By Porter Lang
Start with the sums, because every walker who gets lost gets lost in them. On an Explorer map, 4 cm on the sheet is 1 km on the ground. Everything else follows.
The scale
OS Maps, the Ordnance Survey’s own mapping site, says the Explorer series is at 1:25,000, so 1 cm on the map is 250 m on the ground, and 4 cm is 1 km. The map’s cover reads “4cm to 1km”. The same page says grid lines are 4 cm apart, so each grid square is 1 km by 1 km. It describes Explorer as showing most paths and individual buildings in enough detail for walking and mountain biking, and calls it the most popular choice for walking and cycling. It lists Landranger, at 1:50,000, for cycling and holidays and says it shows roads, larger paths and some individual features. Working from that scale, Landranger comes to 2 cm to 1 km. That figure is our sum, not the page’s.
The grid
A grid reference tells you where a square is, and it has a fixed order. Eastings first, then northings. The Ordnance Survey’s own quick guide puts it as reading the eastings “along the corridor” and the northings “up the stairs”. A four-figure reference joins the easting and northing numbers that meet at the bottom-left corner of the square. A six-figure reference splits each side of the square into ten and uses the tenths to place you inside it.
Wikipedia’s article on the grid says a six-figure reference is the most common form and identifies a 100 m square. Its example is the summit of Ben Nevis, NN 166 712. It adds that the letters can be dropped if the context makes the 100 km square clear, and that spaces are optional, so NN166712 is also valid. Macs Adventure, a walking-holiday company’s blog, says that on a 1:25,000 map a four-figure reference covers a whole square kilometre, that grid lines are faint blue, and that a six-figure estimate is “not a particularly precise art”. It also says it is only possible to work out your grid reference if you know where you are. We treat that as common sense from a promotional source.
The letters
Wikipedia says Great Britain is covered by 500 km squares lettered A to Z without I, of which only S, T, N and H hold much land. Each splits into 25 squares of 100 km, also lettered, and the two letters form the prefix, such as NN. The numbers run from the south-west corner of the 100 km square, and each coordinate uses two to five digits. The grid’s origin lies west of the Isles of Scilly, with a false origin placed south-west of them to avoid negative numbers, making a grid of 700 km by 1,300 km. Locations in Orkney and further north may need up to thirteen digits, according to the same article.
Where the grid came from
Wikipedia’s article on the Ordnance Survey says the National Grid, technically OSGB36, was introduced after the retriangulation of 1936 to 1953. Maps use it in place of latitude and longitude. Short, usually square concrete or stone pillars, about 1.2 m high, were placed on prominent hilltops during that retriangulation as fixed points for theodolites. The same article says the Survey is for Great Britain and that Northern Ireland has its own agency.
Where the Survey came from
The Survey was military first. Wikipedia says the Board of Ordnance began the national military survey in 1790, starting with the south coast of England, and gives 1791 as the founding year in its infobox. The text says work began in earnest in 1790 under William Roy, whose measured baseline of 5 miles (8 km) across what is now Heathrow Airport was the base for mapping begun in 1791. The article dates the Principal Triangulation of Great Britain to 1783 to 1853. The first one-inch map, of Kent, came out in 1801, published privately, and stopped at the county boundary. The Essex maps that followed were published by the Survey and ignored county borders. The Board of Ordnance was abolished in 1855 and the Survey was placed under the War Office. Since 1 April 2015, Wikipedia says, it has operated as Ordnance Survey Ltd, wholly in public ownership.
The height reference for mainland Britain, Ordnance Datum Newlyn, is based on mean sea level measured at Newlyn in Cornwall between 1915 and 1921, according to the same article, and was redefined in 2016, raising it by about 25 mm.
Symbols, and the legend
OS Maps says its guide to symbols is organised in categories, among them public rights of way, public access, access land in England and Wales, roads, railways, boundaries, heights and natural features, vegetation, tourist and leisure information, and archaeological and historical information. It points readers to the map legend for the meaning of each symbol. An outdoors website’s article on map lines says the closer together contour lines are, the steeper the ground. The sources we read were inconsistent or silent on the colours and dash styles of paths, so we do not describe them here. Read the legend on your own sheet.
Dates in order
- 1783 to 1853: the Principal Triangulation of Great Britain, according to Wikipedia.
- 1790: the Board of Ordnance begins the national military survey, according to Wikipedia.
- 1791: founding year given in the infobox, according to Wikipedia.
- 1801: the first one-inch map, of Kent, according to Wikipedia.
- 1855: the Board of Ordnance abolished, according to Wikipedia.
- 1915 to 1921: sea level measured at Newlyn, according to Wikipedia.
- 1936 to 1953: the retriangulation behind the National Grid, according to Wikipedia.
- 1 April 2015: Ordnance Survey Ltd, according to Wikipedia.
- 2016: Ordnance Datum Newlyn redefined, according to Wikipedia.
Side by side
The Survey’s start is 1790 in the article’s text and 1791 in its infobox. The grid lines are described as faint blue by a walking company and are 4 cm apart on the Survey’s own page. A six-figure reference is a 100 m square in one source and an estimate within a square kilometre in another. These are the same fact from two sides. Our reading: for a walker, the useful part is the order, eastings then northings, and a habit of checking which square you are in before you check the tenths. That is our inference.
What kind of sources these are
The Ordnance Survey’s own pages, which are the producer speaking for itself; Wikipedia, which anyone can edit; a walking-holiday company’s blog, which is promotional; and an outdoors website’s article. We read no mountaineering body, no rescue service and no retailer. We give no prices, no sheet numbers and no advice about equipment.
Questions a buyer would ask
Is the sheet an Explorer, at 1:25,000, or a Landranger, at 1:50,000? Does the cover read “4cm to 1km”? How recent is the survey data, and does the sheet show a revision date? Does it cover the area you will walk, with some margin? We recommend no sheet and no edition, and we say nothing about waterproof or paper versions.
What we left out
We left out the contour interval, since none of our sources stated it. We left out path colours and dash styles for the reason above. We left out the Survey’s chief executive, ownership transfer between departments and every price. We left out any statement about magnetic north or compass use, since the sources we read did not cover it.
Roam first. Shop later.