add constants that define what the keys are, for portability

src/fallbacks.jl

@@ -95,8 +95,11 @@ issimple(t::AbstractMultiPointTrait, geom) = allunique((getgeom(t, geom)))
 issimple(t::AbstractMultiCurveTrait, geom) = all(issimple.(getgeom(t, geom)))
 isclosed(t::AbstractMultiCurveTrait, geom) = all(isclosed.(getgeom(t, geom)))
 
-crs(::Nothing, geom) = _get_dataapi_metadata(geom, "GEOINTERFACE:crs", nothing) # see `metadata.jl`
-crs(::AbstractTrait, geom) = _get_dataapi_metadata(geom, "GEOINTERFACE:crs", nothing) # see `metadata.jl`
+"The key used to retrieve and store the CRS from DataAPI.jl metadata, if no other solution exists in that format."
+const GEOINTERFACE_CRS_KEY = "GEOINTERFACE:crs"
+
+crs(::Nothing, geom) = _get_dataapi_metadata(geom, GEOINTERFACE_CRS_KEY, nothing) # see `metadata.jl`
+crs(::AbstractTrait, geom) = _get_dataapi_metadata(geom, GEOINTERFACE_CRS_KEY, nothing) # see `metadata.jl`
 
 # FeatureCollection
 getfeature(t::AbstractFeatureCollectionTrait, fc) = (getfeature(t, fc, i) for i in 1:nfeature(t, fc))

src/interface.jl

@@ -38,6 +38,11 @@ automatically delegate to this method.
 isfeaturecollection(x::T) where {T} = isfeaturecollection(T)
 isfeaturecollection(::Type{T}) where {T} = false
 
+"""
+The key used to retrieve and store the geometrycolumns from DataAPI.jl metadata, if no other solution exists in that format.
+"""
+const GEOINTERFACE_GEOMETRYCOLUMNS_KEY = "GEOINTERFACE:geometrycolumns"
+
 """
     GeoInterface.geometrycolumns(featurecollection) => (:geometry,)
 
@@ -47,7 +52,7 @@ which contain geometries that support GeoInterface.
 This is always a `Tuple` of `Symbol`s.
 """
 function geometrycolumns(featurecollection)
-    gcs = _get_dataapi_metadata(featurecollection, "GEOINTERFACE:geometrycolumns", (:geometry,)) # see `metadata.jl`
+    gcs = _get_dataapi_metadata(featurecollection, GEOINTERFACE_GEOMETRYCOLUMNS_KEY, (:geometry,)) # see `metadata.jl`
     return _aftercare_geometrycolumns(gcs)
 end