2. How to Find a Grid Reference

Although far from essential, a compass with Romar scales is a convenient instrument to estimate the O.S. grid references. Both the 'Silva 54 combi' and the 'Silva expedition 54' have them.

On a 1:25,000 scale map the interval between lines representing 1 km are 4cm and on a 1:50,000 these intervals are 2 cms. The Romar scale on your compass can be used to subdivide these intervals. Splitting the interval into tenths can usually be assessed by eye. Using the Romar scale on the compass, splitting into hundredths is just possible, especially on the 1:25,000 scale map, as in the diagram below.

Using Romar scales on compass to find map reference.

The Romar scale for the 1:25,000 scale map are the two sets of graduations at right angles on the outer edge of the baseplate of the compass. Place the junction of these graduations over the location on the map, whose grid reference is required, as shown in the diagram. Point 'P' on the above example is at - NY 604 923.

If the values are rounded to the nearest digit, then the 6 digit answer will be within ±50 metres and the 8 digit answer will be within ±5 metres.

The Romar scale for the 1:50,000 scale map is placed away from the edge of the baseplate and so one should try to look directly over the graduations to avoid parallax errors.

Some compasses have baseplates with the Romar scale in one direction only. This is not so convenient because the baseplate has to be turned through 90° between measurements.

A convenient device, which can be carried in your top pocket, is a grid reference finder or Romar. This is made of transparent, rectangular plastic and has square sections calibrated in 1 : 25000 and 1 :50000 Romar scales.