Heathrow Airport honours the work of Major General William Roy

You may know that the origins of Ordnance Survey go back to a triangulation survey carried out for King George III and The Royal Society between 1784 and 1790. The survey was determining the relative positions of the Greenwich Observatory and L’Observatoire de Paris, and measuring the distance between the two observatories.  Major General William Roy FRS RE, carried out the survey under the authority of the Master General of the Board of Ordnance, and Roy’s first action was to measure a survey base-line across Hounslow Heath during the summer of 1784.

The two terminals of this base-line are marked by contemporary military cannon set in the ground muzzle upward. The north-western portion of the base-line is now occupied by Heathrow Airport.  As a result, the north-western terminal now lies just north of the Northern Perimeter Road of the Airport, adjacent to Nene Road. It’s approx. 200 metres east of the northern entrance to the tunnel which provides access to Terminals 1 -3 of the Airport and also within 150 metres of the Bath Road, the route of the ancient Great West Road. King George III and other dignitaries of the time would have travelled between London and Windsor Castle using this road, and would have used it to visit William Roy and his survey party as the 1784 survey work progressed.

Until recently the presence of the cannon and the significance of the site were also marked by an inscribed slate plaque set into the south wall of the Nene Road Metropolitan Police Station some 100 metres to the north of the cannon.  Recent redevelopment plans have resulted in the relocation of the Police Station and the demolition of the building.  However thanks to the generosity of Heathrow Airport Ltd., their contractors have remounted the plaque on a specially made masonry plinth within 20 metres of its original position, on the north western side of the adjacent Nene Road Roundabout.  Through this act of goodwill by the Airport, you can still visit and identify the site which has importance to the history of British national mapping.

Katie Benn and Phil Watts from Corporate Office were recently invited to Heathrow by the Airport authorities to view and approve the re-sited plaque and its relationship to the cannon, which, with its own smaller information tablet, remain in situ in a small partially enclosed “garden” area nearby.

The baseline was measured by Roy in 1784, using glass rods, at 27,404.01 feet.  It was re-measured in 1791 by Captain (later Major General) William Mudge FRS RA, as the first work of the then newly established Ordnance Survey, and found to be 0.23 feet (2¾ inches) longer.

A further re-measurement by Ordnance Survey geodesist Alexander Ross Clarke in 1858 determined the length to be 27,406.19 feet (5.19 miles; 8.35 Km).

The south-eastern terminal is situated at the end of the aptly-named Roy Grove, Hampton on Thames, and is also marked by a cannon and plaque.

18 Responses

  1. Dianne Cuming

    I live next to the Roy Grove Cannon that forms part of the Ordnance Survey Mapping. It is with regret that this monument is not maintained by anyone and has been left to rot! If it was such an important part of OS, why is the Heathrow monument more maintained? I have over the last 23 years made attempts to keep the plaque in place using several strategies and my own money!
    It would be good to see it given the same prominence as Heathrow!
    Dianne Cuming

    1. Hi Dianne

      I just had a chat with Katie (mentioned in the article) as she travelled up to Heathrow to see the re-located plaque at the time. It was actually the contractors at Heathrow who re-sited the large commemorative plaque that had been mounted to the wall of the police station and put it on a plinth near to where the police station used to be, with the marker canon about 100 m away. We’re very grateful that they took the time and effort to do that and to speak to us about it too. On the same day, Katie also visited the Roy Grove Canon and we do appreciate that it has not been maintained to the same degree. We’ll raise this again internally Diane.

      Thanks, Gemma

      1. Ol Rappaport

        I visited both ends of the Baseline today (4/6/16), I was very shocked at how much the Roy Grove plaque has deteriorated. It is in urgent need of rescue: the wooden stakes have rotted at ground level and it is now tied to the fence with blue polypropylene rope. The plaque has been painted but this is now flaking very badly. This compares very badly with the ‘Heathrow’ plaque. I have written to the ward councillors alerting them of this Since it is Grade II listed the council has a duty of care to maintain it.

      2. Paul Doe

        Hi just tweeted on this. Any joy on the OS helping with better maintenance and an information table in Roy Grove

      3. Paul Doe

        Hi Gemma at OS
        I asked the OS if they could help with the refurbishment of this site two years ago. I have now decided to put in a bid to Richmond council for a civic pride grant if £1000 to install an Interpretation Board at the site. Would OS assist with funds to refurbish the plaque or resite it? I have asked Rachel Hewitt author of ‘Map of a Nation’ to endorse my bid.

      4. Martin Humphreys

        I visited the SE terminal today. The tablet did look a bit sorry for itself and at this time of year, the nettles had partly covered it. The Canon however stood proudly on recently mown grass.

  2. James Lofts

    I’m interested in how the baseline fits in with today’s map of the area. Does any modern map show the position of the line?

    1. Lawrence

      Both the end points of the base line are shown on OS (1:25,000) maps. The western end is quite clearly marked as “Cannon West End of General Roy’s Base (site of)” at grid reference TQ 077 767. This is just north of terminals 1,2 and 3 of Heathrow. The other end is shown merely as “Meml” at grid reference TQ 137 709. This is at the end of appropriately named Roy Grove, Hampton.

  3. R W (Bob) Howes

    Hi guys, just read a book called ‘Map Addict’ by a Mike Parker wherein he eulogises on the OS. He also describes the details of the Baseline and this, as an old ‘Survey’ hand rekindled my interests. Many years ago, at an Antiques Fair, I happened upon an old OS bracket type brass bench mark for 50p and it spent many years as a paper weight on my desk at the Romsey Road office. After I retired I became interested in a certain steam railway near Winchester and the BM has now been re-sited at Medstead Station, as this type were mostly used for line of levels near railways and canals, so continues its history in the right vein. Unfortunately it never has had and probably never will have a value in its new position. Regards Bob Howes.

  4. Peter Martinez

    This article is mostly about the measured baseline near Heathrow, but I would very much like to know if there is a documented measurement for the line between the two observatories at Greenwich and Paris mentioned in the first paragraph. This figure would be very useful in cross-checking distance-measurement websites such as Google Maps.

    Peter Martinez

  5. Peter Martinez

    Jocylyn, thank you for your reply. That article makes fascinating reading, but I cannot see in there any figure for the distance between the Greenwich and Paris observatories, which is the figure I was hoping to discover. Is there perhaps a more recent figure for this distance, or a figure for a similar distance between any two easily-spotted points, that I can use to verify that the various on-line distance-measurement apps can be trusted?
    Peter M

    1. Jocelyn

      Peter, sorry this did not answer your question. I have since spoken to a colleague who has advised me further. We don’t think a point to point distance measurement was calculated. The Greenwich-Paris triangulation was mainly to determine the longitudinal separation of the meridians running through the two sites. We have found an online publication from the Royal Society about it here – https://royalsocietypublishing.org/doi/full/10.1098/rsnr.2008.0029#d11274494e1

      To check (or at least compare) distance calculation tools, the coordinates of our OS Net stations could be used. A map of the stations is on this page – https://www.ordnancesurvey.co.uk/gps/os-net-rinex-data/ and a text file of accurate coordinates is here – https://www.ordnancesurvey.co.uk/documents/resources/osnet-coordinates-file.txt

      Hope this helps, Jocelyn

  6. Peter Martinez

    Jocelyn: Yes I realised later that I was wrong to suppose that the Greenwich/Paris survey was aimed at establishing the distance between the two places. I went on to calculate distances between points for which I knew reliable lat/long figures, using the Vincenty technique with WGS84, and comparing these with distances read from Google maps. The agreement was satisfactory for my purpose (predicting the time taken for radio signals to travel). Your further links will enable me to refine this process much further. Thank you.


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