The national grid as shown below enables any position in Britain to be fixed. This is done with global lines called longitude and latitude. Longitude run from north pole to south pole whilst latitude run around the circumference of the earth going north or south of the equator. Greenwich Observatory in London is on 0º Longitude with lines going either west or east and the equator is set at 0º with lines going either north or south.
Our starting point for the National Grid in Britain is 2º West of Greenwich which the central meridian for Britainand 49º north of the equator. The intersect point of these two lines is known as True Origin. However for the grid to be used effectively the bottom south west corner must be south and west of mainland Britain. For this reason true origin is moved 100 km north and 400km west to produce a point known as False Origin.
The National Grid is now formed as shown below with each square being 100km by 100 km
You will notice that the main letters are based on ST JOHN and that many of these squares are in sea areas. Each main letter covers 25 squares and in the case of T, O, H and J part of their grid is omitted. In the cases of S and N the 25 squares have a second letter which begins with A at the north west, uses all the letters of the alphabet except I and ends at the south east with letter Z. So if we look at the Isle of Man much of the south of the island is in SC but the northern tip is in NX. This two letter reference system will identify an area 100km by 100km.
Each square can now be divided into 100 squares (ten in each direction) now each 10 km by 10km. A square within this grid is identified by starting at the south west corner and first moving east (this is know as an easting) and then secondly moving north (this is know as an northing)
Hence we may move 60km east and 40km north so if we were in square SP this would be SP 64. SP64 could then be divided again into 100 squares now with each now 1km by 1km with the bottom south west corner being 60 moving east and 40 moving north as show below. If we move 7km east and 8km north the grid reference is now SP 6748
This process can continue until we reach the level of accuracy required. Details on the National Grid and the Ordnance Survey can be found at the website below.
If we can identify co-ordinates points near our site using an Ordnance Survey map then we can establish several control points around the site which relate to the National Grid. Once these control points are established we can find any point for which co-ordinates are known from one of our control points as shown below.
To find the setting out point we need to move 7.873m east and 6.453m north
Hence we have a triangle formed:-
We form the triangle this way to calculate the clockwise angle from north which is known as a whole circle bearing (WCB). This is then calculated :-
Tan O 7.873/6.453 = 50 deg 39 mins 38.5secs
So calculations in the NE quarter will give WCBs of 0 – 90 whilst SE quarter will be 90 – 180, SW will be 180 – 270 and NW quarter will give whole circle bearing between 270 and 360.