Grid References

The UK Ordnance Survey maps, which include the Isle of Man, show an overprinted grid which allows unique reference within a designated square of any point within the British Isles.
The true origin of this grid in terms of the Transverse Mercator projection used is at latitude 49° north and longtitude 2° west; however a false origin 400km west and 100km north of the true origin is used to keep all eastings positive and all grid northings on the British mainland under 1,000,000 m.
The format of this Grid Reference has three components:

• Two Grid Letters indicating a 100km square
• a three (or 4) figure Easting giving a 100m square(10m if 4 figures) reference
• a three (or 4) figure Northing giving a 100m square(10m if 4 figures) reference

The map below shows the two sets of Grid Letters (NX and SC) and the Eastings and Northings for a conventional 6 figure Grid Reference.

The grid reference of Snaefell is SC398881 (to 4 figures it is SC39848813); South Barrule is SC258759

Although the Island falls within two major Grid Squares (SC & NX) the Eastings and Northings are unique and thus in many Island Grid References the initial Letters are often implied.

Distances

Simple trignometry can be used to determine the straight line distance between any two points - using Pythagoras's rule:
Distance = Square Root ((difference in Eastings)² + (difference in Northings)²) x 100m [assuming 6 figure ref]
thus bee-line distance between Snaefell and S. Barrule is SquareRoot((398-258)² + (881-759)²) = SquareRoot(240x240 + 121x121) ~ 170 x 100m = 17km or 11.5miles

Note that on the 1:50000 and 1:25000 maps Eastings and Northings are marked as 1km squares and the user must estimate the nearest 100m reference.