75th anniversary of the Ridgeway Base observations

Guest blog by  Graham Pennington, Geodesy & Positioning team

Today (Friday 7 December) is the 75th anniversary of the completion of measurements on the Ridgeway Base.  The Ridgeway Base runs from White Horse Hill (grid ref. SU3008386375) to Liddington Castle (grid ref SU2098279752) with trig points marking each end.  The baseline was one of several measured sides in the network of observed triangles that made up the triangulation network for the re-triangulation of Great Britain.  A triangulation network requires at least one measured side in order to control scale and to fix the size of the network to “the real world”. 

At its simplest, the baseline was a straight line measured between two fixed points, measuring just over 11 km in length and divided into 18 bays approx. 3/4 km in length.  However, in these days of satellite surveying at cm accuracy and laser distance measurement at mm accuracy, it is easy to take for granted the accuracy of the results achieved 75 years ago and the effort involved in measuring the baselines.  The measurements were taken using little more than tapes measuring 24 m in length and just 3 mm wide.  Each bay was measured 3 times and the measurements only accepted when they agreed to within 0.2 mm.  This is an incredible tolerance even today and the overall accuracy of the whole length was estimated to be just 1 cm.  When the length was checked in 1951, with superior equipment, it only differed from the original measurement by just over 6 mm! 

The Ridgeway measurements took about a month to complete (starting on 10 November) and traversed across many difficult obstacles including a ravine over 100 m wide and 15 m deep, deep ditches and ramparts of the Iron Age Forts located at the terminals of the baseline.  The measurements were made over steeper terrain than conventionally accepted for primary baselines and an advance party went first to clear the way of other obstacles including hedges, traffic signs and even a very large pile of manure.  At one point the baseline went through an obstacle!  Odstone Barn stood exactly on the line of the baseline so the surveyors simply cut holes in the walls and threaded the tapes through the building and carried on measuring. 

Ironically, in the end the Ridgeway Base measurement was not actually used in the computations of the re-triangulation of Great Britain (which gives us our current OSGB36 datum and National Grid) as it was decided the scale and orientation would be set by fitting it instead to 11 existing trig pillars of the Principle Triangulation.  However that does not diminish what an amazing achievement it was.  The surveyors’ efforts all those years ago are worthy of a small celebration since they contributed to the British coordinate system still in use today.

4 Responses

  1. Edward Ford

    Sorry Gemma your date is a little out, I recently came across a photo I took of the building of the towers to cross the ravine, I was part of the advance team led by Mr. C Lucas. We did indeed go out the week before the main party and spent the week on clearance, on the arrival of the main party our job was to set the tripods on line for the measuring party . In all I was five weeks on site. I joined the OS in 1947 having spent the war years in the Merchant Service, being 90 now I find it hard to fix the year but it must have been between 1949 and 1952, The Caithness Base was taped the following Year . If you e-mail me I will send you a copy of the photo.

    1. Hi Edward

      Thanks for getting in touch. I think you are probably referring to the re-measurement which took place in 1951, rather than the original Ridgeway Base measurements which took place from 10 November-7 December 1937. There’s a brief mention of the 1951 measurement in a book we link to on our website. You can find the mention on page 129 via this link: https://www.ordnancesurvey.co.uk/docs/ebooks/map-makers-to-britain-since-1791.pdf

      We’d love to see a copy of your photo though if you’d like to email it over. You can use pressoffice@os.uk to send it across to us.

      Many thanks

  2. Aylmer Johnson


    I was a little puzzled by the last paragraph of your excellent account, above. I know the re-triangulation was scaled and positioned so as to give a best fit with the Principal Triangulation, but wasn’t the scaling done by simply choosing a suitable value for the central scale factor? And surely, the Ridgeway base measurement must have been used to determine that value?

    1. Hi Aylmer

      Sorry for the delay in getting back to you, I needed to speak to my colleague in the Geodetic team and he was out of the office. Mark’s confirmed that the scale of OSGB36 comes from the fixing of the coordinates of 11 points adopted from the Principal Triangulation and not from any baseline measurements. The central meridian scale factor of the National Grid is a separate issue from the scale of the OSGB36 datum realised by the Retriangulation. The CM scale was selected to maximise the usable width of the projection whilst still keeping projection scale distortion inside acceptable limits. Projection scale can be considered separate from datum scale.

      Many thanks

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