Determining Declination

[x_theory]

Hello all! I'm very new to the sport of GeoCaching, and have some questions about determining declination settings on my GPSr in relation to the particular cache I'm out to find.

 

Do you periodically check your declinations (relation to magnetic and grid north offset) on a cache by cache basis since Magnetic North is in constant movement? Since I know accuracy of civilian GPSr's vary quite a bit from military pluggers, having an accurate declination setting is paramount in making sure your coords are as accurate as you can get with selective availability. What are some good tips from the veteran GeoCachers out there in regards to declination, or even elevation settings for that matter, too. Thanks alot

[Kerry.]

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.... Since I know accuracy of civilian GPSr's vary quite a bit from military pluggers

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Without SA that's Not so.

 

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.... having an accurate declination setting is paramount in making sure your coords are as accurate as you can get with selective availability

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What does selective availability have to do with things these days, it's been discontinued for over 3 years.

 

Also what has declination have to do with coords derived by a GPS, there is no connection. Declination is relative to bearings/headings etc not a single position.

 

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.... or even elevation settings for that matter, too.

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what about elevation settings?

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+IVxIV]

I think most GPSr's have an option setting that allows you to choose bearings to be given using either magnetic or true North reference, and it would take care of declination variance for you. But as Kerry mentioned, this is only an issue when describing bearing/heading data and not coordinate location

 

Slightly off topic,, I know about SA being lifted from consumer GPSr's a few years ago (yayy!! ) but does that mean consumer receivers are now JUST as accurate as miltary ones now?

[+Touchstone]

You might check out this first: How a GPS works.

 

I've never seen elevation settings for caches, but I've done some hunts where I wish they did

 

Be not afraid of greatness: some are born great, some achieve greatness, and some have greatness thrust upon them. The rest go geocaching.

[x_theory]

You'll have to forgive me on my ignorance about civilian GPSr's. I've been a longtime user of the AN/PSN-11 GPS "plugger" from my Army days. I've always used one in conjunction with a map, in which declination needed to be figured in. It seems that if your going by GPS alone you don't need to factor declination in because you will arrive at the same coord location regardless (trying to rehash my land nav training, so bear with me) Since declination is only pertinant to bearing, does that mean if for some reason the GPSr did not obtain the correct declination, or if it was manually put in incorrectly, that you'd still arrive at your location eventually, but your directional indicator may be a bit off key?

 

As far as my question about elevation, would it not be pertinant when your figuring in distance to target if your terrain is not flat like a map (i.e. traveling up hills or through valleys that aren't flat)?

 

So I basically gathering that as long as your datum is correct, and the waypoints are correct, you should be able to just turn on your GPSr and it will lead you exactly to the coordinates you plugged in, plus or minus the accuracy rating of the unit? I will be recieving a Magellan 4000XL GPSr here within a few days and while reading the online manual I was just correlating my knowledge of the AN/PSN-11 to this one and figuring out which settings were automatically obtained, or which ones I had to plug in to make sure that my headings were as dead on as they could be, to include distance to my waypoints.

 

Thanks for entertaining some questions from a vet who's more accustomed to navagating with compasses and ranger beads

 

To answer your question, I did some research and yes, Clinton had removed SA from GPS sattelites for civilian use. The AN/PSN-11 pluggers are still a little more accurate, but apparently not by much anymore to thier civilian counterparts.

[+Olar]

Greetings x-theory!

 

Numerous cachers like to utilize a magnetic compass when with-in 50 meters or so of the cache coordinates using the bearing readout from their GPSR. One would have to make sure that their unit is set for magnetic and, if in a location away from home, verify it is using a reasonably close declination value.

Now if you want to also add a topographical map to the mix then you would need to know the exact declination from True North or even from Grid North if extreme accuracy is required in order to make the correction to your magnetic compass reading.

I have in the past adjusted my compass for declination and set my GPSR at True North simply to make map reading in the field easier and to avoid trying to remember when to add and when to subtract. Fortunately it is rare to need that kind of accuracy in geocaching but it is still good to know how to do it.

 

Cheers, Olar

 

"You are only young once but you can stay immature forever"

[+gbod]

Plug in your coordinates, hit "goto", and follow the arrow. It's that simple.

 

Declination is important only if you will be triangulating the position of your cache with the help of a magnetic compass. It's probably easier, in my opinion if you leave the GPSr to point to true north, and make the declination adjustment on your compass. Even then, I've never had to do that to find a cache.

[+Touchstone]

True on elevation question. I'm not sure how far off you'd be in most circumstances. Most of the caches in our are are within a few tenths of the road/trailhead, so it doesn't add much to the distance if you go over a couple of hills. Things like switchbacks and detours around cliffs and such have more of an effect on the final mileage IMHO.

 

Be not afraid of greatness: some are born great, some achieve greatness, and some have greatness thrust upon them. The rest go geocaching.

[Bobthearch]

As everyone said, declination won't affect the GPS coordinates. If your GPS has a magnetic compass, the declination is automatically set.

 

If you're using a 'real' compass, the declination is written at the bottom of any USGS topo map.

 

Best Wishes,

Bob

[dsandbro]

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Originally posted by Bobthearch:

 

If you're using a 'real' compass, the declination is written at the bottom of any USGS topo map.

 

Best Wishes,

Bob

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Many USGS topo maps were last updated 10, 15, or more years ago, so the declination published on the map can be off a bit. As a rough rule of thumb in North America you can use a figure of 1/2 degree change per ten years, but the actual value varies depending where you are. A net search for a downloadable freeware program called Geomagix will give you the declination for any point and time. Several web sites will do it. Or any current aeronautical chart (know any pilots? -- ask to look at their charts).

 

===========================================================

"The time has come" the Walrus said "to speak of many things; of shoes and ships and sealing wax, of cabbages and Kings".

[Bobthearch]

Dave54,

 

Thanks for the software tip - I enjoy downloading and trying Freeware. I actually have quite an accumuation...

 

As far as the declination migrating over time, that's something I've heard, but need to read more about. Anyway, a degree or two isn't going to make much difference, unless you're going very long distances.

 

I bet over the distance of 30 feet, a 2 degree error is going to only be a few inches off, certainly close enough when geocaching. Perhaps a mathematically gifted person could give a real number...

 

Later,

Bob

[Kerry.]

The north magnetic pole is a bit of a funny one but considering the distances generally involved its erratic behaviour isn't a major issue.

 

Generally the north magnetic pole is moving about 40km west per year but in any one day it generally wanders around within a 80-100km circle.

 

".... but does that mean consumer receivers are now JUST as accurate as miltary ones now?"

 

On a comparative basis one would be hard pressed to really find any critical difference in straight accuracy using SPS. But just like some civilian systems augmentation systems are also used in some military units and military units have additional "things" to prevent "interference" etc.

 

The main advantage with dual freq units is that atmospherics can be determined in real time where as single freq units (including single freq military PLGR's and recreational/civil units) have to use modeled atmospherics. The new dual freq military DAGR (PLGR replacement) unit will have much more capability than the old PLGR. But then the DAGR is much more than simply a GPS.

 

".... does that mean if for some reason the GPSr did not obtain the correct declination, or if it was manually put in incorrectly, that you'd still arrive at your location eventually, but your directional indicator may be a bit off key?"

 

That's about it, the destination point wouldn't change but in the process possibly the bearing to waypoint, course over ground values etc might not be relative or meaningful to say your compass etc.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+gbod]

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I bet over the distance of 30 feet, a 2 degree error is going to only be a few inches off, certainly close enough when geocaching. Perhaps a mathematically gifted person could give a real number...

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About a foot actually. A good approximation can be determined by the formula:

 

H = D * Deg * (pi/180)

 

Where H= horizontal error (the number you are trying to determine)

D= distance from the cache

Deg= angle of error in degrees

pi= 3.14159..

 

If you plug in the numbers 30 feet and 2 degrees, the answer is pretty close to 1 foot.

 

Disclaimer: I'm not a math expert, I just happen to remember these kinds of problems from trig class. Also- this formula actually gives you the arc length. The true horizotal distace is the chord length, which will be a shorter since it is a straight line. For small angles the approximation works fine for most applications.

 

And yes, certainly close enough for geocaching.

 

[This message was edited by gbod on October 15, 2003 at 04:00 PM.]

[x_theory]

Excellent replies! Thank you very much for the information. Finding declination on an even outdated USGS topo was never a problem for me, however I just wanted to make sure that for my older GPSr (Magellan 4000XL) that I wouldn't have to rely on a USGS topo declination diagram to plug it in correctly to my unit. I'm still going to play with it a little more, but I'm looking at moving to a SporTrack unit for a little better accuracy and better sat. reception. Once again, thanks for all your replies. What a great group of people you all are

[+IVxIV]

My first GPSr was a Magellan 4000XL. It was a new unit at the time I bought it, and I was pretty impressed with it's abilities. However one issue that started to really bug me was it's ability to hold satellite lock while in the woods.. the problem was (I think) that while the 4000XL is a 12 channel receiver it isn't a parallel receiver so it would take a while to capture/retain lock in hilly or dense terrain..

 

I hope yours works better. I wound up selling mine to my boss @ work who wanted one for fishing. Out on a lake a non-parallel receiver works no problem

[Kerry.]

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Originally posted by IVxIV:

.. the problem was (I think) that while the 4000XL is a 12 channel receiver it isn't a _parallel receiver_ ....

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The 4000XL was not necessarily a 12 channel receiver as there were 2 different versions of the 4000XL and they both looked the same.

 

The real 12 channel version of the 4000XL was a true 12 channel parallel receiver, then there was a 2 (two) channel parallel model that could/would scan up to 12 satellites.

 

The 12 channel receiver apparently has 2 different signal strength screens, one with a bar graph and the other is a numeric version. The North indicator on the 12 channel version is in the upper right hand corner, while the 2 channel version the North indicator was in the upper left hand corner and doesn't have the bar graph display.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[x_theory]

quote:

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Originally posted by Kerry:

The 4000XL was not necessarily a 12 channel receiver as there were 2 different versions of the 4000XL and they both looked the same.

 

Cheers, Kerry.

 

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Kerry,

 

Could you tell me how I'd go about figuring out which version of the 4000XL I have, and if it's a true 12 channel parallel, rather than a two 2 channel parallel reciever? Thanks!

[Kerry.]

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Originally posted by x_theory:

Kerry,

 

Could you tell me how I'd go about figuring out which version of the 4000XL I have, and if it's a true 12 channel parallel, rather than a two 2 channel parallel reciever? Thanks!

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As mentioned above that's about all I know about them. Check the satellite strength display and nore the type of information displayed and the location of the North indicator.

 

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The full 12 channel receiver apparently has 2 different signal strength screens, one with a bar graph and the other is a numeric version. The North indicator on the full 12 // channel version is in the upper right hand corner, while the 2 channel version the North indicator was in the upper left hand corner and doesn't have the bar graph display.

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Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+ScottJ]

Make it easy on yourself. Most GPS units have an "automatic magnetic declination" feature, just turn it on, and then forget declination altogether ... it'll take care of itself. Leave your compass's declination setting (if it has one) zeroed, because with the GPS set this way, you're always going to work in magnetic bearings.

 

Contrary to what some others have said, it is important to make sure that detail is taken care of. Having a good compass and knowing how to use it can really make a difference in finding some tough caches.

 

--

Scott Johnson (ScottJ)

[Kerry.]

"automatic magnetic declination" is purely based on predictions and with the north magnetic pole wandering 40 odd km west each years and arounbd 80-100 km round in circles in any one day these predictions can change.

 

From time to time most manufacturers will release software updates, which contain updated declination tables as even predictions change over time.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go