Today, hundreds of millions of people around the world use Google Maps every day. As smart software in our vehicles and pockets increasingly navigates for us, we give hardly a thought as to which way is north. For centuries, though, knowing north was essential to navigation, especially on the oceans. But what exactly do we mean by “north”?
First, there is “true north”, also known as geodetic north or geographical north, i.e., the direction along the earth's surface towards the geographic North Pole. The earth rotates around the axis defined by the North and South poles. Extending true north into space points to the so-called “north celestial pole”. Conveniently for folks in the northern hemisphere, the north celestial pole currently is within a degree of the bright star Polaris, which means the north point on the horizon is always very nearly directly below the north star and that star’s altitude angle is very nearly equal to the observer's latitude. In the southern hemisphere, it is less simple. While there is the Southern Cross, it doesn’t align with the south celestial pole, however, its main axis points to it.
Then there is "magnetic north". No prize for guessing that this is where a compass needle points. It would be convenient if the earth’s magnetic field was aligned perfectly with the earth’s rotational axis but it is not. The difference in angle between magnetic north and true north is the magnetic declination or magnetic variation, which varies around the world and is published on charts on the compass rose or map legend. Most of the world’s maps are based on true north, although maps used in orienteering sports requiring a handheld compass are usually based on magnetic north. If you have compass directions, then you need to correct them to make them true. The mnemonic is “correcting add east”. For example, in my part of the world, the magnetic declination is +7, meaning true north is 7° east of magnetic north.
Incidentally, William Dampier, the first Englishman to sail to Australia, did pioneering work on magnetic declination during his global circumnavigations from 1679.
Most of the maps we use are two-dimensional grid maps with upward vertical lines pointing to the third type of north, namely “grid north”. Grid maps are typically based on a variant of the Mercator projection, the most common being Universal Transverse Mercator (UTM). Technically, it is not a single map projection but instead divides the earth into sixty longitudinal bands, each 6° of longitude in width. Between latitudes 80°S and 84°N these longitudinal bands are in turn divided into large grids. Grid north is only identical to true north at the central meridian of each grid and diverges as one moves east or west but, by virtue of using narrow bands, the angular distortion is minimal. Shapes and sizes, however, are distorted the further one goes from the equator, making Greenland look bigger than Australia – despite being over three times smaller. Despite that, Mercator projections are popular because angles and directions are accurate, a fact that sailors certainly appreciate.
This brings me back to Google Maps, which in 2005 adopted a variant of the Mercator projection known as “Web Mercator”. This projection differs from the standard Mercator projection in that it treats the earth as a sphere, not an ellipsoid – in order to simplify computations. One property of a spherical projection is that all points on the top of a Google Maps' grid represent north. This "north" is sometimes called "Google Maps North".
Alas, the popularity of Google Maps means we’re probably creating a generation of humans that doesn’t know or care which way is north!
PS I worked for Google Australia from 2007 to 2018. The Google Maps logo is circa 2012.