Grid references define locations on maps using Cartesian coordinates.
Grid lines on maps define the coordinate system, and are numbered to
provide a unique reference to features. This reference is normally
based on projected easting and northings
Although professional map-making and use of the grid had existed in
China before, the Chinese cartographer and geographer
Pei Xiu of the
Three Kingdoms period was the first to mention a plotted geometrical
grid reference and graduated scale displayed on the surface of maps to
gain greater accuracy in the estimated distance between different
locations. Historian Howard Nelson asserts that there is
ample written evidence that
Pei Xiu derived the idea of the grid
reference from the map of
Zhang Heng (78–139 CE), a polymath
inventor and statesman of the Eastern Han dynasty. The American
Robert K. G. Temple asserts that
Zhang Heng should also be
credited as the first to establish the mathematical grid in
cartography, as evidenced by his work in maps, the titles of his lost
books, and the hint given in the
Book of Later Han
Book of Later Han (i.e. Zhang Heng
"cast a network of coordinates about heaven and earth, and reckoned on
the basis of it").
Types of grid referencing
Grid systems vary, but the most common is a square grid with grid
lines intersecting each other at right angles and numbered
sequentially from the origin at the bottom left of the map. The grid
numbers on the east-west (horizontal) axis are called Eastings, and
the grid numbers on the north-south (vertical) axis are called
Numerical grid references consist of an even number of digits.
Eastings are written before Northings. Thus in a 6 digit grid
reference 123456, the Easting component is 123 and the Northing
component is 456.
Grids may be arbitrary, or can be based on specific distances, for
example some maps use a one-kilometre square grid spacing.
A grid reference locates a unique square region on the map. The
precision of location varies, for example a simple town plan may use a
simple grid system with single letters for Eastings and single numbers
for Northings. A grid reference in this system, such as 'H3', locates
a particular square rather than a single point.
Points can be located by grid references on maps that use a standard
system for Eastings and Northings, such as the Universal Transverse
Mercator used worldwide, or the
Ordnance Survey National Grid
Ordnance Survey National Grid used by
Ordnance Survey in the UK. These points can then be located by someone
else using grid references, even if using maps of a different scale.
Universal Transverse Mercator
Universal Transverse Mercator (UTM) system, grid reference is
given by three numbers: zone, easting and northing. In the UTM system,
the earth is divided into 60 zones. Northing values are given by the
metres north, or south (in the southern hemisphere) of the equator.
Easting values are established as the distance from the central
meridian of a zone. The central meridian is arbitrarily set at 500,000
metres, to avoid negative numbers. A position 100 kilometres west of a
central meridian would have an easting of 400,000 metres. Due to its
popularity, and worldwide cover, the UTM system is used worldwide by
NATO as well as many countries, including Australia and the USA.
In the United Kingdom, a proprietary grid system is used. In Ordnance
Survey maps, each Easting and Northing grid line is given a two-digit
code, based on the
British national grid reference system
British national grid reference system with origin
point just off the southwest coast of the United Kingdom. Since the
Eastings and Northings are one kilometre apart, a combination of a
Northing and an Easting will give a four-digit grid reference
describing a one-kilometre square on the ground. The convention is the
grid reference numbers call out the lower-left corner of the desired
square. In the example map below, the town Little Plumpton lies in the
square 6901, even though the writing which labels the town is in 6802
and 6902, most of the buildings (the orange boxed symbols) are in
The more digits added to a grid reference, the more precise the
reference becomes. To locate a specific building in Little Plumpton, a
further two digits are added to the four-digit reference to create a
six-digit reference. The extra two digits describe a position within
the 1-kilometre square. Imagine (or draw or superimpose a Romer) a
further 10x10 grid within the current grid square. Any of the 100
squares in the superimposed 10×10 grid can be accurately described
using a digit from 0 to 9 (with 0 0 being the bottom left square and 9
9 being the top right square).
For the church in Little Plumpton, this gives the digits 6 and 7 (6 on
the left to right axis (Eastings) and 7 on the bottom to top axis
(Northings). These are added to the four-figure grid reference after
the two digits describing the same coordinate axis, and thus our
six-figure grid reference for the church becomes 696017. This
reference describes a 100-metre by 100-metre square, and not a single
point, but this precision is usually sufficient for navigation
purposes. The symbols on the map are not precise in any case, for
example the church in the example above would be approximately 100x200
metres if the symbol was to scale, so in fact, the middle of the black
square represents the map position of the real church, independently
of the actual size of the church.
Grid references comprising larger numbers for greater precision could
be determined using large-scale maps and an accurate Romer. This might
be used in surveying but is not generally used for land navigating for
walkers or cyclists, etc. The growing availability and decreasing cost
GPS receivers enables determination of accurate grid
references without needing a map, but it is important to know how many
GPS displays to avoid reading off just the first six
GPS unit commonly gives a ten-digit grid reference, based on
two groups of five numbers for the Eastings and Northing values. Each
successive increase in precision (from 6 digit to 8 digit to 10 digit)
pinpoints the location by a factor of 10. Since, in the UK at least, a
6-figure grid reference identifies a square of 100-metre sides, an
8-figure reference would identify a 10-metre square, and a 10-digit
reference a 1-metre square. In order to give a standard 6-figure grid
reference from a 10-figure
GPS readout, the 4th, 5th, 9th and 10th
digits must be omitted so it is important not to read just the first 6
^ Thorpe, I. J.; James, Peter J.; Thorpe, Nick (1996). Ancient
Inventions. Michael O'Mara Books Ltd (published March 8, 1996).
p. 64. ISBN 978-1854796080.
^ Needham, Volume 3, 106–107.
^ Needham, Volume 3, 538–540.
^ Nelson, 359.
^ Temple (1986) 30.