Kent and Medway Biological Records Centre

Grid References

1. Introduction

As with identification, it is important to find accurate

grid references

for where you have observed species.

It can cause problems if you record a species with an innacurate grid reference - for instance you may get newts swimming in the channel!

This help guide is designed to explain how to use grid references from the basics upwards. Please get in touch with us if you'd like further help.

Whilst recording, there are many different ways of describing the location of a species. You can provide a description of the place (e.g.

by the Leeds Castle junction of the A20

), a name of the area (e.g.

Oare Marshes Kent Wildlife Trust Nature Reserve

), a postcode (a grid reference can be obtained from the internet using a postcode, for example on the Multimap website), or a National Grid Reference (see section 2 below).

2. UK National Grid System

Along with the internationally recognised latitude and longitude system, which is based on angles and distances north or south of the Equator and east or west of the Greenwich Meridian, Britain has its own

National Grid System

. This system is based on metric measurements (

kilometres and metres

) from a false origin point off the Scilly Isles.
Figure 1Figure 1; the National Grid System






To begin with, the whole country is divided into

100km x 100km squares

(

figure 1

). These are each labelled with two letters, which form the first part of every National Grid Reference. Kent falls within two of these squares:

TQ

(west Kent) and

TR

(east Kent).



These letters can be found at the top corner of an Ordnance Survey (OS) map.

Figure 1Figure 2; TQ subdivided into 10km squares



Each of these 100km squares can be further subdivided into 10km x 10km squares (

figure 2

).


The names given to any square within this grid (or any further subdivided grid) is a reference to the origin of that square (the bottom left-hand corner),

reading firstly from west to east, and then from south to north

. For example, the highlighted square below (

figure 3

) has a grid reference of

TQ75

, and any record within that square would have that reference. Equally, a record with a grid reference of TQ75 could be located

ANYWHERE WITHIN THAT SQUARE

. Consequently, two records with the same grid reference could theoretically be over 14km apart if they were located in diagonally opposite corners.

Figure 3Figure 3; Highlighted square is TQ75



TQ75 (

highlighted square in figure 3

) represents a 10km x 10km square, which can be further subdivided into 100 equal squares of 1km x 1km, as demonstrated below (

figure 4

).



If every single 10km x 10km square within the 'TQ' square were subdivided into 1km x 1km squares (

figure 4

), there would be 100 squares both along the bottom and up the side of the grid. Each of these squares would be labelled 0-99, and these are the numbers you find along the bottom and sides of an Ordnance Survey map.

Figure 4Figure 4; TQ can be subdivided into 1km x 1km squares



Now you know that each square within a standard OS map refers to a 1km x 1km area on the ground. As before the names given to any square within this grid is a reference to the origin of that square (the bottom left-hand corner), reading firstly from west to east, and then from south to north.

Figure 5Figure 5; Highlighted square is TQ7356




For example, the highlighted square in

figure 5

has a grid reference of TQ7356, and any record anywhere within that square would have that reference.

3. Standard 6-figure Grid

A 6-Figure Grid Reference is generally used as the standard and refers to a further subdividing grid. It is the predominant grid reference used both in biological recording and given as a point on an Ordnance Survey (OS) map to define a location, such as a meeting point.

If the highlighted grid square in

figure 5

(TQ7356) was further subdivided into 100 equal squares, each square would measure 100m x 100m and would allow you to give a relatively accurate grid reference for a point, or at least give someone else a reasonable chance of finding that point again with the aid of an OS map, especially with further site information.

Figure 6Figure 6; Locating a point within TQ7356
However, to draw these lines on a map would make it very difficult to see any other details. Consequently, you have to judge for yourself approximately how many tenths along and up the square the point is. For example, if we take the highlighted TQ7356 square above, we can mentally subdivide into 10 along the bottom and up the side, as below.

We then need to locate a point within the square that we want to find a grid reference for, as below (X marks the spot as always)

We can then see that the 'X' is approximately 6 tenths along the bottom and 9 tenths up the side, as below.

You now have the information you require to give a complete 6-figure grid reference, using the following method, again always reading west-east first followed by south-north (which is sometimes remembered by the phrase 'along the hall and up the stairs').
Figure 9The 6-figure Grid Reference

4.aAccuracy of Different Grid References

As mentioned above, different grid references have different accuracies. In general, the longer the grid reference, the more accurate the reference. The most often used ones are as follows:
TQAnywhere within the 100km x 100km square
TQ11Anywhere within the 10km x 10km square
TQ1212Anywhere within the 1km x 1km square
TQ123123Anywhere within the 100m x 100m square
TQ12341234Anywhere within the 10m x 10m square *
TQ1234512345Anywhere within the 1m x 1m square *


* = It should be noted that these grid references are only possible to obtain with the use of a Global Positioning System unit (GPS).

5. Non-standard Grid References

It should be noted that although the above mentioned grid references are often used, there are a couple of other non-standard ones used, but not so widely. All of the above references are obtained by subdividing grid squares into 100 equal squares, each one have a length and height exactly one tenth of that of the original. Other systems do not use this subdivision, but other types, and are usually only used for one particular size of grid.

5km Grid

Figure 7Figure 7; The 5km Grid
As the name suggests, this involves splitting a 10km x 10km square into a 5km x 5km grid. This is done by dividing the 10km square into 4 equal squares, (

figure 7

)

The squares are named by adding the relevant letters (based on compass points) to the end of the name of the 10km square. For example, if we assume that the 10km square above has the grid reference TQ75, then the highlighted 5km square would have the grid reference TQ75NE, as it occurs in the north-east quadrant of the 10km square. This form of grid reference is rarely used, but it is useful to know in case it is encountered. This form of subdivision could theoretically be used at different measurement levels, and thus a grid reference of TR6437SW would refer to anywhere within the 500m x 500m square located in the south-west quadrant of the 1km square TR6437.

6. DINTY Tetrads

This form of subdivision is generally seen as non-standard, but is encountered frequently within the world of recording, specifically for local distribution maps. This form of grid reference involves the production of 2km squares, and so divides one 10km square into 25 equal squares, (

figure 8

).
Figure 8Figure 8; DINTY Tetrads
To label each 2km tetrad square, all letters of the alphabet are used with the exception of 'O' (as it may get confused with a zero, and thus be confused with a different type of grid reference with a number missing). As can be seen in

figure 8

, the labelling commences in the bottom left-hand corner, and follows the alphabet moving up the left side, moving one column to the right once the top of the square is reached. As with the 5km squares above, the name of any individual 2km tetrad is the name of the 10km square it resides in followed by the single letter shown above. The highlighted tetrad above has a grid reference of TQ75G. The 10km square can be seen to be TQ75 by the labels along the sides, with all west-east numbers beginning with 7, and all south-north numbers beginning with 5. Any record ending with a single letter could be located anywhere within that 2km square, or in this case within any of the 1km squares TQ7252, TQ7253, TQ7352 or TQ7353. The reason this system is sometimes called the DINTY tetrad system, is because the 'word' DINTY is spelled out reading across the second row down.

As with the 5km squares (

see 5. Non-standard Grid

)
, this form of subdivision could theoretically be used at different measurement levels, and thus a grid reference of TR6437B would refer to anywhere within the 200m x 200m square located within the 1km square TR6437 using the same grid labelling method shown in the 2km tetrad method above.

7. Uses of Different Grids

You may be asking why we need so many different types of grid reference. This is because each has their own use, and some methods are more appropriate than others to use whilst recording.

Remember, grid references can always be 'dumbed-down' later without the help of the original recorder, but cannot be made more accurate.



If a record has a grid reference at the 1km level of TR1246, you can determine that it resides in the 10km square TR14, but you cannot determine which 100m square the record resides in. Consequently, you should always record as accurately as possible when appropriate.

Uses of different grid references levels are as follows:

  • 100km level -

    only ever used (although rarely) for distribution mapping for very large areas, e.g. European distribution (although it should be stated that the rest of Europe does not use the British National Grid system).
  • 10km level -

    usually used for national distribution mapping of a species.
  • 5km level -

    this method is not often used in recording, but is used to divide large digital maps or aerial photographs for ease of use.
  • 2km level -

    this method is mainly used by recorders wishing to map species at a local level, such as the Kent distribution. It is useful in showing the rarity of a species or broad abiotic associations, but not for locating individuals. It is also used by some national recording schemes, e.g. Botanical Society of the British Isles, for monitoring purposes.
  • 1km level -

    this method is also used for mapping local distributions, giving a more accurate picture, although it is sometimes the most accurate grid reference that can be determined from a map, depending on the scale of the map and the amount of detail present.
  • 100m level -

    this is the most accurate grid reference you can reliably obtain from a standard OS map. It is also the level obtained from the internet using the postcode to determine location. This is the level at which most species are recorded.
  • 10m level -

    a GPS unit is required to accurately record at this level. Most GPS units record at the 1m level (see below), although if you cannot get a good accuracy due to environmental factors, or if something covers a 10m x 10m area, then this is the best level to record.
  • 1m level -

    a GPS unit is required to record at this level, and this is the most accurate location possible. It should be noted that although this is theoretically recording at the 1m x 1m level, this is rarely the case, as GPS units always carry a level of inaccuracy due to environmental factors. Consequently, when recording using a GPS, you should always note the level of accuracy given by the GPS unit.


Although it may be possible to record certain species at very accurate levels with the use of a GPS, this is not always appropriate due to the species' ecology. If you have located a rare plant, the most accurate location possible should be given, as plants are generally not mobile, and so an expert could use the locational data to find the species and verify or invalidate that record. Highly mobile species do not need to be recorded at such an accurate level. Birds are often recorded only at the 1km level, unless the area of habitat they are found in is small, such as a small reedbed, or if recording a breeding site. However, this does not always apply and also depends on the state or aspect of the species. A fox may be recorded at the 1km or 100m level, but if it is found dead on the side of the road, or you have located a fox den, a more accurate location should be noted.

Ultimately, if you have any questions about recording the location of a species, or any questions about recording in general, please contact the KMBRC.