Most know this, but it can't hurt to point out some distinctions.
When I say "semantic(s)" I mean "what we wish to convey." Or,
"higher-level meaning." When I say "syntax" I mean "how we say it;
the grammar and characters we type to utter it in a well-formed way."
Here, "it" is a particular semantic thing to convey.
So, semantic structures include:
"various ways humans use land" and
"groupings of vegetative or rocky/sandy/muddy/watery land coverings" and
"category-names that political bodies give to areas of land for
human, plant or animal conservation"
Syntactic structures include:
The : as a way for the more general tag "park" (not used as an OSM
key to the best of my knowledge) to be extended into park:type as the
name of a key holding values like county_park or state_wilderness.
Grouping, in general. So, landuse=[farm, forest, meadow,
industrial...] is a syntactic structure for putting RELATED "things"
into a container that holds them, BECAUSE they are related.
Numbering, but with some careful distinctions: integers used as
values in a key-value pair are syntax, AND have semantic meaning. An
example: 2, 4, 6 and 8 are admin_level values to mean nation, state,
county and city. The numbers in the key-value pair are syntax, but
what they mean are semantic.
Also helpful to keep in mind is the concept of a rendering toolchain
that starts with a semantic idea in the mind of a human ("I want to
see Acme Park on the map"), this "falls into" one (and should be only
one) semantic bucket of "means this and only this," this gets turned
into a syntactic sentence (grammatically correct in OSM-speak), like
"landuse=forest + name=Acme Park" as tags on a simply polygon, this
utterance goes through some behind-the-scenes server magic (like how
osm2pgsql allows OSM -> PostgreSQL -> mapnik) and eventually Acme
appears as "named, colored park on rendered map."
Item in real world -> sensible semantic object -> syntactic OSM
utterance -> toolchain -> map rendering.
Again, I know it may be obvious, but we are talking about many things
on many levels here. This is a useful hierarchy/nomenclature, and
these paths really do exist. Let's try to keep them straight.
SteveA
California
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