Handheld GPS Receivers Map Datum Settings

For common handheld-type receivers, the format of the datum transformation parameters might be slightly differ with "7-parameters". For example, GARMIN receivers use the parameters as follow:

• Map Datum:
  
  DX = -156
  DY = -270
  DZ = -189
  DA = -251
  DF = -0.0000142

• User UTM Grid:
  
  Longitude Origin = E 114 deg 10.713 min
  Scale = 1.0000000
  False E (mt) = 836694.1
  False N (mt) = -1649325.9

Please refer to your receiver manual for how to set above parameters for your GPS receiver.

Suggested Procedures for GPS Height Transformation within a small local area

In GPS surveys, the surveyed coordinates are computed in the World Geodetic System 1984 (WGS84) coordinates which is a geocentric coordinates system, expressed in geodetic coordinates (i.e. latitude, longitude and ellipsoidal height) or Cartesian coordinates (i.e. X, Y and Z). In other words, the height values obtained from GPS surveys are in ellipsoidal height referenced to the surface of the WGS84 ellipsoid but not the orthometric height above the geoid surface.

However, the height values we use in daily survey works are in orthometric height above Hong Kong Principal Datum, HKPD. The following documents outline some suggested procedures on how to convert the GPS surveyed height values (in WGS84 Ellipsoidal Height) to the local height above HKPD within a small local area:-

Web-based Datum Transformation Tool

The Web-based Transformation Tool provide instant conversion between HK 1980 Grid Coordinates, HK 1980 Geodetic Coordinates, WGS84 Latitude and Longitude (ITRF96), UTM Grid Coordinate or UTM Grid Reference (ITRF96). The Transformation Tool uses the conversion methods, parameters and formulas listed in the "Explanatory Notes on Geodetic Datums in Hong Kong" (PDF) and the "Datum Transformation and Transformation Parameters" (The "7-parameters") (PDF). It can convert up to 10 point coordinates at one time. It is only applicable for coordinates within Hong Kong.

Note: Users SHOULD NOT use coordinates derived from datum transformation for applications requiring precise point positions. Transformation between datums does not improve the accuracy. In most cases, the transformation coordinates would be less accurate because of the errors in the transformation and projection computation would be added to the results. Please seek advice from professional land surveyors.

Historical Records of Geodetic Surveys in Hong Kong