Ordnance Survey Benchmarks in Tickhill
The 1901 Ordnance Survey map of Tickhill, whose 1902 edition was reproduced in 2004, shows exactly where the benchmarks were with their associated height above sea level. Curiously, the height above sea level was given in feet and tenths of feet, not inches. For example, the height at the benchmark on St Mary’s Church was given as 60.6 feet. Height above sea level was measured initially from a reference point in Liverpool, using, in particular from 1844, the mean sea level at Victoria Dock. The Ordnance Survey replaced this measurement with the height of mean sea level at Newlyn after taking measurements of sea levels every 15 minutes, day and night, between 1915 and 1921.
Parts of Tickhill were among the first surveyed in England, in the mid 19th Century, to establish height above sea level. What was known as the first primary levelling took place between 1840 and 1860. One route through Tickhill was included in the Thorne to Stalybridge levelling between 1845 and 1846. (In contrast, the measurement of the London to Doncaster route was not undertaken until 1848-1851.) The precise places where measurements were made through Tickhill were listed in an 1861 publication Ordnance Survey abstracts of the lines of spirit levelling in England and Wales by Colonel Sir Henry James. Here are the locations of benchmarks from the Toll House in Sunderland Street to Worksop Road:
Mark on South East angle of Tickhill Toll House: 1.33 feet above centre of the road.
Mark on jamb-stone of door of John Grayson’s house Tickhill: 1.28 feet above centre of the road.
Mark on centre of floor of Tickhill Market Cross: 2.29 feet above base of Cross.
Bolt in West face of St Mary’s church tower Tickhill: 3.16 feet above surface.
Mark on stone near bottom of wall adjoining small cottage at junction of roads: 1.71 feet above centre of the road.
Mark on the 9th milestone from Worksop 8th from Doncaster East side of road: 2.94 feet above centre of the road.
Some of these benchmarks have survived especially the benchmark just to the right of St Mary’s west door, the only one in Tickhill to include a bolt made from gunmetal in the middle of the horizontal bar above the arrow head shape. Another is close by at the bottom of the wall near the junction of Church Lane and Westgate. Other benchmarks have been obscured or removed. For example, the pebbledashing of the Toll House has covered its benchmark, while the benchmark on the centre of the floor of the Buttercross has been removed and an unmarked paving stone placed in there.
Another series of benchmarks follows the Northgate route. One can be seen at the lower edge of the front wall of 30 Northgate. It is partly obscured by a drainpipe.
Measurements on maps from different periods are not necessarily consistent. In part this is due to remeasuring: the second levelling in England took place between 1912 and 1921 while a third set of measurements were taken between 1950 and 1968. However, another reason showed the unreliability of using benchmarks on a variety of structures from walls to church towers to gate posts to mile stones: subsidence. In mining areas variations in height above sea level could change over time by as much as 8 to 10 feet. The Ordnance Survey finally abandoned the benchmark network in 1989 when it started to use satellite technology known as the Global Positioning System.