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Is there some specific constraint regarding vertical and geodetic datums within a compound Coordinate Reference System (CRS)? E.g. if the ellipsoid model should be the same or not, or if the vertical datum can be barometric, etc.

As also engineering CRSs can be compound with geodetic 2D CRSs, then I don't believe there are such sort of constraints.

Thanks for any comment.

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This question is probably best asked on the MetaCRS mailing list, which might give a more practical answer based on community experience. In short, I do believe that the vertical and horizontal datums of a compound CRS need not be the same, though in practices this isn't such a great idea. It's more a matter of what software can consume, and not so many softwares are so great at consuming compound CRSs anyway right now.

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Thank you Howard, I was not aware of the MetaCRS project: I'm joining right now. – Campa May 21 '13 at 8:47

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