On top of the inner ring, just to the north of the main visitors' entrance to Old Sarum, is an information board describing how Ordnance Survey set out to check the accuracy of the mapping of Southern England. It explains how, in 1794, the Ordnance Survey set out to check the accuracy of the first mapping of Southern England, which it had begun 10 years earlier.
The task of verifying the accuracy of the first stages in the mapping of Britain was entrusted to a young army officer, Lieutenant William Mudge. He went on to become the first Director General of the Ordnance Survey. The theodolite used by Lieutenant Mudge was made in 1791 by Jesse Ramsden, one of Europe's finest instrument makers. Made specially for the Ordnance Survey, it was the largest and most accurate instrument of its type in the world.
From a point just below Old Sarum, Lieutenant William Mudge laid out a baseline 36,574 ft (11,253 m) long. From each end of the line, the positions of distant places were plotted. The accuracy of the process, which was repeated all over England, depended on Jesse Ramsden's craftmanship and William Mudge's surveying skill in setting out the first baseline from Old Sarum.
The nearer end of Mudge's line is marked with an inscribed stone beside the modern A345 in front of you.