>This line of reasoning makes sense to me (and I don't see anyone
>really disagreeing with it?)
>
>Does this mean that we should recommend that each datum value that's
>defined provide a definition
...and IANA registration (right?)...
>for both a 2D (AT=0,2) and 3D representations (AT=1)? These could
>be the same thing, but it might be useful to note that sometimes a
>datum value implicitly refers to two things, distinguished by the AT value.
>
>--Richard
>
>
>Alexander Mayrhofer wrote:
>>>> Q: is altitude type 0 valid?
>>>It has to be, if the server can't figure out what the altitude of
>>>the client is, then the easiest way to to tell the client this is
>>>to use datum EPSG:4326 and altitude type of '0'.
>>>
>>>We could sidestep having 2 WGS-84 datums registered and simply
>>>have 4979 delivered with an altitude type of '0', meaning servers
>>>only ever have to remember 1 WGS-84 datum (4979) and if there is
>>>an altitude, include the type (and altitude value). If there is no
>>>altitude value, set the altitude type to 0, which the client
>>>converts to 4326 if it informs another entity its location.
>>I think it would be logically flawed to return 4979 without an altitude
>>- that's what 4326 is for. Plus, i feel it would also be incorrect to
>>use 4979 with Altitude Type 2 (floors), because that's not what 4979
>>uses as the value for the third dimension.
>>Therefore, i think allowing only the following makes sense:
>>- 4326 with AT 0 or 2 - 4979 with AT 1
>>Everything else does - strictly speaking - not "fit" the definitions of
>>the reference systems.
>>
>>>> Q: what meaning is attached to altitude type 0?
>>>IMO - that there is no altitude given (or guessed or whatever)
>>Agreed.
>>It's a different story what a consumer of such data may assume. For
>>example, what we use in the "geo" URI is that the client MAY assume that
>>this means "on earth's surface" (on ground, or on surface of water
>>bodies), which seems "natural" to what people would expect from
>>coordinate pair without altitude.
>>
>>>> Q: what is the impact of having no altitude on the existing datum
>>>> definitions?
>>>In WGS-84, the difference is merely either you're in 2D or 3D.
>>+1
>>
>>>We could extend that rule to being applicable to all datum pairs.
>>>For example, NAV83 (horizontal) paired with NAVD88 (vertical).
>>Seems logical, but - obviously - someone more familiar with those
>>reference systems than me would need to take a look at this :-)
>>Alex
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