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.

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