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There is a convention that non polar maps are drawn with North towards the top of the map and South towards the bottom. Near the poles the concepts of North and South begin to lose their meaning and alternate orientations must be applied.
Very localised maps such as those for shopping malls or parks are often oriented around the main point of entry. This makes it easy for people to navigate to their objective from the entrance – the right and left turns can be clearly understood. Unfortunately, this does not always help everyone find their way back again.
Maps, such as road maps, present a problem to a significant percentage of the population when they are used to follow a route “down” the map. If you are following a route South on a conventional road map the rights and lefts you need to take in the real world are the reverse of those you see on the map. Those with less well developed “special awareness” find this a difficulty. One solution is to turn the map upside down – but then all of the legends (road numbers, names etc.) will be upside down as well. If you are designing a “how to find us” map then it may be possible to create a map showing multiple routes (perhaps from major highways approaching your area) with an orientation chosen to match the direction of travel.
So where is North then?
Before the advent of GPS (Global
Positioning System) devices the most common means of determining the direction of north was the magnetic compass. Most people are probably aware that magnetic north (the direction a compass needle points) is not quite the same as true north but ignore the differences for many purposes. Navigators using a magnetic compass do need to take the difference into account if they are to arrive at their intended destination or to avoid navigational hazards.
It is thought that the earth’s magnetic field is generated by electrical currents that themselves are generated in the hot, liquid, outer core of the planet. The flow of electric currents in the core is continually changing, so the magnetic field produced by those currents also changes. This means that on the surface of the Earth, both the strength and direction of the magnetic field will vary over time. This gradual change is called the secular variation of the magnetic field.
The magnetic north pole was first located in 1831 by James Clark Ross. Subsequent expeditions found that the magnetic pole moves continuously and is currently changing location by about 15 kilometres a year. There is also a small additional local variation in the location of the magnetic pole as it follows a roughly elliptical daily track. You can find out more about the location and behaviour of the magnetic north pole at http://www.geolab.nrcan.gc.ca/geomag/e_nmpole.html
Many maps present a grid - perhaps based upon the UTM grid or on another, national, survey grid. The grid lines will only run north/south along the one line that tracks the central meridian of the grid system. The angular difference between a meridian of longitude and a grid north is called convergence.
The horizontal variation between true north and magnetic north is called variation. The variation changes continuously but for practical navigation with a magnetic compass the cumulative annual changes for a particular map location must be taken into account.
There is also a vertical angular difference between true and magnetic north that is called declination but this may be ignored for most navigational purposes.
A map used for accurate navigation should show the angular convergence of the grid on the sheet being used as well as the local magnetic variation. In addition, the variation value should be dated and an indication of the annual change provided.
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