Geodetic Survey Control System of Hong Kong Simon KWOK, SLS/G James WONG, LS/G(NT)

Contents Introduction History of development Types of Geodetic Survey Control Station Accuracy Standards Calibration baselines and network Specification and Practice Guide for establishing GPS control stations Q and A

Development History Horizontal Control Network Vertical Control Network GPS Control Network

Horizontal Control Network 1845 Triangulation stations first appeared on the map of Hong Kong produced by Lt. Collinson R.E. 1899/1900 Appeared on a map produced by Mr. Tate 1903/1904 Appeared on another map compiled by Mr. W.J. Newland

1845, 1899/1900, 1903/1904 Although triangulation stations are shown on these maps, there is no survey record found for the triangulation.

Horizontal Control Network 1928/29 A military map of scale 1/20,000 was produced from air photographs taken in 1924/25 by the R.A.F. with ground controls provided by the 2nd Colonial Survey Section R.E. These ground controls were adjusted by the Geographical Section in 1928-30 1946 The above-mentioned ground control were re-adjusted by the Crown Lands and Survey Office in 1946. It was adopted as the Main Triangulation of Hong Kong on which all surveys were based up to 1963.

Horizontal Control Network 1963 A re-triangulation was carried out because the network of that time could not meet the requirements of large scale mapping and cadastral surveys. Hong Kong (1963) Geodetic Datum Reference Ellipsoid : Clarke 1858 Datum Origin : Old Trig. “Zero” Hong Kong (1963) Grid System Map Projection : Cassini Projection Projection Origin : Old Trig. No.2

Horizontal Control Network 1975-77 Metrication policy in the 1970s Imperial grid was converted to metric unit of measure Grid origin was further shifted 3550 m to the west

Horizontal Control Network 1978-1979 Introduction of EDM Re-survey the distances between hilltop stations for improving the consistence and accuracy of the control network HK 1980 Geodetic Datum Reference Ellipsoid : International Hayford (1910) Hong Kong 1980 Grid System Map projection : Transverse Mercator Projection

Hong Kong 1980 Geodetic Datum Reference Ellipsoid : International Hayford (1910) Semi-major axis (a) = 6378388 m Flattening (f) = 1 / 297 Origin : Old Trig. “Zero” at the Observatory Latitude : 22° 18’ 12.82” N Longitude : 114° 10’ 18.75” E Azimuth Trig. 67.2 to Trig. 94 : 292° 59’ 46.5” Determined by astronomical Observations in 1960 by a team of visiting Geodesists who assessed the accuracy to be ±0.2”

Hong Kong 1980 Grid System Reference Ellipsoid : International Hayford (1910) Map Projection : Transverse Mercator Projection Origin : Old Trig. 2 Latitude : 22° 18’ 43.68” N Longitude : 114° 10’ 42.80” E Northing : 819069.80 m Easting : 836694.05 m Scale Factor Unity along central meridian at old Trig. 2

Vertical Control Network 1866 The surveyors of the surveying vessel “Rifleman” fixed a bench mark (Rifleman’s Bolt) for surveying the foreshore of Victoria Harbour. The highest point of the Bolt was taken as 17 feet 10 inches above the zero level which is now known as the Hong Kong Principal datum (HKPD). All heights and levels on land refer to this datum.

Vertical Control Network Rifleman’s Bolt Originally fixed on the wall of No.12 storehouse of Naval Dockyard in Admiralty Upon demolition of the Dockyard, the Bolt was relocated and refixed on the wall of Blake Block in H.M.S. Tamar Naval Base in Central Preserved for its historical value and not to be used as a bench mark In 1997, the British Forces donated the Rifleman’s Bolt to SMO

Original Location of Rifleman’s Bolt

Rifleman’s Bolt

Rifleman’s Bolt

GPS Control Network 1991 GPS Network 2000 GPS Network Satellite Positioning Reference Station Network

1991 GPS Network First Hong Kong GPS Network Joint Survey : Hong Kong, Macau and British Forces (512 Specialist Team Royal Engineers (STRE)) Hong Kong Network : 15 Stations 13 existing triangulation stations 4 satellite Doppler stations Absolute position of Doppler stations as origin Reference frame : WGS84 (STRE 91)

1991 GPS Network Purpose Problem of the 1991 GPS Network Link the local datum HK80 to WGS84 (STRE91) Enable GPS equipment to be used for surveying Problem of the 1991 GPS Network On the top of mountains Difficult to access

2000 GPS Network Densification of 1991 GPS Network on low ground near vehicular access reduce logistic cost

2000 GPS Network 46 stations Average distance between stations : 10km Average Observation Time of each GPS baseline : 3 hours No. of repeated baselines ≥ 3 Relative accuracy : ± 3mm ± 1 ppm

2000 GPS Network

Comparison of Internal Accuracy (1991 vs 2000) 1 ppm scale error in the 1991 GPS Network Reasons: Full constellation of NAVSTAR satellite was not available before 1994 GPS hardware and software are improving during this decade 2000 GPS Network is more accurate.

Reference Frame WGS 84 (STRE 91) ITRF Origin at the centre of Earth Doppler positioning measurement technique 1 to 2 meter absolute accuracy ITRF More accurate realization of the reference frame Use combination of space techniques GPS, VLBI, SLR, DORIS cm level absolute accuracy Nowadays : Broadcast and precise ephemeris are based on ITRF system.

Connection of 2000 GPS Network to ITRF96 2 Hong Kong Permanent GPS Reference Stations Fanling Kau Yi Chau 6 Global Stations of International GPS Services for Geodynamics (IGS) Cocos Islands (Indian Ocean) Guam (Pacific Ocean) Lhasa (Western China) Shanghai (Eastern China) Tsukuba (Japan) Yarragadee (Australia)

Connection of 2000 GPS Network to ITRF96 2 months continuous GPS observation data Baseline length : 1200 km to 5000 km Accuracy of the ITRF96 coordinates determined in this survey is better than 2 cm Hong Kong 2000 GPS Network shall be the GPS reference datum for all GPS surveys in Hong Kong

Station Summary of 2000 GPS Network Station

Position Difference between Reference Frames

Satellite Positioning Reference Station Network 2001 – Phase I 6 stations were constructed and operating. 2004 – Phase II Other 6 stations shall be established.

Satellite Positioning Reference Station Network Territory-wide array of permanent GPS reference stations Total 12 stations Station Separation : 10 to 15 km In most of the Hong Kong area, users can measure baselines from at least 2 permanent stations which are within a distance of 10 km. Relative Accuracy : ± 3 mm ± 0.2 ppm

Types of Geodetic Survey Control Stations Horizontal Control Stations Vertical Control Stations Satellite Positioning Reference Stations

Horizontal Control Stations Number of Stations in Hong kong Main / Minor Triangulation Station : 230 (Approx.) Main / Minor Control Traverse Station : 3500 (Approx.)

Horizontal Control Stations Type A Triangulation Monument Type B Triangulation Monument Picket Box (fixed in open ground) Picket Box with concrete platform Picket Box (fixed on rock surface) Urban Survey Mark

Type A Triangulation Monument

Vertical Control Stations Number of Bench Marks in Hong kong : 1500 (Approx.)

Vertical Control Stations Stainless Steel Rod Bench Mark Stainless Steel Staple Bedrock Bench Mark

Stainless Steel Rod Bench Mark

Bedrock Bench Mark

Design Drawing of Bedrock Bench Mark

Satellite Positioning Reference Stations Existing 6 Stations Siu Lang Shui (HKSL) Lam Tei (HKLT) Kam Tin (HKKT) Fanling (HKFN) Shatin (HKST) Stonecutters Island (HKSC)

Siu Lang Shui (HKSL)

Lam Tei (HKLT)

Kam Tin (HKKT)

Equipment Box

Equipment Box

Fanling (HKFN)

Equipment Cabinet of HKFN

Stonecutters Island (HKSC)

Accuracy Standards

Calibration Baselines and Network Steel Band Baselines Legislative Council Building, HK Hong Kong Science Museum, Kowloon North District Government Offices, NT Plover Cove EDM Calibration Baseline GPS Equipment Calibration Network at Ap Lei Chau

Plover Cove EDM Calibration Baseline

GPS Equipment Calibration Network at Ap Lei Chau Location : Hill-top near Lei Tung Estate, Ap Lei Chau Consist of 4 concrete pillars Trig. 252 Trig. 253 Trig. 254 Trig. 255

GPS Equipment Calibration Network at Ap Lei Chau

Break

Discussion on the Specifications and Practice Guide for Establishing GPS Control Station

Q & A

Thank you !!