Terminology doc bugs

[c42f]

According to https://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf (an excellent read), the word datum should be identified with a TRS rather than a TRF (is this standard? need to read something definitive on the matter). The terminology docs probably need more work anyway.

Perhaps this also means the abstract type Datum should be ReferenceFrame instead since positions are always relative to an actual realization. A pity since it's longer, though possibly less jargon-y.

[c42f]

Uh oh. The IERS says a datum is a "geodetic reference frame"

ftp://tai.bipm.org/iers/conv2010/tn36_gl.pdf

So maybe the technical community doesn't really agree, still needs further digging.