Make Sea Level an individual. Make other consequent changes to Sea Level, Altitude, and Ratio Measurement Information Content Entity

[michaelrabenberg]

Apropos of #883 ...

1. I suggest making https://www.commoncoreontologies.org/ont00000722 (Sea Level) an individual.

Relatedly:

2. I suggest replacing Sea Level's skos:definition with the following: "The Fiat Surface that divides the spheroid composed of Earth and its atmosphere and that occupies the two-dimensional spatial region that corresponds to the mean level of calm water in the Earth’s oceans."

3. The skos:definition of https://www.commoncoreontologies.org/ont00000787 (Altitude) is the following: "A Height that inheres in a Site that externally connects an Independent Continuant to either the surface of the Earth or the Earth's mean Sea Level." In light of how Sea Level is defined, I suggest replacing this skos:definition with the following: "A Height that inheres in a Site that externally connects an Independent Continuant to either the surface of the Earth or Sea Level."

4. One skos:example in https://www.commoncoreontologies.org/ont00001022 (Ratio Measurement Information Content Entity) is "a measurement of the barometric pressure at 1,000 feet above sea level." I suggest capitalizing 'sea level' in this example.

[alanruttenberg]

Altitude should be renamed "Altitude above sea level", since altitude alone is ambigious to (at least) whether the height is measured from the geoid, sea level, the surface of the earth, or the ellipsoid. Or remove sea level from the definition and leave something fuzzy (not sure what - maybe "surface or proxy for surface of Earth".). I'd rather the latter, since that would move what surface is being used over to the measurement (ICE) side.

Also, why does it inhere in a site rather than being a relational quality between an independent continuant and Earth, which seems to me clearer than having to invent a site.