Automatic conversion between geometry types

[evetion]

On Slack, @jaakkor2 provided this:

import GeoJSON, LibGEOS
a = """{"type": "FeatureCollection", "features": [
    {"type": "Feature", "geometry": {"type": "Polygon", "coordinates": [[[0.0, 0.0], [1.0, 0.0], [0.0, 1.0], [0.0, 0.0]]]}}
    ]}"""
poly1 = GeoJSON.read(a)[1].geometry
LibGEOS.area(poly1)    # fails

Note the assumption that geometries "just work" without conversion. Is that something we could do better? Means mostly that we don't just provide a convert, but also to automatically take the correct type to convert into. A GeoJSON.Polygon becomes a LibGEOS.Polygon automatically.

[rafaqz]

It would be nice if that did just work... we need to make sure convert works everywhere I guess. And LibGeos.area either needs to be written in a way that triggers convert, or calls it.

[evetion]

Note that there's actually also GeoInterface.area

[rafaqz]

But I guess if you are trying to run area specifically with LibGeos you will need to use LigGeos.area?

The GeoInterface.jl method will try to use the package matching the object type? I'm not actually sure how and where these implementations are. How do we deal with dispatch?

[yeesian]

I think the (burden of the) work will be in the individual packages to implement their methods for GeoInterface types/traits.

[rafaqz]

Note that I've implemented this for ArchGDAL and LibGEOS, it's just waiting on the wrapper types PR for testing.
JuliaGeo/LibGEOS.jl#156
yeesian/ArchGDAL.jl#366

It's surprisingly easy to implement.

[rafaqz]

The LibGEOS example works now