A visit to Major-General Roy's Baseline (1784)

All who walk or cycle in Britain owe a debt of gratitude to the Ordnance Survey (OS), without whose maps we would be lost - literally. Even those who don't use OS Maps directly, use apps that probably would not exist were it not for the definitive mapping provided by the OS. Whilst I use Google Maps in this blog, I always use OS mapping on my walks and cycle rides (courtesy of the Memory Map app).

The nearest things to shrines for OS devotees are the two ends of General Roy's baseline in West London, one of which is located in a non-descript cul-de-sac in Hampton, the other on the Northern Perimeter Road of Heathrow Airport. Here they are on the OS Explorer 1:25K map. I show the baseline as a blue line, and fittingly, the OS marks the memorials at both ends.

The baseline on OS 1:25K Explorer Mapping

The ends are marked on the ground by cannon barrels, as explained below.

South Eastern end (left), North Western end (right)

Major General William Roy (1726-1790) was perhaps the 'John the Baptist' of the Ordnance Survey, since his work led to the organisation's foundation in 1791, a year after his death. He was a pioneer of the method of survey by triangulation, and a leading advocate of the need for an accurate survey of Britain. The story of the baseline is recounted in Rachel Hewitt's 'biography' of the Ordnance Survey, 'Map of a Nation' (Granta, 2010). In 1783 it was agreed with the French that a more accurate survey was needed to determine the relative locations of the observatories in Greenwich and Paris. (This Anglo-French cooperation was all the more remarkable given that fear of invasion by the French was a main driver for Roy's concept of a national survey.)

It was decided that the initial baseline for the triangulation should be on Hounslow Heath, West of London, presumably because it was a flat area near London and the whole length of the baseline could be cleared to allow the measurement process. One end was based at the Hampton Poor House (now Roy Grove), and the other end was at King's Arbour, conveniently close to the Bath Road, a distance close to 5 miles (26,400 feet). The North Western location is now on the Heathrow Airport Northern Perimeter Road, and the Bath Road is the A4.

The initial painstaking measurement took place over 75 days in 1784 using 20 foot glass rods (!), arriving at a value 27, 404.01 feet. A revision in 1858 gave a value of 27,406.19 feet. This measurement formed the basis for the national triangulation by the Ordnance Survey which took place over the next few decades. (The values are those given on the plaques at the baseline ends - see photos below. Hewitt says the length measured by GPS is 27,376.8 feet.)

The two ends were originally marked by Roy with wooden posts, but these soon rotted. Given the importance of the baseline, in 1791, these were replaced by cannon barrels buried upright. The barrels have proved long lasting and are still there to this day.

Here is a map of how I visited the sites. (OK, it's a Google map!)

The South Eastern end of Roy's baseline is located in the eponymous Roy Grove, a cul-de-sac in suburban Hampton. This can easily be visited on foot (the 285 bus from Kingston stops nearby, for example) or by car (park briefly and considerately in Roy Grove itself). A plaque was added in 1926, marking the 200th anniversary of Roy's birth, explaining the history, and a further information board was added in recent times. (Maybe a third plaque will be added in 2026!)

I expected the other end to be behind the perimeter fences of Heathrow Airport, but actually it is quite accessible. On foot, you can follow the pavement beside the North Perimeter Road from the access spur from the A4 Bath Road. There are frequent busses along the A4 to nearby bus stops. By bike, there are shared use pavements alongside the A4. I parked free in the car park of Cranford Country Park and cycled the 3 miles from there.

[For a cycle ride along the Crane Valley from Cranford Park, see my companion cycling blog.]

You can also park in the public car park adjacent to the Airport Employment and Skills Centre, and make the short walk from there. There is a charge, but not the crazy rates for airside parking (check out the APCOA website or app). Incidentally, there is a public viewing stand here, where aircraft spotters gather to watch planes taking off about 100 meters away, and the airport's cycling hub (Runway Cycling) is also here.

Take care crossing the various junctions.

The spot itself is tucked away screened by bushes. Once again, there is an upright cannon barrel (painted silver this time, rather than black), and an identical 1926 plaque to it's twin.