Graticule

A graticule is a network of geographic lines. We use it to refer to the rectangular^[1] zones between the latitude and longitude lines, each 1°×1° in size.

A graticule may be divided into 100 centicules in a 10×10 grid.

Shape

The shape and size of a graticule as measured over the ground (in miles or kilometers) varies with distance from the equator. A graticule near the equator (latitude 0) is almost exactly square shaped (roughly 111×111 km or about 69×69 miles); other graticules are still 111 km in north-south direction, but become narrower and narrower as one goes further away from the equator. In other words, the length of one degree of latitude is 111.0 ± 0.2 km, with the last digit uncertain because the Earth isn't a perfect sphere. One degree of longitude corresponds to (111.0 ± 0.2 km) * cos(L), where L is your latitude. For example, the Groningen graticule, at +53° latitude, is 66.8 ± 0.1 km in the east-west direction. Graticules touching on the North and South Poles actually have the shape of a piece of pie, since the polar "border" of such a graticule has length 0. Google maps, however, does not cover latitudes below -85° or above +85°. This is okay, though, because the geohashing algorithm would probably not work very well for these regions (giving graticules that were 111km north-south but less than 10km in the east-west direction) anyway.

Numbering

Graticules are numbered with a pair of numbers based on the corner closest to N0°, E0°, so that the graticule a location belongs to can be determined by truncating the degree fraction.

Note that in this numbering 0 is not the same as -0: graticules immediately west of the Greenwich meridian have the east/west part -0°, and graticules immediately south of the equator have the north/south part -0°. For example, graticule (52, 0) is Cambridge, United Kingdom, whereas graticule (52, -0) is the next graticule westwards, Northampton, United Kingdom.

There are 360 x 180 = 64,800 graticules on the globe. So far only a fraction of these have been named and an even smaller fraction actually geohashed. A majority lie in open water.

Footnotes

1. ^ It isn't entirely true that the graticules mark out a rectangular chunk of ground. The side of the graticule closer to the equator will be larger than the one closer to the pole, leading to something more akin to a trapezium. In the limit, at the north and south poles, the graticules become triangular. Add to this the additional complexity of the curvature of the earth and any discussion of the shape of a graticule is either hideously technical or an oversimplification. On the Mercator projection used by Google Maps however, the earth is distorted so that the shape is, in fact, rectangular.

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