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A Reference Card or "Romer" is a device for increasing the accuracy when reading a grid reference from a map. Made from transparent plastic, paper or other materials, they are also found on most baseplate compasses. Essentially, it is a specially marked-out ruler which matches the scale of the map in use. The scales are laid out in reverse, such that by lining up the numbers given in the grid reference with the gridlines for the square in question, the corner of the romer lies on the location whose grid reference you wish to read. Some transparent versions have a small hole at the origin when this is not at the corner of the Reference Card. This allows access to the map such that the location could be marked with a pencil if using the Reference Card in reverse having been given a grid reference to start with. They are used in many types of land navigation.
Romers can easily be made yourself; by hand, by using a computer or by finding a website with instructions.
They describe only an approximate location. The smaller the scale on the map, the less accurate the romer reading. On 1:25,000 maps, such as the British Ordnance Survey Explorer Series, a 6 figure grid reference gives an area of 10,000m2. This is larger than a football pitch (soccer field). A more popular and accurate device is a Grid Reference Tool.
Invented and developed for car navigational rallies by car rally partners Eric Gardner and John Cridford during the early 1950s, the 'Garford Romer' was available for both Imperial and Metric OS maps and is still sold today. Although a registered design when it was first made and sold it was and still is much copied.
The illustration below shows how the romer is used. Here, we are plotting the reference 696018. The marks corresponding to (6, 8) on the romer are lined up along the gridlines (69, 01). The hole near the corner yields the exact point, the church at Little Plumpton.