Implement CRSmode so we can dispatch on the type of crs.

[evetion]

Partially solves #30.

As a follow up to JuliaGeo/GeoInterface.jl#140. With this PR we can implement crstrait for GFT types, which can then be set and used across the ecosystem when reading crs information.

[asinghvi17]

AFAICT, this won't work for the CoordinateReferenceSystemFormat types?

Maybe we need either a wrapper as indicated in the issue, or an extra type param in the type definition for those

[rafaqz]

Yeah, I guess they will all need a type parameter.

[rafaqz]

Hmm @asinghvi17 your comment in the issue makes a good point that wrapping the other way is simpler, instead of adding heaps of type parameters to every object here we just compose the objects here as-is with other singletons.

[evetion]

Can you show a MWE for a wrapper? I'd like to adjust the existing types to use them as they're implemented across the ecosystem.

[asinghvi17]

I was thinking of something on the lines of:

struct CRS{T, CRSTrait} <: CoordinateReferenceSystemFormat
	val::T 
	CRS(x::CoordinateReferenceSystemFormat, ::CRSTrait) where CRSTrait = CRS{typeof(x), CRSTrait}(x)
end

CRS(x::CoordinateReferenceSystemFormat) = CRS(x, GI.crstrait(x))

[asinghvi17]

then, the GI.crs of a geometry might be CRS{EPSG, GeographicCRSTrait}(EPSG((4326,)))

[asinghvi17]

That MWE probably won't work as is, but you get the idea

[rafaqz]

I'm not totally clear on where these will be set to something besides Unknown.

Is the idea that somewhere some package calls isgeographic from ArchGDAL or Proj when a file containing some crs is read?

Like is this for Rasters/ArchGDAL/GeoDataFrames to set? Shapefile/GeoJSON etc wont have that option. GeometryOps.jl only has Proj in an extension so it cant be default behaviour to add this.

Probably also Proj and ArchGDAL should set this too whenever they return a GeoFormat crs? It could be good to write those small PRs to see if this actually makes sense.

[asinghvi17]

I guess we could also use the GeoInterface crstrait stuff directly here?

[evetion]

Sadly, GeoInterface depends on GFT, so we can't, but they are indeed 1 on 1 linked.

Producers should set these formats directly, so AG, Proj, Rasters and GeoDataFrames. GeometryOps and GeoMakie could consume and choose correct algorithms.