leave the complex work to the navigator.
> 2) Why don't land maps, such as USGS topo maps have compass roses on them?
Because the rose is big and a land map does not have a lot of space to
print it?
These may just be reasons things have come this way. In any case, the
current declination here is 1 degree I think. Neither method may have a
large advantage, but it is probably grown historically.
Besides, on land a compass probably is used at lower resolution (follow
this path in the northeast direction) than on sea (keep a course of 282
degrees).
Thomas
Charlie Sorsby
Precision Navigation (http://www.precisionnav.com/index2.html)?
What do you think?
Also, I queried here a while back about general recommendations for
a good (and reliable) vehicle compass (mechanical or electronic)
but haven't seen any responses. Hope I didn't miss any.
A Cc: to c...@swcp.com will be sincerely appreciated.
Thanks.
--
Best regards,
Charlie "Older than dirt" Sorsby Edgewood, NM "I'm the NRA!"
c...@swcp.com www.swcp.com/~crs USA Life Member since 1965
Dave Martindale
>I'm certainly NOT advocating that anyone go out in the woods without a map
>and the knowledge to use one. I hope I didn't imply that.
No, you didn't - just that hikers probably don't measure bearings from
maps anymore.
>The difference between magnetic and true north in my area is
>only about 11 degrees, so unless you're on an extremely long hike, you would
>still be able to find your way out of just about any situation even without
>correcting for it. In some areas, though, I guess you might have more to
>worry about.
Here, it's almost 20 degrees. On the west side of Vancouver Island,
it's something like 23 degrees. If you ignore it, your track is 23
degrees off. If you try to compensate for declination and get it backwards,
your track is 46 degrees off. Oops.
>Yes, on my boat, I always used the good old compass rose to convert to
>magnetic bearings, before Loran and GPS came along. My boat's compass
>didn't have a declination adjustment on it. Now that you mention it, this
>brings two questions to mind:
>1) Why don't most marine compasses have declination adjustments on them?
>2) Why don't land maps, such as USGS topo maps have compass roses on them?
Marine (and aircraft) compasses, as well as many lens/prism sighting
compasses, are of extremely simple construction - a rotating card with
magnet attached, and a "lubber line" to read the bearing against. The
card and magnet are inside a sealed capsule, so you can't get at them
to crank in an offset between the magnet and zero degrees on the card.
About all you can do is offset the lubber line. This might work fine
on a marine compass with a wide viewing angle, but doesn't with the
sighting compass where you have to have your eye in a specific place to
see anything at all.
Baseplate compasses have the advantage that the bearing scale is
physically separate from the magnetic needle. They are a bit harder to
use when taking a bearing, as you have to rotate a part of the compass
to line up the needle with a mark on the compass. But the mechanism can
easily be made to offset the line-up mark and the zero degree point on
the bearing scale, thus providing declination adjustment.
As for compass roses: Marine charts usually have large areas of water
where you can plant a compass rose without obscuring anything. The charts
are also issued relatively frequently, so the slow change of magnetic
declination isn't a problem. (Same for aeronautical maps). Topo maps
seem to be replaced only very slowly, so a printed compass rose could be
several degrees wrong by the time the map is reissued.
Dave
John Galt
Dave Martindale wrote:
> >1) Why don't most marine compasses have declination adjustments on them?
>
> Marine (and aircraft) compasses, as well as many lens/prism sighting
> compasses, are of extremely simple construction - a rotating card with
> magnet attached, and a "lubber line" to read the bearing against. The
> card and magnet are inside a sealed capsule, so you can't get at them
> to crank in an offset between the magnet and zero degrees on the card.
The main reason that standard marine compasses don't have declination adjustments
is because a boat can traverse several degrees of declination in a day whereas a
hiker usually can't. I have hiked in the Arctic where declination changes were
close enough that I did cover a couple of degrees in a day or two, but that is
certainly not the norm. For marine uses, imagine having to adjust the
declination every few hours -- it's easier to just do it when you calculated your
position and course.
--
I do not exist to serve the state.