Web-based Datum Transformation Tool
The Web-based Transformation Tool provides an instant conversion between various coordinate systems / datums. 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) as well as the Geoid Model established by the Hong Kong Polytechnic University. 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 the coordinates derived from the Transformation Tool for applications requiring precise point positions. Transformation between coordinate systems / datums does not improve the accuracy. In most cases, the transformed coordinates would be less accurate because the errors in the transformation and projection computation would be added to the results. Please seek advice from professional land surveyors if necessary.
Coordinates Transformation API 
This service is used to perform coordinate transformation through the Application Programming Interface (API). For details, please refer to user manual.