Ordnance Survey 'Bench Marks' on the Isles of Scilly

 

In 1887, Ordnance Survey set about recording heights above the 'Ordnance Datum' on the main islands of Scilly.  The Isles of Scilly Datum is on the New Pier, Hugh Town, St Mary's.  This local datum arose because the main datum at Newlyn could not be transferred across to these offshore islands as they were beyond the range of the levelling methods available.  The St Marys datum is based on the mean sea level, which, being based on a shorter period of observation than the mainland are a little less accurate.  The marks, known as 'Bench Marks' (BM), are generally cut into buildings or other semi-permanent features. If one BM is known then the exact height of the next can be found by measuring the difference through a process of spirit levelling.  All those currently found on the islands were levelled and last confirmed in 1887 and are no longer maintained.  65 have been recorded from the OS archive, but so far very few have been recorded here.  The Ordnance Survey archive is as follows, all are cut marks unless indicated.  Not all the shorthand is currently fully understood.  Good searching.

St Mary's

SV Blocks
89/10; Most of the Garrison
89/09; Woolpack Point, NONE
90/10; Hugh Town
90/09; Morning Point Garrison & West Peninnis Head
91/12; Centre, North of Island, Bar Point down to Newford
91/11; Centre of the Island, Longstone HC
91/10; Rocky Hill down to Old Town - NONE?
91/09; East Peninnis Head
92/12; Northeast Corner, Watermill Cove
92/11; Centre East St Mary's, Green Farm to Lunnon
92/10; Centre of area Porth Hellick  
[92/09 Church Point NONE]
[93/11 Tolls Island to Deep Point NONE]

 

Grid Square Easting Northing Description interpreted from the Archives Height above Island Datum Height of Mark above ground in Metres
SV 8952 1028 Angle Wall West Side Road [Garrison Wall No. 2]] 17.8582 0.5
SV 8971 1001 Angle Wall Southwest Side of Road [Garrison Wall No 1?] 15.5143 0.5
SV 8975 1049 Rock SE Side Rd 36.1127 0.1
SV 8980 1033 Twr (Tower?) Garrison Hill 41.7241 0.8
SV 8993 1066 Wall Steps Entrance to Star Castle 35.6342 0.4
SV 9006 1065 Building Entrance Garrison South Face 2.1M Southeast Angle 21.7140 0.6
SV 9008 1058 Wall E side Rd Opposite White Horses 27 2278 0.5
SV 9009 1031 Wall Opposite Trinity Cottages  Side of Rd 15.1973 0.7
SV 9014 1070 Public House Southeast Side Pier Entrance West Angle Northwest Face 4.9256 0.8
SV 9064 1058 Tower St Marys Church South Angle 11.9482 0.6
SV 9067 1069 Sch (School?) North Side Road 11.5611 0.5
SV 9086 1008 GTP (Gatepost?) 1.2M South Fence Junction West Side Road 33.7200 0.5
SV 9005 0993 Wall Angle - Morning Point? 16.4623 0.6
SV 9098 0962 STO (Stone?) Base Fence 48M SE Junction Fences NE Side Path 25.9690 0.4
SV 9125 1206 Stone West Side Tower East Side Telegraph Rd 50.7492 0.3
SV 9175 1216 Gatepost NE Side Watermill Lane 32.6685 0.5
SV 9183 1234 West Face Building 21 feet N from Northwest Angle Trenoweth Farm 36.9905 0.7
SV 9195 1204 North Angle Building West Side Road 34.6923 0.6
SV 9108 1102 Gatepost East Side of Road 4.4013 0.5
SV 9126 1175 Gatepost North Side Porth Loo Lane 42.2575 0.5
SV 9193 1149 Building South Side Road Northwest Angle 15.2979 0.2
SV 9199 1160 Building West Side Road East Face 2.2M Southeast Angle 19.9004 0.5
SV 9102 0976 Stone Door Peninnis Mill DIS (Disused??) 35.1343 0.3
SV 9114 0988 Gatepost 4.3M West of Fence Junction 17.4803 0.8
SV 9231 1201 Green Farm House East Side, Green Lane Southwest Side 4M West Angle 34.6680 0.6
SV 9208 1167 Building Northest Side of Road Southwest Face 32.2844 0.7
SV 9275 1145 Wall Mount Todden FM (Farm?) 5M East Junction 36.7010 0.5
SV 9277 1110 Stone Wall West Side of Road 19.8272 0.4
SV 9262 1072 Gatepost 11M South of Junction Walls 4.2794 0.6

Tresco

SV Blocks
88/13; NONE;
88/14; Plumb Island to Appletree Point
88/15; Northwest - Castle Down to New Grimsby
88/16; North Tresco
89/13; NONE
89/14; Abbey Farm, Both Pools & Down to Heliport
89/15; Northeast Tresco - Hotel down to North of Abbey

 

SV 8888 1462 Gatepost Southwest Side of Appletree Road 11.2898 0.5
SV 8892 1428 Rock 1M East Angle Fence West Side Appletree Road 20.2357 0.2
SV 8896 1488 Building Abbey Farm Northwest Side TK (Track?) East Angle 6.9921 0.2
SV 8817 1595 Wall Cromwell's Castle Southeast Angle 6.9220 0.0
SV 8867 1556 Gatepost Wall Northeast Face 33.1287 0.8
SV 8882 1534 Number 1 Palace Row Northwest Face North Angle 4.8768 0.4
SV 8883 1546 Gatepost Wall Junction East Side TK? 19.7571 0.7
SV 8887 1572 GTP 2.0M North Wall Junction 32.8940 0.5
SV 8896 1583 Wall 23M Northeast Fence Northwest Face 9.0830 0.5
SV 8825 1611 Wall North Side Entrance King Charles Castle 43.5985 0.8
SV 8833 1601 Stone Fence Square Enclosure 3.0M Southeast Angle 19.8608 0.5
SV 8914 1431 Base Dorrien Monument 34.4150 0.4
SV 8975 1491 Rock Northeast Side of the Road 31.6108 0.0
SV 8921 1527 Gatepost Angle Wall Northeast Face 20.0558 0.5
SV 8922 1540 Tower St Nicholas Church Southeast Face Angle 5.2212 0.6
SV 8931 1587 End Wall Southeast Face Island Hotel 13M South Angle 7.2756 0.5
SV 8934 1562 Ocean View west Side of Road Northeast Face North Angle 4.8219 0.5
SV 8936 1500 Gatepost Southeast Side Junction of Road TK Northwest Face 24.4450 0.5

St Martins

SV Blocks
91/16; West of Island
92/15; Centre East
92/16; North Island

92/17; (White Island NONE);
93/15; Centre East
93/16; NONE;
94/15; NONE;
94/16; Chaple Down, Day Mark
94/14; Great Ganilly NONE

SV 9163 1620 The Elms North Side of Road Southeast Angle 15.2766 0.7
SV 9168 1631 Rock Northeast of Lookout 38 6456 0.0?
SV 9189 1617 Building North Side of Road Southwest Angle 10.4089 0.6
SV 9209 1614 Building Middle Town West Side of Road Northeast Face 6.2M North Junction Building and Glass House 12.9631 1.1
SV 9419 1609 Flush Bracket - No. 10696 TP (Top?) St Martin's Head 47.1400 0.4
SV 9419 1610 Base West Side Door, Day Beacon Tower 48.8800 0.5
SV 9284 1562 St Martin's Church North Face, 10M Northeast Angle 32.5069 0.3
SV 9288 1539 Carrow Farm House Southeast Corner 22.3815 0.8
SV 9315 1554 Cottage Northwest Side of  South Angle 28.6634 0.7
SV 9343 1574 Gatepost North Side ofPath 26.4323 0.5

St Agnes & Gugh

SV Blocks
87/07; (St Agnes, South of West, Long Point Area);
87/08; Northwest St Agnes
88/08; (Northeast inc West Gugh);
89/08, East Gugh NONE);  
Annet 86/08 NONE

SV 8775 0830 Stone St Agnes Church Southest Angle 9.1623 0.5
SV 8793 0838 Gatepost Low Town Farm East side of Road 6.2667 0.6
SV 8827 0782 Wall East Side Barnaby Lane 2.3M South Prodn Wall Opposite 18.7178 0.6
SV 8802 0815 Wall Junction Opposite Downs Farm Entrance South Angle 26.5176 0.5
SV 8825 0833 Hedge Wall 1.0M Southwest Junction 17.3157 0.6
SV 8832 0821 Building Road Junction South Side Road NE Angle 22.6345 0.2

West Tean

SV Block
90/16

SV 9093 1641 Stone SW Angle Old Building Tean Isle 5.9863 0.6

Bryher

SV Block
87/14 (South of Island)
87/15 (North of Island)
Samson NONE

SV 8758 1475 North Angle of Bank Cottage 9.5433 0.5
SV 8797 1491 Tower, All Saints Church, Northwest Angle 6.6233 1.4
SV 8789 1502 House East Side of Road Northwest Angle 19.5255 0.2
SV 8796 1533 Wall 0.7M Southwest of Junction 18.2331 0.3

BENCHMARKS FOUND SO FAR

WATERMILL

Bench Mark behind the stream on route to
Watermill Cove; SV 92109, 12161

  

Second Picture

Bench Mark near Watermill House - SV 91959, 12050

THE GARRISON

Bench Mark at the Guard House
BNG: SV 90070, 10660

 

 

Bench Mark near Trinity House Cottages, Garrison Wall
BNG: SV 90097, 10317

 

Bench Mark at Morning Point

BNG: SV 90057, 09944

 

Bench Mark Garrison Wall 1

BNG SV 89720, 10024

 

Bench Mark Garrison Wall 2

BNG SV 89532, 10293

PENDRETHEN

Bench Mark on the path west from Pendrethen Quay.

SV 91448, 12717; Not on the database, may have been moved?

PENINNIS MILL
 

Bench Mark on the base of Peninnis Mill
SV 91023, 09772

 

Bench Mark on Giants Castle
(Not on the 1908 Map)

HUGH TOWN

Rear of Town Hall, near the drain.  SV 90328, 10503.  Not found in archive.

 

The Bench Mark at the Mermaid Pub, near the Quay

SV 90147, 10713

ST MARTIN'S

Bench Mark on the Day Mark at St Martin's

SV 9419, 1610


Can you find others?

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