Geodesic Datum's

25.10.2004

Submission:

I asked my map people to give me the datum of my maps. They came back with a reference point in "HK 80" with reference to the "International 1910 (Hayford)" ellipsoid and a formula to convert to and from WGS84 format. They explained that the map was made with reference to"HK80" and all my data entries should be in "HK80" format. As all my navigational data is in WGS84 format, shall I convert the reference point into WGS84 before entering data in WGS84 format? Alternately, shall I do it the other way round as suggested by the map people i.e. enter the map reference point in HK80 format and convert all the WGS84 data into HK80 format before data entry? Please advise.

Mr. R. Li Procedures Specialist Hong Kong

Answer or Commentary:

(I:W: This is a common problem that is affecting most procedure specialists but unfortunately many do not realise it. Previously this datum problem was a pilot navigation one. This was solved by the introduction of WGS84 but in solving the problem for navigation it created one for the procedures specialist. Procedure specialists usually have the situation where our nav. aids and aeronautical information is in WGS84 while our topographical maps are still in our old local datum. When we design a procedure it must be done in a single datum not a mix of datum's. So there are two choices either to convert the topographical map to WGS84 or convert the aeronautical information to the local datum. Usually the second way is the easiest to do and sometimes if you are lucky you will still have the aeronautical position information (Lat./Long.) in the old local datum. Once you have everything in one datum you can design an IFR procedure. Once completed any new position information (fixes, FAF, etc.) will have to be converted back to WGS84 and the new bearings must be recalculated.)