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!