Read the number of the vertical grid line to the left of the location for which you want to find the grid coordinate. If the location does not lie exactly on the line, estimate the distance between the line and the location by dividing the grid square into tenths.
Write down the line number and the estimated number of tenths to the next line. For example, if the line to the left is 25 and you estimate the location to be another three tenths of the way between that line and the next, write down the numerals 25 and 3, to read 253.
Read the number of the horizontal line below the location for which you want to establish the grid coordinate. If the location does not lie exactly on the line, estimate the distance between that line and the location by dividing the grid square into tenths.
Write down the line number and the estimated number of tenths up to the next line. For example, if the line number is 44 and you estimate the location to be up another six tenths of the way between that line and the next, write down the numerals 44 and 6, to read 446
Combine the two grid coordinates to form the grid reference, writing the vertical coordinate first, followed immediately by the horizontal coordinate. In this example, the grid reference is 253445.
