Jonathan,
Quite right. Even longitude and latitude depend on the choice of datum, whether that be an ellipsoid or a geoid.
To add to the fun, you can have mixed mode coordinates, where the horizontal coordinates use one datum, and the vertical uses another (an extreme example is instantaneous water surface, where the vertical datum has no fixed relationship to the body of the Earth at all!)
Grace and peace,
Jim
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On Mar 17, 2014, at 12:39 PM, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
> Dear Jim
>
> Yes, I suppose you're right, on reflection. I was thinking of a situation in
> which you were interested in the ellipsoid only for the sake of a vertical
> datum, rather than as a horizontal datum (which is its usual purpose in
> grid_mapping). However, I now understand from the discussion and explanations
> from Jon Blower why you cannot actually refer heights to a vertical datum
> without implying a particular latitude-longitude coordinate system. So the
> existing grid_mapping_name of latitude_longitude will be appropriate if you
> want to identify the geoid or ellipsoid as a vertical datum, I agree.
>
> My understanding is now that the height is taken perpendicular to the surface,
> and the lat-lon is defined on the surface, so the lat-lon of the point whose
> height is being measured depends on the choice of surface. Please correct me
> if that's not right.
>
> Cheers
>
> Jonathan
>
>> I thought this was already defined. In the second paragraph of section 5.6, it says that if you aren?t specifying a projected coordinate system (or, I assume, a Cartesian coordinate system such as ECF), then use the name ?latitude_longitude?. I haven?t noticed anything we?ve talked about that would invalidate this usage. We are talking about adding vertical datum specifications and such as further attributes to the variable, but even latitude and longitude values can shift depending on the ellipsoid and/or geoid being used, so these should specified even when there is no projected coordinate system.
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Received on Mon Mar 17 2014 - 10:47:28 GMT
