Grid reference skills
QIf I need to convert an eight- or 10-figure grid reference to a sixfigure reference, should I round up or round down the surplus digits?
You should always round down a grid reference. Remember, a grid reference indicates the bottom-left corner of the grid square it references. The area size the grid square represents is determined by how many figures compose the reference, so a four-figure grid reference indicates a 1km square, a six-figure grid reference indicates a 100m square, an eight-figure grid reference indicates a 10m square, and a 10-figure grid reference indicates a 1m square.
Grid references also always include a twoletter descriptor at the start, indicating the particular 100km square area of an OS map in which the reference lies. These letters are marked in blue in the corner of an OS map.
If we have a 10-figure grid reference, for example from a satellite navigation device, and we want to round it to a six-figure, always round down the last two least significant digits from each side to zero. In other words, we ignore the last two digits on each side. Likewise, with eight-digit grid references, round down the single least significant digit on each side to zero. So, if you have a 10-figure grid reference that reads SK12599
95799, it would become SK125957 as a sixfigure reference.
Even though the location referenced is numerically nearer to the SK126958 grid square in terms of distance on the ground, the location is still deemed to be in the
SK125957 grid square.
Paul is co-founder of the UNS – a charity providing navigation courses for hillwalkers, with all profits going back into the hills. Trail readers can currently enjoy 10% off the School’s weekend navigation courses! Simply go to www.ultimatenavigationschool.co.uk and enter ‘trail18s’ at the checkout for your discount.