Possibly the most difficult step in wildlife recording (once you recognise a handful of species) is the 'where' - establishing the unique OS Grid reference of the place where you made the observation. Fortunately these days there are a number of electronic aids for both obtaining and checking grid references, from websites to GPS-enabled phones, which make the process much easier and less error-prone. However there are still likely to be times when you need to refer directly to a paper map, either to read off or look up a location as a grid reference.

Working with OS Grid references

This mini-tutorial aims to cover most aspects of working with OS Grid references. The text is divided into the following four sections:

1. Understanding Grid references;
2. Working out the grid reference of a location;
3. Finding the map location from a Grid reference, and;
4. Notes about format and precision.

Note: The locations and maps shown here are for illustration only (you cannot look them up on UK maps), and are intended for use solely in the context of this document.

1. Understanding a Grid Reference

The three parts of a Grid Reference

Before working with grid references, it is important to understand their format and be able to separate out the 3 parts that they contain:

1. Grid square identifier (2 letters)
2. Eastings value (numbers)
3. Northings value (numbers)

Some parts of the Grid reference layout are always the same:

• the Grid square identifier always consists of exactly two-letters
• the 'eastings' value always has the same number of figures as the 'northings' value.

But some parts of the Grid reference are flexible, to allow them to be used in different ways:

• the number of digits (eastings+northings) can vary in length (generally between 4 and 10 digits)
• the Grid reference may include spaces between the separate parts, or may be written as a continuous string (depending on preference).

As a result there is no single 'correct' way of writing a grid reference, although the 6-figure grid reference (eg. SW 726 182) is probably the most widespread and suited to many requirements.

What do the parts of a Grid Reference mean?

The three parts of a grid reference contain sufficient information to identify any point within the UK grid area:

The Two-letter grid square identifier

The Ordnance Survey grid divides Great Britain into a series of 100km x 100km squares, each of which is given a two-letter code for quick reference. The two-letter grid-square identifier shows which of the 100km grid squares the Grid reference refers to.

Eastings and Northings values

The eastings and northings values (the first half and second half of the figures respectively) indicate how far east (towards the right) and north (towards the top) the point lies within the designated grid square. To work out how far right or up the point is, the first figure in the Easting/Northing gives the relevant offset in 10ths of the whole grid-square, the next figure the additional offset in 10ths of each of the larger divisions, the next figure the additional offset in 10ths of a the preceding division, and so on. For example,

The effect of increasing or decreasing the number of digits in a Grid reference is described in more detail in Section 4 (below).

2. Working out the Grid Reference for a Location

This section describes the various methods for establishing the grid reference of a location - using maps and other means.

2(i). Working out the Grid Reference using a Map

Calculating a grid reference is relatively easy, consisting of three steps.

Identify the OS Grid square that the location is in.

The 2-letter code identifying the OS Grid square is given in large text at the bottom left-hand corner of the sheet.

Work out the Eastings value for the location.

The eastings value (distance from the left of the grid-square) can be read off the top or bottom margin of the sheet, using the gridlines for reference. Choose the gridline that is either directly on or immediately to the left of the location. On OS Explorer Maps (1:25000 scale) the gridlines are labelled in 2-figures (00 - 99), which provide the first two digits of the Easting value The third digit is obtained by calculating how far the location is to the right of the chosen gridline, using units of 1/10 of a grid-square. Using a ruler (or romer) may help with this step.

Work out the Northings value for the location.

The northings value (distance from the bottom of the grid-square) is found in the same way, using the gridlines on the left or right margin of the sheet for reference. Choose the gridline that is either directly on or immediately below of the location. On OS Explorer Maps (1:25000 scale) the gridlines are labelled in 2-figures (00 - 99), which provide the first two digits of the Northing value The third digit is obtained by calculating how far the location is above the chosen gridline, using units of 1/10 of a grid-square.

Put together the parts

The completed OS Grid reference is obtained by combining the OS grid square identifier, eastings and northings values (in that order):

2(ii). Other methods of establishing the Grid Reference of a location

Hand-held GPS units

A number of hand-held GPS-enabled devices (including many mobile phones) can be used during fieldwork to display the OS grid reference of the current location. The grid reference value is often supplied in to a very high resolution (8-figure or 10-figure format) - in some cases choosing a lower precision setting may be more relevant to recording requirements (see Section 4).

If your device will only display the latitude and longitude of the current location, use FieldNotes coordinate-conversion tool (menu: Tools | Coordinate converter) to translate values to the equivalent OS grid reference.

Online tools

A number of websites provide interactive tools that display the OS Grid reference of any selected point:

Generally the sites allow you to navigate within the map to the required location, then either create a marker at this point and lookup the associated map information, or give the Grid reference for the centre of the displayed map. The exact mechanism varies from site to site and you may have to try a few to find one that suits your requirements.

3. Finding the Map Location from a Grid Reference

This section describes the various methods for finding the location that a Grid reference refers to - on either a paper or online map.

3(i). Finding the Location from a Grid Reference using a paper Map

The first step in finding the location is to identify the three parts of the Grid reference - the two-letter Grid-square identifier, eastings value and northings value. The two-letter grid-square identifier is self-evident. The eastings and northings are obtained simply by splitting the figures into two parts of equal length.

Identify the relevant Map sheet

Two-letter Grid-square identifier indicates which OS grid-square, and hence which map, contains the location. In some cases (where a Grid-square is split across several maps) you will also need to refer to the first two digits of the eastings and northings as well.

Identify the general region in which the location lies

Assuming that you have selected the correct map, look first at the eastings value to establish how far to the right (East) the location lies, then at the northings value to establish how far up (North) the location lies. On larger-scale maps (eg. OS Explorer 1:25000 series), the first two digits of the eastings and northings correspond directly to the gridline labels.

Identify the general area of the location by moving right to match the first two digits of the eastings value, and up to match the first two digits of the northings value.

This information allows you to highlight the area of the map (bounded by gridlines '44' - '45' horizontally, and gridlines '05' - '06' vertically) in which the point is located.

Identify the precise location of the Grid reference point

Next, identify the exact location by looking at the additional figures in the Easting/Northing values.

The additional information allows you to locate the point quite precisely: in this case on the coast at Gilly Head.

3(ii). Finding the location of a Grid reference using Online map resources

FieldNotes includes a tool for resolving OS Grid references using online map resources (menu: Tools | Grid reference locator) and also from the 'Observations' and 'Site' data-entry forms . This allows you to quickly check the location of any OS Grid reference in your browser.

Many (but by no means all) of the map-resource websites will display the location of a OS Grid reference, although they vary considerably in how easy it is to enter the relevant data.

The exact mechanism varies from site to site and you may have to try a few to find one that suits your requirements.

4. Precision - effects of varying the number of digits in a Grid reference

As the map square identified by the two-letter prefix is always a fixed size (100km x 100km), increasing the number of digits in the eastings and northings simply increases the precision with which the point is defined. Each successive figure in the grid reference affects the precision of the designated point as follows:

OS grid references are most usually given in six-figure format (eg. AA 123 123). This level of precision resolves to a 100m-square on the ground, which is adequate for most applications. However grid references can be specified to other levels of precision; for example, many GPS units and online mapping resources provide 8-figure or 10-figure OS grid references as standard.

When reporting locations as OS grid references, the relevance of precision is often overlooked. The number of figures that used in the grid reference convey important information about the precision with which the location is specified. For example, consider the following grid references - are they all equivalent (ie. freely interchangeable)?

SW 12 12

SW 120 120

SW 1200 1200

SW 12000 12000

Although the four grid references refer to the same point, their format additionally indicates the confidence (ie. precision) with which they identify the location. At the extreme ends of the scale, the lowest precision value (SW 12 12) refers to a 1km by 1km square surrounding the relevant point (see Table 1 above). In contrast the highest precision value (SW 12000 12000) resolves the point with a precision of 1m by 1m.

Obviously the values are not directly equivalent, and some care should be taken in choosing the most appropriate format when reporting grid reference values.