Google Maps Projection
When features on the curved surface of the Earth are represented on a flat surface, their shape or size will always be subject to distortion in one way or another. Methods for representing these features on paper are known as map projections.
Google Maps use a Spherical Normal (equatorial) variant of the Mercator projection for its map images. The Mercator map was developed in 1569 by Gerardus Mercator as a navigation tool based on a rectangular grid with parallel lines of latitude and longitude. The Mercator map was designed as an aid to navigators since straight lines on the Mercator projection are loxodromes or rhumb lines - representing lines of constant compass bearing - perfect for 'true' direction.
The Mercator projection exaggerates the size of areas far from the equator. For example, Greenland is presented as having roughly as much land area as Africa, when in fact Africa's area is approximately 14 times greater than Greenland. Google Maps cannot show the poles as the Mercator projects them at infinity. Instead it cuts off coverage at 85° north and south.
The first launch of Google Maps did not use Mercator, and streets in high latitude places like Stockholm did not meet at right angles on the map the way they do in reality. While the Mercator projection distorts a 'zoomed-out view' of the world, it allows close-ups (street level) to appear more like reality because it preserves street angles.
True North on Google Maps is not shown, but for a normal Spherical Mercator projection, grid north and true north will coincide and it will follow any vertical line (or meridian) to the top of the map.
Ordnance Survey Map Projection
Ordnance Survey maps are based on a similar type of map projection to Google Maps called the Transverse Mercator Projection which is designed to keep the unavoidable distortions within acceptable limits. The Transverse Mercator Projection is especially suitable for the UK, a country with its greatest extent in a north-south direction. Grid north and true north do not coincide, with variation smallest along the central meridian (vertical line) of the map, and greatest at the map edges. This variation is illustrated below in an extract from an Ordnance Survey map key.
|Ordnance Survey map showing Grid North, Magnetic North and True North variations for Purbeck UK, 1977