> All, we've received privately feedback about a potential error in RFC
> 5870. Could someone who is more familiar with reference systems
> review Jose's email, and advice whether we'd need to add errata to
> the RFC?
[…]
> When using WGS84, the altitude is set in relation to the reference
> ELLIPSOID (which is the GRS80 ellipsoid), but not to the GEOID. You
> can see this, for example, in the definition of the CRS with code
> EPSG 4979. In other words, any GPS navigator can offer you the
> "ellipsoid height " over the GRS80 elllipsoid (there are many
> ellipsoids), but no any height referred to the geoid (there is only
> one geoid). This two figures of the earth are completely different.
[…]
Not that I have much expertise in this field, but…
--cut: http://en.wikipedia.org/wiki/World_Geodetic_System --
[…] It comprises a standard coordinate frame for the Earth, a
standard spheroidal reference surface (the datum or reference
ellipsoid) for raw altitude data, and a gravitational equipotential
surface (the geoid) that defines the nominal sea level.
--cut: http://en.wikipedia.org/wiki/World_Geodetic_System --
Thus, WGS-84 defines both a reference ellipsoid and a geoid,
which gives the nominal sea level.
--cut: http://en.wikipedia.org/wiki/Altitude --
[…] As a general definition, altitude is a distance measurement,
usually in the vertical or "up" direction, between a reference datum
and a point or object. The reference datum also often varies
according to the context. Although the term altitude is commonly
used to mean the height above sea level of a location, in geography
the term elevation is often preferred for this usage.
--cut: http://en.wikipedia.org/wiki/Altitude --
Thus, I guess that the term "altitude" may refer either to the
distance above the reference ellipsoid, or to the distance above
the geoid.
Therefore, I find it believable that there may be CRS that rely
on one or another definition of altitude, and I find believable
that there may be equipment that reports the altitude
corresponding to one or another definition of altitude.
So, I believe that no change is necessary to the specification
regarding this issue. However, a note of warning for
implementors to be cautious about the exact nature of the data
to be written in the geo: URI may be desirable.
--
FSF associate member #7257
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