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Etrex20 magnetic variation setting


Fangamon
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I'd guess it is just that most people would round it up or down to a whole degree anyway... in use if not by intent. For local navigation it won't make much difference over part of a degree... longer hauls are a different matter. Many people can't follow a bearing within two degrees anyway... at least with a compass.

Since the E20 doesn't have a compass, the magnetic variation (declination) would only allow comparison with a compass set the same way. I think you get automatic table results to true as default (if my memory holds). The table is pretty approximate and not always up to date. Another thing is that local fields shift even during the same day, so an approximation is usually all you end up with anyway. The online calculators can't do much better even if they claim more decimal points... I'm out there often and simply adjust my compass for a wide area average declination... Pain to keep changing it every time I go somewhere. I let the GPS give me it's idea of True and it works out. Anyway, I think it is interesting they provide a decimal point on the pad but you say it doesn't accept one... that is interesting and I will take a look if I can find one.

Edit: OR is that a minus sign to match the plus on the right side?

Doug 7rxc

Edited by 7rxc
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Why do you want to do it manually? The GPS takes care of it based on your location. As stated by 7RC it isn't going to make any difference when looking for caches. Even if it was off by a few degrees the gps will still take you to the cache. The degree heading might be off a little but that doesn't affect navigation when going to a lat/lon.

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Why do you want to do it manually? The GPS takes care of it based on your location. As stated by 7RC it isn't going to make any difference when looking for caches. Even if it was off by a few degrees the gps will still take you to the cache. The degree heading might be off a little but that doesn't affect navigation when going to a lat/lon.

When working multicaches that require shooting a lot of bearings in the field, and where the author has decided to use mag north as the reference, the difference in just a degree or two declination over the distance between waypoints can make a lot of difference, especially if they accumulate. Had to remind a CO about that once on an older cache. That said, the declination for any location changes from year to year, and I was not aware that Garmin was providing updated tables to compensate for this (perhaps in firmware updates???). Our declination here in the Denver area has shifted down a full degree since I bought my first Summit HC.
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Why do you want to do it manually? The GPS takes care of it based on your location. As stated by 7RC it isn't going to make any difference when looking for caches. Even if it was off by a few degrees the gps will still take you to the cache. The degree heading might be off a little but that doesn't affect navigation when going to a lat/lon.

When working multicaches that require shooting a lot of bearings in the field, and where the author has decided to use mag north as the reference, the difference in just a degree or two declination over the distance between waypoints can make a lot of difference, especially if they accumulate. Had to remind a CO about that once on an older cache. That said, the declination for any location changes from year to year, and I was not aware that Garmin was providing updated tables to compensate for this (perhaps in firmware updates???). Our declination here in the Denver area has shifted down a full degree since I bought my first Summit HC.

For some reason my old analog calculator - also known as a 10" wooden slide rule - happened to migrate to the top of the heap nearby. I don't remember taking it out of the case and really looking closely at it for years. Just now I wondered if I could see a half degree difference on the trig scales. Oh yes, in mid range. It even has an expanded ST scale (sine or tangent are about the same at smaller acute angles) that's usable up to 5 1/2 degrees spread over the 10 inch display. . . :)

 

Oh yes. The original leather belt loop with snap is still in good condition for field work. Funny coincidence: While watching a US football game last night on TV I caught a glimpse of the dormitory from which I once walked to classes with that slide rule attached to my belt. Long ago, it was.

Edited by 39_Steps
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@PigSti

Agreed, one digit after the decimal might be handy in just the right circumstance (and none of the Garmins I've owned allow for this), but as someone pointed out earlier, the distance to the next waypoint will determine the lateral offset you suffer from the lack of precision in the declination. Over a distance of a couple hundred yards, it's not too awful, but as you start getting out a ways, the difference in the resulting position can become significant. How did you happen to need this feature? Multi-cache or puzzle cache? Got a GC code so we can see what got you into this ditch? I'm still trying to find the one that bit me.

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Sounds about right. We were about 1 mile away from the next projected waypoint on the multi I mentioned, the CO had tried to trick us by sliding in the words "magnetic north" in the description, and I had to make an additional 1 degree declination correction on my old unit that would have otherwise put us most of 100' from the cache... in (ouch) a heavily wooded area.

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Still, I would like to be given the choice on that option, instead I'm denied it!

 

I am sure you would, but magnetic north measurements are only good to a degree or two so you would be using a precision that was too high for the accuracy of the device.

 

I am constantly astounded by people who seem to think compasses are accurate instruments for finding north. They are not. Even the very best compass under ideal conditions has an accuracy no better than .5 degrees; in the real world, with magnetic materials and other noise, the true accuracy I would expect would be no better than 2 degrees or so.

 

To make things worse, the magnetic pole moves, so your declination is going to be different next year.

 

Short answer: understand your device and its limitations.

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Even the very best compass under ideal conditions has an accuracy no better than .5 degrees..

 

To make things worse, the magnetic pole moves, so your declination is going to be different next year.

 

I'm assuming here that the unit is just a radio receiver/decoder and that the real accuracy is determined by the satellite atomic clocks and separate externally reported measurements of the geomagnetic variation.

 

If I need that 0.5 degree accuracy for an immediate measurement why can't I be allowed that function as the space weather reports' are at twelve decimal places not zero. I'm talking about altering that one decimal point number on a weekly basis, not annually!

 

I ain't rocket science fellas, it's a subroutine.. :laughing:

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Even the very best compass under ideal conditions has an accuracy no better than .5 degrees..

 

To make things worse, the magnetic pole moves, so your declination is going to be different next year.

 

I'm assuming here that the unit is just a radio receiver/decoder and that the real accuracy is determined by the satellite atomic clocks and separate externally reported measurements of the geomagnetic variation.

 

If I need that 0.5 degree accuracy for an immediate measurement why can't I be allowed that function as the space weather reports' are at twelve decimal places not zero. I'm talking about altering that one decimal point number on a weekly basis, not annually!

 

What are you talking about? What does declination have to do with GPS position?

 

The accuracy of magnetic north (for a GPS with a compass) is completely dominated by the accuracy of the magnetometer. It has nothing to do with the GPS satellites.

 

The accuracy of declination is dominated by local magnetic effects, and (once again) has nothing to do with GPS or satellites.

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Even the very best compass under ideal conditions has an accuracy no better than .5 degrees..

 

To make things worse, the magnetic pole moves, so your declination is going to be different next year.

 

I'm assuming here that the unit is just a radio receiver/decoder and that the real accuracy is determined by the satellite atomic clocks and separate externally reported measurements of the geomagnetic variation.

 

If I need that 0.5 degree accuracy for an immediate measurement why can't I be allowed that function as the space weather reports' are at twelve decimal places not zero. I'm talking about altering that one decimal point number on a weekly basis, not annually!

 

What are you talking about? What does declination have to do with GPS position?

 

The accuracy of magnetic north (for a GPS with a compass) is completely dominated by the accuracy of the magnetometer. It has nothing to do with the GPS satellites.

 

The accuracy of declination is dominated by local magnetic effects, and (once again) has nothing to do with GPS or satellites.

 

WAIT, you have missed the point here!

 

Declination is the angular deviation value reading of a compass needle from the true north GPS reading.

 

http://magnetic-declination.com/what-is-magnetic-declination.php'>http://magnetic-declination.com/what-is-magnetic-declination.php

 

positive-declination.gif

 

Ok, then explain to me how my Etrex 20 calculates magnetic north without an internal compass then? It CAN'T, it does it by inputting a magnetic variation value and then comparing it to it's own internal GPS true north value...

 

2hr3d7a.jpg

 

I'm assuming that because Etrex 20 is totally reliant on calculating the angular deviation of a compass needle from true north (Satellite reading) from an input from an external magnetometer reading, then a multi decimal point declination input would give better results!

 

The accuracy of declination calculated on the Etrex 20 is totally dependent on the readings of an external magnetometer, then why not allow a multi decimal point input option.

 

If magnetic variation increases by E 1' per annum then why are we limited to whole degree only inputs!

 

-----------------------------------

 

Readings for Darwin Australia

 

http://magnetic-declination.com/

 

Latitude: 12° 34' 33.6" S

Longitude: 130° 54' 47.1" E

Magnetic declination: 3° 11' EAST

Declination is POSITIVE

Inclination: -39° 49'

Magnetic field strength: 46239.9 nT

 

106n6th.jpg

 

Why is something so simple, so hard with Garmin gear!

 

:blink:

Edited by PigSti
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To clarify...

 

There's no "true north GPS reading" unless either a) an internal magnetic compass is employed, or B) there is motion on the part of the user that can be used to compute a direction of motion by GPS signal. When moving slowly or still, and in the absence of the internal magnetic compass, there is no directional reference at all, and there is no way of determining true north, much less magnetic north.

 

So the original "have to do with" question posted to PigSti by Fizzy was correct. What does this have to do with GPS position? Nothing. That's one dimension too few to the job It has to do with a bearing based upon the delta in GPS position, but only produces a good result if there's a reliable 'delta' to work with. This requires that the user maintain a relatively constant direction of motion at the requisite speed, and that there be insufficient drift occurring in the geolocation of the user (due to PDOP, reflection, tree cover, etc.) to avoid errors in computation of direction.

 

Fizzy got it right. Location is irrelevant. Change in location, on the other hand, can provide some clue about true north, against which a declination can be applied to determine magnetic north.

 

Just keep your feets a'movin ... if you can. But that same motion will very likely have a lot more than a 0.5 degree angular error unless you're pretty well fixed on some point on the horizon. Don't trip over anything or walk across any creeks or ...

Edited by ecanderson
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To clarify...

 

There's no "true north GPS reading" unless either a) an internal magnetic compass is employed, or B) there is motion on the part of the user that can be used to compute a direction of motion by GPS signal. When moving slowly or still, and in the absence of the internal magnetic compass, there is no directional reference at all, and there is no way of determining true north, much less magnetic north.

 

So the original "have to do with" question posted to PigSti by Fizzy was correct. What does this have to do with GPS position? Nothing. That's one dimension too few to the job It has to do with a bearing based upon the delta in GPS position, but only produces a good result if there's a reliable 'delta' to work with. This requires that the user maintain a relatively constant direction of motion at the requisite speed, and that there be insufficient drift occurring in the geolocation of the user (due to PDOP, reflection, tree cover, etc.) to avoid errors in computation of direction.

 

Fizzy got it right. Location is irrelevant. Change in location, on the other hand, can provide some clue about true north, against which a declination can be applied to determine magnetic north.

 

Just keep your feets a'movin ... if you can. But that same motion will very likely have a lot more than a 0.5 degree angular error unless you're pretty well fixed on some point on the horizon. Don't trip over anything or walk across any creeks or ...

 

Ok, so explain why there is any need to input the magnetic variation into an Etrex 20?

 

What does it do and what result does it give?

 

There are these settings here..

 

334oy92.jpg

 

.. why :blink:

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Ok, so explain why there is any need to input the magnetic variation into an Etrex 20?

 

What does it do and what result does it give?

Already asked and answered above.

 

IF a device CAN, through whatever means, determine its current 'true' orientation, a declination value can then be used to compute its magnetic orientation. Just some simple addition is involved.

 

In the case of the eTrex20, its only means for knowing its current physical orientation requires motion, not position (hence Fizzy's comment). Using differences in position as determined by geolocation from GPS data, the eTrex20 can approximate its own current orientation vs. true north providing 1) you point the device directly along your current path, and 2) providing that you keep moving fast enough and straight enough to allow it to successfully determine the direction of your motion with some level of accuracy (all of which will undoubtedly prove to be a LOT less accurate performed than the original 0.5 degrees that was mentioned).

 

If it is capable of correctly determining its orientation through those means, it can then offset that angle by a declination value to provide its magnetic orientation instead of its 'true' orientation.

 

Since accurately maintaining the orientation of the unit vs. direction your while walking, and maintaining a bearing accurately while walking both determine the accuracy of the device's understanding of its own physical orientation vs. north (either true or magnetic), the chances of its 'compass pointer' pointing in any particular direction (either to north, or toward a computed location for a cache) with any real accuracy is always questionable. It also explains why you see cachers drifting left and right a fair bit if they are attempting to follow the pointer on a device without an electronic compass whose computations require neither a steady position in your hand vs. your path, nor a steady direction of walking. The electronic compass knows how to point to the cache regardless of either since it does not depend upon YOU to keep assisting it in figuring out where 'north' is.

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And actually, there is a more appropriate response to your question, but I don't think it answers the question you think you were asking, since exactly the same question could and should be asked about the eTrex30.

 

One would think that these units would contain tables of declination values that would take into account both your current location (determined by GPS) and time period. At least in the short (10 year?) term, I'm pretty sure that theoretical declination can be determined with fair precision. So why SHOULD it be necessary to enter those values into any of these units if tables can be built to assure the correct number is being used?

 

I can think of one good reason for this -- and that is if you are using a map in conjunction with the GPS. If you're using a rather dated map whose grid is aligned to magnetic north but provides declination data, then in order to match up any bearing you might want to shoot with the GPS (again, something not very much fun to do with a unit without an electronic compass unless you're damned good at walking while juggling a map and a GPS at the same time) it would be convenient to operate the GPS in magnetic north mode after having adjusted the declination to match that of the map.

 

There may be other reasons (e.g., the tables I suspect exist do in fact NOT exist - which would be a shortcoming in the firmware, to be sure) and other applications where an adjustable declination makes sense, but I haven't personally run across them.

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It also explains why you see cachers drifting left and right a fair bit if they are attempting to follow the pointer on a device without an electronic compass whose computations require neither a steady position in your hand vs. your path, nor a steady direction of walking. The electronic compass knows how to point to the cache regardless of either since it does not depend upon YOU to keep assisting it in figuring out where 'north' is.

 

While I agree with the rest of what you've posted here I don't believe this is true. Whether you have a GPS with electronic compass, one without or even just using a 'normal' compass folks tend to drift left or right 'off line'. Where the GPS scores in this respect is that it pretty soon becomes evident when you are 'off line', not the case with a 'normal' compass when trying to follow a bearing.

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Using differences in position as determined by geolocation from GPS data, the eTrex20 can approximate its own current orientation vs. true north providing 1) you point the device directly along your current path, and 2) providing that you keep moving fast enough and straight enough to allow it to successfully determine the direction of your motion with some level of accuracy (all of which will undoubtedly prove to be a LOT less accurate performed than the original 0.5 degrees that was mentioned).

 

Everything you wrote was good except for this. The velocity and direction of motion are determined by Doppler measurements, not by successive position measurements, so (aside from the inevitable Kalman filter) they are instantaneous.

 

In fact, the accuracy of GPS directional and speed measurements is quite good. But, as you point out, it is hard to keep walking the same direction within a degree or two for any length of time, so the accuracy of the "compass" in the GPS is limited.

 

As for the magnetic declination: My main objection here is that the magnetic model used to determine declination (as shown by PigSti to an arcsecond) gives an artificial precision far beyond that of the actual declination! It is based on a global geomagnetic model. The problem is that local variations in magnetic fields cause the actual direction of the magnetic field to deviate considerably from "true" magnetic north. Additionally, the absolute accuracy of the magnetometer in the GPS is likely no better than a couple of degrees. The best specs I have seen for a consumer-grade solid-state magnetometer are for an accuracy of up to 0.3 degrees in a static environment. The accuracy will be far worse in a dynamic environment, like geocaching.

 

So, to answer PigSti's question: No, putting in a higher precision declination will not help your GPS point to the correct magnetic north, because the error is completely dominated by other things.

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You guys have been busy while I've been asleep!

 

I'm not following your use of the term 'Doppler' in this context.

 

It's the Kalman filter that the device applies to the location datapoint history that provides the final determination by the unit of its bearing and velocity. Had a discussion about 6 months ago with developers of a new sports watch (not Garmin, but big) about the parameters of such a filter and how it was impacting both the 1 second track that the device was creating and the displayed and recorded speed of the wearer. It's a mathematical tussle between eliminating most of the drift and other normal short term inaccuracy factors vs. the fact that the wearer can actually be making some abrupt moves, especially on trails. We finally reached a happy compromise as was needed due to environments like running in the woods and urban canyons where the geolocation was a bit rougher to begin with. Handhelds in the woods don't have the advantage of playing 'road snap' games to correct for positional issues, and the ability of a trail runner to make abrupt changes in bearing (as compared to a vehicle - unless it hits something!) can create a bit of a challenge if the runner expects to see his track correspond tightly to the satellite shot of the trail he was traveling. More critically, runners get pretty fussy about the accurate recording of total distance, so accumulated errors get their attention very quickly. Not that some of them have any more accurate idea of their circuit's actual distance than what the device they last owned or fellow runner told them, but you'd swear they were all out there with a Rolatape MeasureMaster taking measurements just to be sure!

 

Anyway - enough of the digression. Would be interested to hear your additional thoughts on 'Doppler'.

 

Fully agree with your comments regarding local magnetic anomalies, and was only attempting to demonstrate that there were yet more variables involved in the accuracy of such readings. Devices that depend upon consistent orientation in the hand and consistent direction of motion just complicate any accuracy issues further.

 

As for accuracy of a magnetometer in a Garmin GPS, there's something we don't know with any certainty (or perhaps you do?). They can be considerably better than 'a couple of degrees' if treated properly in the field (I've got a long story about coarse positioning of a rapid deployment 24" telescope on a trailer), but I have no clue what (or even whose) '3-axis' chip Garmin is using in their handhelds. At least with a decent chip (vs. none at all), one can avoid many of the orientation and motion issues by holding the device at rest and shooting a bearing that way. In fact, not only is it a convenience to be able to stop and shoot a bearing (something I've mentioned enjoying earlier), it's also going to produce a more accurate result. '3 axis' notwithstanding, jostling these chips around while walking certainly doesn't improve the result.

 

My only gripe about Garmin's use of their 3-axis chips is what seems to be their continued use of the non-regulated (straight off the battery) Vcc on their boards to power these chips, and the calibration issues that this can present to the user. It's why I switched to NiZn cells on my Oregon some time ago. Those cells look like a lithium cell for voltage but with an almost dead flat discharge profile, and they have kept me from having to fuss over any compass calibration on my 450 for a very long time now. Unfortunately, PowerGenix didn't seem to be able to get any traction in the consumer AA cell space, so they bailed and went back to making bigger batteries. When the ones I bought long ago become too tired to use, and several already have, I know of no other chemistry that comes close to it.

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One would think that these units would contain tables of declination values that would take into account both your current location (determined by GPS) and time period. At least in the short (10 year?) term, I'm pretty sure that theoretical declination can be determined with fair precision. So why SHOULD it be necessary to enter those values into any of these units if tables can be built to assure the correct number is being used?

 

I can think of one good reason for this -- and that is if you are using a map in conjunction with the GPS. If you're using a rather dated map whose grid is aligned to magnetic north but provides declination data, then in order to match up any bearing you might want to shoot with the GPS (again, something not very much fun to do with a unit without an electronic compass unless you're damned good at walking while juggling a map and a GPS at the same time) it would be convenient to operate the GPS in magnetic north mode after having adjusted the declination to match that of the map.

 

There may be other reasons (e.g., the tables I suspect exist do in fact NOT exist - which would be a shortcoming in the firmware, to be sure) and other applications where an adjustable declination makes sense, but I haven't personally run across them.

 

I was under the impression that the tables resided not on the units, but in the GPS satellites/servers, and that the declination was part of the position data just as the local time of day is (and sunrise-sunset times, tide calculations, etc.)

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I was under the impression that the tables resided not on the units, but in the GPS satellites/servers, and that the declination was part of the position data just as the local time of day is (and sunrise-sunset times, tide calculations, etc.)

 

Nope.

 

None of the above are transmitted by the satellites. The only thing close is the GPS UTC time.

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I'm not following your use of the term 'Doppler' in this context.

 

GPS receivers calculate your instantaneous velocity vector using the Doppler frequency shifts of the satellite signals. See this, for example.

 

For longer times, average speed is calculated by time over distance, hence the algorithm used by your developer friends.

 

More critically, runners get pretty fussy about the accurate recording of total distance, so accumulated errors get their attention very quickly. Not that some of them have any more accurate idea of their circuit's actual distance than what the device they last owned or fellow runner told them, but you'd swear they were all out there with a Rolatape MeasureMaster taking measurements just to be sure!

 

This is a side issue, but distance is a fractal measurement; the total distance run is a function of the scale of the measurement. If you talk to your friends about this again, I recommend the use of Savitsky-Golay smoothing to match the smoothing length scale to the runner's stride length scale. I think it works better than Kalman filtering, but that's just my opinion.

 

As for accuracy of a magnetometer in a Garmin GPS, there's something we don't know with any certainty (or perhaps you do?). They can be considerably better than 'a couple of degrees' if treated properly in the field (I've got a long story about coarse positioning of a rapid deployment 24" telescope on a trailer)

 

I went to several manufacturers and looked at the specs. 0.3 degrees in a static environment (i.e. "treated properly") was the best quoted accuracy I saw. Seems about right to me.

 

But be careful not to confuse accuracy with repeatability. Especially in a static environment, the repeatability should be better than the accuracy.

 

When the ones I bought long ago become too tired to use, and several already have, I know of no other chemistry that comes close to it.

 

If you REALLY want good voltage regulation, get one of those relatively cheap USB backup batteries for iphone, etc. and regulate the output voltage with an LM75xx and a big capacitor. Of course that means wandering around with two devices stuck together, but hey, when you need it you need it!

 

Personally, I don't use the magnetic compass in my GPS. I rely on the motion pointer, which is far more stable, and automatically adjust for the accuracy of my direction in my brain.

 

It's my opinion that proper use of any instrument (GPS, compass, etc.) requires an understanding of its accuracy and performance limitations. Even for geocaching, it takes a while to really get comfortable with your device and get an intuitive sense of how it behaves.

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GPS receivers calculate your instantaneous velocity vector using the Doppler frequency shifts of the satellite signals. See this, for example.

 

For longer times, average speed is calculated by time over distance, hence the algorithm used by your developer friends.

Instantaneous velocity vector computed by doppler shift? I'd be interested in seeing some docs on that. Given the errors in timing already involved in just location of a stationary receiver, and the fact that these aren't geostationary satellites (which would then require angular velocity calculations of all sorts against even a non-moving object), it seems an impractical solution for objects not moving at relatively high speed (certainly faster than someone could walk comfortably).

 

This is a side issue, but distance is a fractal measurement; the total distance run is a function of the scale of the measurement. If you talk to your friends about this again, I recommend the use of Savitsky-Golay smoothing to match the smoothing length scale to the runner's stride length scale. I think it works better than Kalman filtering, but that's just my opinion.
Understood. Much was discussed about the various options for smoothing, both in terms of the algorithms appropriate to each type of use and the parameters for each algorithm that might be employed. The problem is that these devices were 'multi-sport' and did not include stride measurement peripheral devices (ANT or BT).

 

As for accuracy of a magnetometer in a Garmin GPS, there's something we don't know with any certainty (or perhaps you do?). They can be considerably better than 'a couple of degrees' if treated properly in the field (I've got a long story about coarse positioning of a rapid deployment 24" telescope on a trailer)
I went to several manufacturers and looked at the specs. 0.3 degrees in a static environment (i.e. "treated properly") was the best quoted accuracy I saw. Seems about right to me.
That's about as good as I've seen for consumer grade chips as well. We wound up with devices made in the UK by an outfit called OXTS. They make some positively nifty inertial+GPS systems (see http://www.oxts.com/). Heading accuracy for a single location of 0.1º, and even tighter looking specs for certain other measurements (e.g., position, velocity, angular rate, etc.). Good enough to take a lot of the fun out of geocaching, that's for sure!

 

If you REALLY want good voltage regulation, get one of those relatively cheap USB backup batteries for iphone, etc. and regulate the output voltage with an LM75xx and a big capacitor.
Since the only external power input on most these things is already USB with a 5V expectation, and the output of those backup batteries you mention are only 5V to start with, I would imagine that by the time you got a sufficient input/output voltage differential to get an LM75XX to regulate (something on the order of 2.5V as I recall), the GPS would be very upset by how low the input voltage was to the USB port. But I get your drift.
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Interesting discussion there but simply for computability sake with regard to in-flight directions, a fractional input would be very welcome. I do notice that my Long/Lat is to 5 decimal places! Surely with all this stuff about single degree magnetic variation accuracy being quite enough.. go figure :blink:

Edited by PigSti
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Interesting discussion there but simply for computability sake with regard to in-flight directions, a fractional input would be very welcome. I do notice that my Long/Lat is to 5 decimal places! Surely with all this stuff about single degree magnetic variation accuracy being quite enough.. go figure :blink:

 

Well, if you are willing to use true North instead of magnetic north, your GPS can do a great deal better. The issue with magnetic north is the magnetic field; true north is defined geometrically, so that the GPS can point to it with much greater accuracy.

 

Perhaps you thought your GPS used declination to determine true north? It doesn't. It gets true north from its motion and uses declination to gibe you magnetic north from that. Think of it as the opposite of a compass. The compass estimates the direction of true north from its measurement of magnetic north, while the GPS estimates magnetic north from its measurement of true north.

 

For aviation I would imagine you would prefer true north. In that case, just ignore the declination setting in the GPS altogether.

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Perhaps you thought your GPS used declination to determine true north? It doesn't. It gets true north from its motion and uses declination to gibe you magnetic north from that.

 

Oh brother! Yes I know this and I regret having ever asked this question. This forum is really a complete waste of time..

 

<_<

Edited by PigSti
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Perhaps you thought your GPS used declination to determine true north? It doesn't. It gets true north from its motion and uses declination to gibe you magnetic north from that.

 

Oh brother! Yes I know this and I regret having ever asked this question. This forum is really a complete waste of time..

 

I apologize. I assumed at your first question that you knew how the GPS determined position and direction and that you knew how it used the declination. As I slowly became aware that you didn't, I probably didn't respond as clearly as I could.

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Perhaps you thought your GPS used declination to determine true north? It doesn't. It gets true north from its motion and uses declination to gibe you magnetic north from that.

 

Oh brother! Yes I know this and I regret having ever asked this question. This forum is really a complete waste of time..

 

<_<

Following these l-o-o-o-n-g repetitive threads on built in magnetic compass aiming to fractions of a degree has been something of a refresher course for me, and I have even gotten a few chuckles about Garmin's pricey new Monterra compass issues. I also found a six year old white paper by Honeywell that was informative as a result (don't remember the link).

 

Have a pleasant year end season, you all.

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Have a good Christmas guys...

 

23 posts to mention that he was using the E20 as a back up instrument for flying... rather than walking or even driving. OK think of it as a rough bearing indicator based on GPS positions. As long as you have your destination waypoint in, it should indicate pretty closely the way to head. Not the heading by the way just to be clear... bearing and heading are different... many of the definitions can be used differently as well depending on the user group... Select your data windows with care, don't put the wrong one on screen... distance to, and bearing to target for sure, just like caching. You have to see what works for you best, heading will tell you what way the nose is pointing, but many days flying skewed almost sideways to counter drift will show the difference there. I bet a Garmin flight GPS would have more decimal places for declination cancelling in mode.

Speed is part of it, since walkers wouldn't have time to induce errors.

 

Thanks to ECA and Fizzy for the tech explanations too. I know things have changed a bit over the years, but I'm pretty sure that the magnetic settings were mostly to adjust the bearing so you could use a magnetic (uncorrected) compass in hand (not an electronic). I used to use it that way, or to correct declination of my compass IF I ran into questionable declination settings in the field... think local anomalys, not electric/magnetic field issues in vehicles. I'm always showing people how to use a small piece of tape to do declination correction on compasses that don't have a mechanism... and how to keep a paper copy with them to use as a field reference. As for declination tables for use by GPS or to calculate from... even topo maps clearly state that their 'calculation' for annual changes is only good for ten years from date of publication. Then they change the magnetic north model field definition. And more than a few years from publication they say that the numbers could be off at that point.

 

Always an interesting topic though... Anyone else do declination by shooting the North Star (sorry PS) I don't think that works from AUS. Not so good in the daytime either, or WX, but for local results it's accurate.

 

Have a good holiday guys.

 

Doug 7rxc

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Etrex 20 shows magnetic heading because it has a built in data base with the magnetic variation on each part of the planet. You will use the feature by just selecting Magnetic aHeading. If you travel to another place with a different magnetic deviation, you will realize that magnetic variation showed by the gps will be different.

 

I am an airline pilot and our navegational systems, Inertial Reference System, uses and internal database to show the pilot the magnetic track based on the true tracked obtained by the aircraft Gps's. Same principle.

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It is a built in isogonic line database. Airliners uses the same principle because, besides the regular backup compass, there are no magnetic navigation instruments in a Boeing737. Magnetic track and heading showed to me is derivated from the True Heading and True Track obtained from the GPS and adjusted by the isogonic lines database.

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I think that when I had my 76csx , when it was set to magnetic , it showed the declination value, and for my location it was incorrect by almost 2 degrees when I last checked it, When I contacted Garmin they told me that there were declination tables built into these units, that were basically for a ten year period.

Edited by Forkeye
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And we have sometimes the same errors in airplanes ... hahahaha.. but it is not a big deal because we fly a straight magenta line based on Gps coordinates and magnetic track info is shown just to verify that we are flying the same magnetic course shown jn the navigational charts.

I want to know if the built in isogonic line database for the etrex 20 could be updated.. did Garmin tell you something about it?

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It is a built in isogonic line database

No argument there. BUT... The concern is that older handheld units have the same old database, and mag north drifts a good bit over time. When trying to project a waypoint for a cache at a considerable distance (and some owners get a kick out of mentioning their use of magnetic in the cache description to fool you instead of true), it can make a large difference to the result if the table is outdated. We have no evidence that even firmware updates also update this table, and some units haven't even had a firmware update in ages.
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I have played around with the user setting before and noticed that when I set it at 000, it reads about 5-8 degrees off true north. I would have thought 000 in user would have been basically true north. Too bad Garmin removed the displayed magnetic value like my 76scx had, kind of a step backward. I always use true settings anyway.

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Setting to 000 declination is telling the unit that true north = magnetic north (no declination), so it makes sense that using 000 while using the mag compass would point several degrees off true unless you just happened to be somewhere that they line up (which does occur in certain parts of North America) .. it's supposed to. It's a little dated, but here's a map I had used previously that shows the error across the world. The green line indicates locations where a 000 would be appropriate. Your mileage (no pun intended) will definitely vary. Just a guess, but where you live, looks like something on the order of 13 degrees.

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It will takea few years to change for more than 1 degree.

 

It is a built in isogonic line database

No argument there. BUT... The concern is that older handheld units have the same old database, and mag north drifts a good bit over time. When trying to project a waypoint for a cache at a considerable distance (and some owners get a kick out of mentioning their use of magnetic in the cache description to fool you instead of true), it can make a large difference to the result if the table is outdated. We have no evidence that even firmware updates also update this table, and some units haven't even had a firmware update in ages.

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