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Agraticule orgrid (from Latin crāticula 'grill/grating'), on amap, is a graphical depiction of acoordinate system as agrid ofcoordinate curves or "lines", each curve/line representing a constantcoordinate value.[1] It is thus a form ofisoline, and is commonly found on maps of many kinds, at scales from local to global.
The termgraticule is almost always used to specifically refer to theparallels andmeridians oflatitude andlongitude, respectively. In modern usage, graticules are contrasted withgrids, which display theeastings and northings of aprojectedcoordinate reference system, such asUniversal Transverse Mercator – usually the coordinate system in which the map is drawn.[2]
Some cartographers have used the term "graticule" to refer not only to the visual lines, but to the system of latitude and longitude reference itself;[3] however, in the era ofgeographic information systems, it is more common to call this thegeographic coordinate system.
The graticule may serve several purposes on a map:[4]
These are usually secondary to the main purpose of the map, so graticules are often drawn to be relatively low in thevisual hierarchy.
The graticule is of ancient origin, being almost as old as the concept of thespherical Earth,coordinate system for measuring geographic locations, and themap projection.Strabo, in hisGeography (ca 20 AD), states that the maps inEratosthenes'sGeography Book 3 (3rd century BC, now lost) contained lines "drawn from west to east, parallel to the equatorial line" (thus the termparallel)[6]Ptolemy'sGeography (ca 150 AD) gives detailed instructions for drawing theparallels andmeridians for his twoprojections.[7]
The works of Ptolemy and other classical geographers were available to the scientists ofmedieval Islam. Some, such asal-Khwarizmi, further developed these works, including creating maps on a graticule of latitude and longitude.
During the EuropeanMiddle Ages, graticules disappeared from the few maps that were produced;T and O maps in particular were more concerned with religiouscosmology than accurate representation of location. Theportolan charts of the 13th to 15th centuries were much more accurate, but usedrhumb lines that were much more useful for sea navigation than latitude and longitude. At the same time, however, the rediscovery of Ptolemy and other classical knowledge of the shape and size of the Earth led to the recreation of some of the ancient maps with their graticules; the earliest extant copies of Ptolemy'sGeography with his maps date to the 14th and 15th centuries.[7] Starting in the 16th century, the graticule has been ubiquitous on global and continental scale maps.
There is some debate over whether the Chinese and other Asians knew the world to be spherical prior to Western contact, but most maps appear to assume regions as flat.[8] Although Chinese maps do not portray any concept of latitude and longitude, cartesian grids appear on some maps dating back to the 11th century.
