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For x/y or lat/lon or heights between different ellipsoid, we can do conversion by using proj.4. But I found that if I want to convert between WGS84 height and local height, I found that it is not so simple.

For example, I try to convert height of EPSG:5738 which is Hong Kong Principal Datum from WGS84 height. As it proj.4 file is blank, I cannot do it.

From the official description of HKPD (http://www.geodetic.gov.hk/smo/gsi/Data/pdf/explanatorynotes.pdf), it just shows the rough relationship.

Does it mean I have no luck with doing it in proj.4 or other tools?

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By "local height", do you mean "height above the ellipsoid"? –  BradHards Feb 23 '13 at 6:45
This might be of help to you: lists.maptools.org/pipermail/proj/2011-August/005844.html –  Devdatta Tengshe Feb 23 '13 at 6:47
I don't mean the height above the ellipsoid. It is the local-defined height. –  Harold Chan Feb 23 '13 at 11:32
Thanks for @DevdattaTengshe providing the link, unfortunately, I can't find HKPD gtx file. –  Harold Chan Feb 23 '13 at 15:08
Harold: you want height above a local datum, but you don't have the datum characteristics? So the problem isn't about tools, its about appropriate data? –  BradHards Feb 23 '13 at 21:17

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