FAQ on Geodetic Datum

Q1 How to convert the World Geodetic System 84 (WGS84) coordinates into the HK 1980 Grid coordinates?

A1 This can be found in the SMO's publication "Explanatory Notes on Hong Kong Geodetic Datum" (PDF), which contain the transformation procedures and formula. To use the "7-parameter" datum conversion, please also refer to the "Datum Transformation and Transformation Parameters" (PDF).

Q2 How to convert the WGS84 coordinates into the Universal Transverse Mercator Grid (UTM Grid) coordinates?

A2 This can be found in the SMO's publication "Explanatory Notes on Hong Kong Geodetic Datum" (PDF), which contain the transformation procedures and formula.

Q3 What is Hong Kong Principal Datum (HKPD)?

A3 In Hong Kong, all heights and levels on land refer to the Hong Kong Principal Datum (HKPD). The HKPD was originated from a copper bolt fixed in Hong Kong Naval Dockyard in the 19th century. The level of the bolt (known as 'Rifleman's Bolt') was determined by the H.M. Surveying Vessel "Rifleman" in 1866 as 5.435m (17'10") above HKPD. The relationship between the HKPD and Mean Sea Level of Hong Kong during the years from 1965 to 1983 was determined by the Royal Observatory with reference to the above 19 years of observation records of the Automatic Tide Gauge situated at North Point, Victoria Harbour. It was found that the Mean Sea Level of the above period is approximately 1.23m above HKPD.

Q4 What is the map projection method applied in the Hong Kong 1980 Grid?

A4 The HK1980 Grid is a local rectangular grid system based on Transverse Mercator projection. Details of the corresponding map projection parameters can be found in the SMO's publication "Explanatory Notes on Hong Kong Geodetic Datum" (PDF).

Q5 Why would there be a difference between the heights fixed by GPS and the height values in HKPD?

A5 The heights fixed by GPS are the ellipsoidal heights with reference to the WGS84 Ellipsoid that does not exactly fit the surface of the geoid (the Earth) covering Hong Kong.

Q6 What are the relationship between the True North, Grid North (HK1980) and the Magnetic North?

A6 True North is the direction of a meridian of longitude which converges on the North Pole.

Grid North is the direction of a grid line which is parallel to the central meridian on a map. It deviates from the True North due to the map projection effect of plotting objects on ground to a map. The Hong Kong 1980 Grid system uses the Transverse Mercator projection. The deviation of Grid North and True North is zero along the central meridian, while the magnitude of the deviation is within the range of + / - 6 minutes (approx.) elsewhere in Hong Kong.

Magnetic North is the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate and currently is west of Grid North in Hong Kong., in a magnitude of 1-3 degrees.