Map Reading - True North

Could be wrong, but I thought true north was for navigating at night using the North Star, whereas a compass and map (set properly for difference between magnetic and grid north) were for daytime use in getting a visual bearing.

You prompted me to check how accurately Polaris indicates North.

I might be tempted to use a torch.

"At present, Polaris is 0.75° away from the pole of rotation (1.4 times the Moon disc) and hence revolves around the pole in a small circle 1.5° in diameter.

Only twice during every sidereal day does Polaris accurately define the true north azimuth; the rest of the time it is slightly displaced to East or West, and to bearing must be corrected using tables or a rough rule of thumb.

The best approximate was made using the leading edge of the "Big Dipper" asterism in the constellation Ursa Major as a point of reference. The leading edge (defined by the stars Dubhe and Merak) was referenced to a clock face, and the true azimuth of Polaris worked out for different latitudes."

The Earth rotates eastwards about a North-South axis. This axis, by definition, passes through the North Pole. This is True North. The Earth and its axis "wobbles" with respect to the sun and the stars. Therefore the North Pole axis doesn't always align with the Pole Star.

The Earth is a "sphere" (well, almost a sphere). Maps are "flat". Representing the spherical Earth (or part of it) on a flat sheet is always a compromise. A bit like peeling an orange and trying to lay the peel flat ! For reference (navigation) the earth is divided E-W by Meridians numbered 0 to 180 at one degree intervals east and west of the N-S meridian that passes through Greenwich. It is divided N-S by ever decreasing circles, parallel to the Equator and at one degree intervals numbered 0 at the equator to 90 at the North and South poles. Many maps show the position of these Meridians and Parallels along their margins.

GPS uses the World Geodetic Survey 1984 (WGS84) as a globe or "Reduced Earth". For aviation and shipping navigation, GPS can represent the layout of this reduced earth more accurately than a flat sheet of paper. In GPS, direct routes from A to B are Great Circles. As they would be around a globe.

Many paper maps are produced using a projection of the "Reduced Earth" onto a conical sheet (which can be opened out and laid flat). One such projection is a Lambert's Conical Projection. The North-South Grid Lines on this type of map converge towards the True North where they meet. We call them Meridians of Longitutde. The East-West grid lines are called Parallels of Latitude. despite being parallel to the Equator, they curve slightly towards the South on the Lambert maps. The Grid is therefore not a series of squares or rectangles, although they might look as if they are to a casual observer.

The Ordnance Survey Maps of the UK are based on a Transverse Mercator Projection, ie the reduced Earth is projected onto a cylinder of paper. The projected N-S Meridians are now parallel to each other and the E-W parallels of Latitude are at right angles to the meridians and straight, rather than curved. The OS maps have a "False Origin" somewhere SW of Land's End. The Reduced Earth is mathematically adjusted from sheet to sheet thereby "forcing" the reduced earth to fit onto flat sheets of paper. For convenience, the OS has superimposed a rectangular (actually square) grid onto their map. Clearly this OS grid cannot be fully aligned with the true N-S and E-W Meridians and Parallels of the reduced earth over the whole of the UK. The OS shows the relationship between True North and Grid North on each of their sheets. The relationship varies from sheet to sheet., but doesn't change with time. Showing True North and Grid North on a map enables users to relate features between different mapping projections and also to the reduced earth.

 A simple (and fairly reliable) navigation tool that is widely available and inexpensive, is a magnetic compass. It aligns itself with a spot pretty close to the True North Pole. Unfortunately, the earth's magnetism changes and the spot to which compass points, rotates around the True North Pole over a number of years. We call this difference "Variation". It changes over time. Most maps state the variation at the time the map was produced, and state by how much it is expected to change in future..

In the UK at the moment, Variation is about 2 or 3 degrees west of True North and reducing year on year. In western Canada it is about 19 or 20 degrees East of True North.

Variation around the world varies from place to place and time to time.The maximum variation is 180 degrees.

If you use a compass for navigation you need to know the variation, ie the angle between magnetic north (ie where the compass points) and the grid on the map, assuming you have measured your direction on the map relative to the mapping grid.

Knowing the relationship between Grid north and True north enables you to relate features in different mapping projections.

Adam - your scepticism about the statements you've highlighted in red is well placed. My recollection (so no guarantees of, well, anything, really) of what the Navy and DofE taught me is:

• True North is the imaginary axis around which the actual planet Earth actually spins. It is, as Adam quite rightly says/quotes very nearly exactly aligned in the night sky with Polaris. This is a very convenient coincidence. The best way of imagining what True North is is that a person standing exactly at True North on the surface of the Earth would appear to an external observer to be turning on the spot once every day.
• Magnetic North is what a compass (or any other magnet which is free to rotate) points to. This is quite a distance away from the position of True North, and changes on a more-or-less random basis over many years, but the angular difference between True and Magnetic North is currently very small - about 2 degrees.
• Grid North is where is gets a bit more abstract. Clearly a flat map cannot truly represent the surface of the curved Earth, so all maps include certain distortions which a good cartographer will endeavour to minimise for a given map so you and I don't spot the difference. The vertical lines on a map - which you implicitly imagine converging at True North - are parallel and don't tend to point exactly towards True North due to the cartographer doing his or her job. The difference between True and Grid North will be very small, unless the cartographer has had problems.

For example, my OS map of my local area has this in the footnotes:

"At the centre of this sheet true north is 0 degrees 7' east of grid north.

Magnetic north is estimated at 2 degrees 26' west of grid north for Jul 2009

Annual change is approximately 9' east"

If that helps, I'll be pleasantly surprised.

Mark

Thank you for taking the time to answer at length.

I know these details but was confused by various people making out that the important variance is between True North and Magnetic North. 

For navigation by map - surely we are dealing with Grid North and Magnetic North - the world our compass indicates and the world as represented on a 2d projection of a 3d oblate spheroid that is the world.

Yes - "Knowing the relationship between Grid north and True north enables you to relate features in different mapping projections." - but I'm trying to navigate on one simple consistent series of maps.

True North doesn't seem to be important for the map and magnetic North (for navigation) only needs to relate to Grid North.

True North seems like some Platonic ideal - but irrelevant to map navigation - where there is only Map (Grid) North and Magnetic North.

It may be a psychological fault of mine but I don't like to base an edifice of learning on an initial confusion.

Back at college I spent a month querying the original assumptions of John Rawl's 'A Theory of Justice' - finally walking out as the lecturer couldn't establish them to my (arrogant?) satisfaction.

The origins of an impressively tenuous 3rd in 'Mental & Moral Science'.

A lot of those who appear on the net to teach basic navigation seem confused themselves. Or I am.

You are not confused. They are.

In aviation, the three basic maps that are used are WGS84 (in GPS), Lambert's Conical and a Transverse Mercator. In all three, True North is used as Grid North. True North is used to define Latitude and Longitude and that is what is used to define a location  True North is much more than a platonic ideal. Isogonals are drawn on the maps to show lines of equal magnetic variation so making that adjustment is easy and accurate.

The OS used a "flat earth" square grid that is a unique construct and deliberately close enough to True North for most hill walkers not to worry about. But, as you rightly say, if you measure a bearing on a map relative to Grid North, then you really need to know the angular difference between Grid North and Magnetic North at that point if you are going to use a magnetic compass to find your way.

However, the relationship between True North and Grid North on the OS maps, is only quoted for the centre of the map (1:25,000) or the four corners (1:50,000). it varies across the map. Likewise, Magnetic North varies across the map, more so than True North You really need to know the relationship between Grid and Magnetic at the places you are walking from and to (and all points in between) to be really pedantic, (which we are in aviation, but then we make it easy to do - see above).If you look at the 1:50,000 OS maps, they show the relationship between Grid North and Magnetic North along the top edge of the map. This, I suggest, confirms your point of view.

Most GPS devices will do the arithmetic for you. Give it UK OS reference points A and B and it will work out Lat/Long, or any other grid co-ordinates it has in its data bank plus bearings relative to grid, true or magnetic.

Probably worth pointing out that the Ordnance Survey isn't just used for hiking maps. For example, the Survey Data is used in construction and land ownership. Relating the Ordnance Survey data to other mapping is important to other users, if not to ramblers.

Don - thank you for that. It encapsulates what previously was floating around - an increasingly confident overview but based on a lot of disparate sources.

My queries (so long after the competences of my DoE meander over Dartmoor) were prompted after looking at the Appalachian and Pacific Crest Trails - each long enough to require an awareness of constantly changing magnetic declination and in areas sufficiently East and West of the present US agonic line to require considerable compensation - in opposite directions.

As the variation varies along the route I thought it would be wise (perhaps lazy) for a traveller to be able to set his (on-compass) adjustment for each mapped sector.

It seems to be a function of the internet that the first ten people offered up on You Tube will be Survivalists - and nine, at least, will have pony tails. Each claimed to make magnetic declination 'easy' - then become hesitant when they got to it.

I have only used maps with a compass in the UK - a small area and small journeys. Here the violence done by imposing a parallel grid onto the chosen projection never seemed to bring these issues up.

I am particularly pleased by your confirming -

"However, the relationship between True North and Grid North on the OS maps, is only quoted for the centre of the map (1:25,000) or the four corners (1:50,000). it varies across the map."

This was something that seemed inevitable from the overlay of two geometries but passed over by the Armageddon hippies.

In passing, I think this is what the BBC were referring to here (mentioned above):

"There is a difference between the two - the result of trying to reproduce a spherical world with a flat map - and that varies with time and depending on where you are in the world."

I have just had a look at my maps for the Canadian Rockies where I do a lot of back-packing and a lot of flying.

The aviation maps use the meridians and parallels of lattitude as a grid. ie they are aligned True N-S. Magnetic variation is quoted with a date and rate of change and is about 18 deg east in 2000, decreasing 10' per year. No need to frig about with grid v true differences, they are one and the same - WGS84

The hiking maps (Gem Trek) show Grid North (NAD83), True North and Magnetic North. The difference between Grid and True is 35' for the one around Banff and Assiniboine. The difference between True and Magnetic is 17 deg and 54' East in 1998 decreasing 10' per year. This suggests the difference today would be about 15 or 16 degrees East. The map doesn't quote the difference between Grid and Magnetic but it is clear from the diagram you need to add the 35' to the current 15 or 16 degree variation. If you forget to apply the Grid correction, you won't get lost. If you forget to allow for the Magnetic correction, or apply it in the wrong sense, you might well get lost !!

Be pragmatic, East is Least, West is Best (i'm sure mods will have me for that statement)

Don is absolutely correct (it's good that pilots don't have to work this out 'on the fly' in the Canadian North) and Adam you seen to have got it; you're not going round the twist, well not in this particular aspect anyway.

Now we've got 'True North', just what is 'truth'?

<ducks quickly>  (no reference to the avatar! )

Adam, I'm sure you've now worked out that True North is not used in map-and-compass navigation. An OS map has N-S grid lines, and a compass reads Magnetic North: you just need to adjust your reading to compensate for the difference between the two. All OS maps give this variation, and how it changes over time, written in the border, so that you can work out how much adjustment to apply. 

I used to teach mountain navigation to young adults, and the term True North often cropped up, due to some half-remembered geography lesson, soon to be forgotten once you become immersed in actual map reading - most of which does not, of course, require a compass!

If you need to brush up your navigation skills, the standard book to read is 'Mountain Navigation' by Peter Cliff. It's an excellent, concise textbook that's been around for years. Of course, there's no substitute for practise, although the skills relevant to navigation in the British hills may not be so relevant on the Pacific Coast Trail. Good luck if you decide to give it a go!

In fairness to myself - I had worked that out but the internet kept throwing up (how apt) 'experts' who hadn't.

I asked here because it is, as any fule kno,  a bastion of reason and good sense ......... That and - some people here know what they are talking about.

I can admit that I asked with a slightly truculent defiance to the net ("And yet it moves" ) and just a bat-squeak of doubt.

My father was a great believer in being able to take things back to first principles. Although I am less equipped than he was in mathematics I do find things easier to grasp, remember and, if necessary, reconstruct if I have an understanding of their foundations.

Small example - I am constitutionally incapable of remembering the simple formula for calculating various percentages, gross and nett. However, I can always write down a formula and swap things around the equals sign.

With the confirmations received here I would be able to 'figure out' the basics of map reading. With the 'information' from the net - it would be a fog of confusion.

Indeed, navigating the web in search of the truth can be a bit like going for a walk in the fog with no compass!

Brilliant analogy !

Try Kinder Scout in the fog without a compass.................

+1  Wouldn't dare!

I have, it's a bit like surfing the net - far too easy to get bogged down somewhere you don't want to be...