Magnatic North movement

[Angla]

Who here did not know that magnetic north moves at a set pace for a given latitude every year?

Back in my military days a bearing to a position on a map had to be adjusted by a certain number of minutes and seconds dependant on the age of the map

This may explain errors when transferring old map co-ordinates to gps co-ordinates because a lot of the time maps are not pictures taken last year but just re prints of 1985 maps with some details enhanced

Does anyone know if our gps units make these adjustments automatically for us?

Anyone in the military currently should know this as it is a fundamental field training skill but not a national secret and may be affecting our skills without us knowing

Angla

[szopen]

Yep,

The Poles are one the move.

Me incuded.
;D ;D ;D ;D ;D ;D ;D ;D

[Morlers]

Does anyone know if our gps units make these adjustments automatically for us?
Hi Angla
I think the lower end models give you true north and not magnetic north bearings. You would have to adjust for the variance when converting to your compass (ie true to magnetic). For my area approx 12 degrees true = o degrees magnetic and this varies yearly. You only get true and magnetic bearings in the more ecxpensive sets depending what you set them up for.

:) :)

Morlers

[HarryO]

Anytime I mention variation and deviation to the
missus, she gives me a funny smirk..... # #::) # #::)

and constantly shifting positions?... Don't ask!

[Angla]

Simple way to work out wether it is true or magnetic is look at the compass and the bearing on the GPS at the same time.

The question is would your fishing spots X move with the GPS and always be accurate within the tolerances

Angla

[szopen]

I hate to say it but the fishing spots should be the least of your worries.

It is the shoals, rock, any safe ports of entry that really matter.

On the water.

On land you still can take a very wronmg turn,

Shall I say adventure???

[MAL]

5% ?

Cheers.

MAL

[MAL]

5 Deg ::)

Cheers.

MAL. ;)

[DICER]

I think most GPS systems would take this into account based on position, triangulation and time.

"GrandMa's Socks" - Grid to Magnetic Subtract and vice versa - that's for those still using maps

Can't wait for North/South reversal. Should screw up a lot of things.

[Mantaray]

Simple way to work out wether it is true or magnetic is look at the compass and the bearing on the GPS at the same time.

The question is would your fishing spots X move with the GPS and always be accurate within the tolerances

Angla

i had the chance to quiz an expert about this not so long ago and the answer was, A single GPS position has nothing to do with changes in true or magnetic. one is a position and the other relates to bearings. it is when running between 2 GPS positions that true or magnetic becomes more important in conjunction with using a compass or plotting a course to travel

would your fishing spots move? answer was No.

[Grand_Marlin]

You are confusing the two issues here.

A GPS works on triangulation of satelites, as well as altitiude to give a 3 dimensional position of lat / lon.

Magnetic variation means nothing to a GPS, only to a compass.

GPS units are usually "factory default" on true north.
You have the option of changing to magnetic north, and manually entering the deviation shown on your chart. This deviation, when entered into the gps is only adjusting a number value to tie up with your boats compass (providing the compass is swung and reading correctly) again, it does not have any impact on how the gps works.

To explain:
A waypoint is obviously a fixed point on the earth - whether done by satellite or by a surveyor, this point has the same coordinates forever, they do not change - this hopefully answers your first question.
A GPS works much the same as you driving down a straight road in your car. You are going from point a (say home) to point b (say shop) and you know exactly how to get there. They are 2 fixed points and nothing changes.
This is how the gps looks at things.

However, If you were to be going from point a (home) to point b (shop) by compass bearing, then from year to year, you would have to adjust your bearing to reach the shop. (keep in mind though that in brisbane it takes 60 years to deviate one degree, as acording to my chart, it decreases by 1 minute annually.

The problem comes when you have to take a lat long position off the chart and steer to it by compass. This is where you have to take into account your magnetic variation. So, again in Brisbane where the magnetic variation is essentially 11 degrees, if I had to steer true north to a position on the map, my compass reading would be 349 degrees (see Dicers GMS rule)

Any errors are most probably coming from the difference in chart datums as compared to the datum your gps is running on.
All gps units should be set to WGS84. (American datum and adopted world standard)
Your map will have the datum written on it that applies to that particular map. All australian maps printed prior to 2000 worked on Datum AGD66 or AGD84, that can vary the difference between your map and the gps coordinate by as much as 180m. (thats handy in Moreton Bay)
The new maps are written in WGS84 or GDA94 - the map will show you. WGS84 and GDA94 are virtually identical, with only centimetres difference between them.
There was a post in the saltwater section a couple of weeks ago on hoe the gps can automatically convert these waypoints created on older datums, to the new datums.
I didnt comment on this point, because in all honesty - I dont know how to convert them.

I hope I havent sent everyone to sleep.

Cheers
Pete

ps, if you are around in 11,473 years, to be travelling true north, you will be reading due south on your compass! - I think I will give it away when this happens.

[DICER]

A GPS unit itself has a primary function of calculating the location on land or water by using satellite signals, rather than the satellite calculating this for your GPS unit! A satellite transmits a message containing only three pieces of information, the satellite number, it's position in space and the time at which the message was sent. Your GPS unit utilizes this information and then makes the calculation for your position.

RE grandma sox analogy; Usually the declination tells you which way to substract to east or west!

Thanks Mantaray+Grand Marlin

[Angla]

That is the answer I was looking for and a big thank you to the knowledge of fellow Ausfishers. I am sure this will also help others too by knowing this information. :o






On a lighter note, and I know this works :D

I was on a spot last month and put a big red cross there .............so I went back there today and when I looked over the side the big red cross was exactly on the side of the boat where I left it. Thats accuracy for you! ;)

Angla

[bidkev]

A very good response Pete, and this clarifies the matter succinctly. My explanation was gonna be, "If you ain't using a compass, then you don't need to worry about mag variation". I think your response is a *little* bit better ;D

kev

Keep your eyes wide open before marriage, half shut afterwards.

[Grand_Marlin]

Fair dinkum Kev,
I wish I had thought of that....would have saved me half an hour !!

Cheers

Pete

[DICER]

Magnetic reversal may come quicker than 11,473 - some parts of africa are already around ~40%. At the momment it's roughly 200, 000 years between reversals. The last reversal was 730,000 years ago; so we're overdue.

The magnetic field is showing worrying signs of reversal again. Not only has the magnetic north pole has wandered by 1100 kilometres over the last 200 years, but the strength is also weakening 5 per cent century.

[szopen]

Pete,

A very well written post.
Regards.

Dicer,
The poles shif probably would wreck a chaos around the world for quite a while.
I do not mind it waiting another thousand years or so.