Intersected Station

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spire SK58/INT2
chimney SD96/INT8
vane NZ42/INT16
pinnacle SD80/T35
water tower SE22/5 not designated as INT as the station also has a roof bolt

An intersected station is one at which the coordinates were derived by intersection, no observations were ever taken at the station. Intersection is a process where rays are observed to the station from many outside points and the coordinates derived from the resulting intersection of those rays

The main distinguishing factor is that intersected stations generally cannot be occupied by a theodolite.

However there is the noticeable exception of St Pauls Cathedral. This was originally intersected during the Principal triangulation and later (during the Principal) a scaffold was erected and the Ramsden 36" theodolite taken to the top and observations taken. [1].

Examples of Intersected marks include Centres, church spires, weather vanes, pinnacles and chimneys. They were often used in conjunction with a nearby mark that could be 'occupied', for example a bolt on the church roof. The station name is only classified as Intersected in the OSGB36 list if it does not include any occupiable marks. If there is a mix of occupiable marks (i.e rivet or bolt) and non-occupiable (centre, church spire, vane, flagstaff, etc) then it is not an Intersected station.

The following table is extracted from [2]. 'count1' counts records where the station name contains "I" (denoting that it comprises only 'Intersected' marks). 'count2' counts records where the the station comprises a mix of Intersected and Occupiable marks.

The inclusion of a block as an intersected station may seem surprising. The single instance is Lochboisdale Pier for which the OSGB36 record states "Centre of conc block on pier". This suggests that it was not an OS block with a metal mark but a pre-existing non-OS concrete block with a mark type of 'Centre'.

The three instances of 'Stone', Chars Stone, East Breakwater North End and West Breakwater East End are all recorded as destroyed but may have been something similar.

It is unclear why the rivet at John Lea Sec Mod Sch Wtr Twr (SK22I035) or the bolts at Park Place Belfry (SU77I011) and Laurence House (NT48I103) were designated as an Intersected Station.

Summary of Ordnance Survey intersection stations by type
type count1 count2
apex 35 40
spire 2523 2555
tower 67 72
flagstaff 268 2353
mast 280 427
chimney 741 751
belfry 93 94
vane 441 961
beacon 41 52
lighthouse 182 186
dome 101 102
cross 37 50
memorial 15 15
obelisk 18 18
cupola 107 118
pinnacle 31 54
monument 107 109
centre 68 1756
pipe 3 27
cairn 13 16
old trig 20 442
pylon 15 15
l'ning c 11 94
pole 6 20
stone 3 4
vent 11 35
aerial 6 40
rivet 1 (1404)
bolt 2 (6032)
block 1 (3289)
turret 4 5
lamp 4 10
total 5255 10421


  1. Email from Ian Wilson to the Trigonomy mailing list, 4 July 2012