Specification of latitude, longitude and altitude variables

[marack]

The site latitude, longitude and altitude variables don't currently specify the ellipsoid they reference.

I'm particularly worried about the altitude variable. The comment states that the value is 'above mean sea level'. In my experience the values for lat/lon/alt used in site metadata are extracted from GPS instrumentation, generally as WGS84 coordinates. There can be up to 100m difference between WGS84 and MSL heights (according to Wikipedia).

Questions:

1. Should we specify all lat/lon/alt values in the standard are in relation to a reference ellipsoid?
2. Should we nominate a fixed reference ellipsoid (WGS84) or allow the user to nominate one through metadata?
3. Should we change the description of altitude from 'above mean sea level' to 'above the reference ellipsoid'?

[kenkehoe]

The reference should be part of standard_name. altitude standard name "altitude" and is currently defined as "Altitude is the (geometric) height above the geoid, which is the reference geopotential surface. The geoid is similar to mean sea level." Is this correct enough?

[mike-dixon]

How about if we add a global attribute (or variable) that specifies the geoid?

[kenkehoe]

If we add an attribute, it should be at the variable level to be attached to the altitude variable (latitude and longitude if necessary) so it does not get separated.