Itchen Navigation: Bradshaw’s Map of 1830
This is an extract covering the Itchen Navigation from Bradshaw’s “Map of Canals, Navigable Rivers and Railways &c” published in 1830. It names the locks from Winchester to Wood Mill and gives the height (in feet and inches) of the Navigation above each lock. The names of some of the locks differ from those used today.
Notes on the heights given by Bradshaw
The heights quoted by Bradshaw were said to be based on a level 6ft 10in (2.08m) below the Old Dock Sill in Liverpool Docks which Bradshaw states is "low water at Liverpool". This not the same as the Liverpool datum measured by Ordnance Survey in 1844 which represented Mean Sea Level. Modern Ordnance Survey heights are related to Mean Sea Level measured at Newlyn, Cornwall between 1915-21. The level of the Old Dock Sill is 4ft 6.5in (1.38m) below Newlyn datum. Bradshaw’s height given for the Navigation above Wood Mill appears to be very approximately 10 feet (3m) higher than levels based on Newlyn datum. Closer examination of these heights shows the height values shown to be increasingly in error travelling north from Wood Mill. On the Newlyn datum, the water level above Wood Mill is about 6 feet (2m) and at Blackbridge Wharf it is approximately 107 feet (32.7m), a difference of about 101 feet (30.5m). Bradshaw's values are 16ft 2in (4.7m) and 129ft 7in (39.5m) respectively, a difference of 113ft 5in (34.8m). Conclusion: Bradshaw's values are unreliable.
It should also be borne in mind that early 19th century surveying equipment / technology was not up to the task of accurately transferring a level from Liverpool to Wood Mill. It is likely that the figures given by Bradshaw were derived from a local measurement or estimation of low water level and assumed that such a height would be the same as that at Liverpool. It was only confirmed in the early 20th century that mean sea level is not constant around the coast of Britain.