Reference Systems ︎︎︎

Within the fabric of London’s urban space there are objects that link a specific location with a network of a larger scale. In the case of our area of study, these are national systems of reference for elevation, systems for telecommunications and railway networks.

In the walk from Primrose Hill tunnel to Alexandra Road Estate are a number of unassuming brick buildings that are easy to overlook. On the corner of St John’s Wood Park and Adelaide Road is a vent shaft for the Jubilee Line. This round brick building allows heat from the tube to escape. In bordering Islington, a new scheme is using old tube vent shafts to provide heat to homes and schools in the borough. There are two vent shafts on the Hilgrove Estate - both are for the Chiltern Mainline which, in addition to the West Coast Mainline, links London to Birmingham. The Chiltern Mainline connects Marylebone to Birmingham via Banbury and Leamington Spa. The tunnel was originally built for the Grand Central railway in 1889. The new tunnels for HS2 will provide a third mainline to Birmingham from London and will run under or very nearly under Chalcots, Hilgrove and Alexandra Road Estates. 

A few hundred metres south along Finchley Road is the disused Marlborough Road Station which closed in 1939 to reduce congestion on the Metropolitan Railway when it expanded its capacity for suburban commuters into the city. Marlborough Road station is now a substation for the new S-Stock trains introduced in 2010 but until 2008, it was a Chinese restaurant.

Walking through the streets of Swiss Cottage we can find at least one ordnance survey benchmark located on the east wall of the Jubilee Line vent shaft. Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.

Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.

Benchmarks are the visible manifestation of Ordnance Datum Newlyn (ODN), which is the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level. This particular benchmark stands at 53.996m above mean sea level.

Another link that we found to a larger network is a set of antennas and dish plates located in the roofs of Farjeon House and Langhorne Court on Hilgrove Estate. In July 2021, the decision to install a set of new antennae, dishes and equipment was approved by Camden on behalf of the MBNL, EE and 3 networks, to replace and install the necessary setup to be able to provide 5G connectivity in the area. Although it is not confirmed that the new equipment has been installed, the antennae in place still represent the hardware of a larger network of telecommunications active in the space.


Benchmark: AIR SHAFT SE SIDE HILGROVE RD NW ANG W FACE 1976  / 53.996m

London Underground Ventshafts