Able to write `LineString` and `Polygon` geometry,

[ErickChacon]

I started a PR in GeoTables (JuliaEarth/GeoTables.jl#15) to be able to write Meshes.jl geometries to .shp using Shapefile.jl. However, we have problems when trying to write non-multi geometries like (LineString and Polygon).

ERROR: KeyError: key GeoInterface.LineStringTrait not found
Stacktrace:
 [1] getindex
   @ ./dict.jl:498 [inlined]
 [2] write(path::String, o::Shapefile.Writer; force::Bool)
   @ Shapefile ~/.julia/packages/Shapefile/TRwfU/src/writer.jl:140
 [3] #write#36
   @ ~/.julia/packages/Shapefile/TRwfU/src/writer.jl:266 [inlined]
 [4] save(fname::String, geotable::GeoTables.GeoTable{GeoJSON.FeatureCollection{2, Float64}}; kwargs::Base.Pairs{Symbol, Bool, Tuple{Symbol}, NamedTuple{(:force,), Tuple{Bool}}})
   @ GeoTables ~/documents/repositories/GeoTables.jl/src/GeoTables.jl:64
 [5] top-level scope
   @ REPL[1965]:1



ERROR: KeyError: key GeoInterface.PolygonTrait not found
Stacktrace:
 [1] getindex
   @ ./dict.jl:498 [inlined]
 [2] write(path::String, o::Shapefile.Writer; force::Bool)
   @ Shapefile ~/.julia/packages/Shapefile/TRwfU/src/writer.jl:140
 [3] #write#36
   @ ~/.julia/packages/Shapefile/TRwfU/src/writer.jl:266 [inlined]
 [4] save(fname::String, geotable::GeoTables.GeoTable{GeoJSON.FeatureCollection{2, Float64}}; kwargs::Base.Pairs{Symbol, Bool, Tuple{Symbol}, NamedTuple{(:force,), Tuple{Bool}}})
   @ GeoTables ~/documents/repositories/GeoTables.jl/src/GeoTables.jl:64
 [5] top-level scope
   @ REPL[1960]:1

We can solve this by transforming the StringLine and Polygon to MultiStringLine and MultiPolygon; however this will required to do by every package that wants to use Shapefile.jl to write to disk. Can that issue be solved here, so other packages using GeoInterface.jl integration can also benefit from it?

[rafaqz]

The reason is that the shapefile format doesn't have Polygon or LinearRing, so there is no direct mapping.
Just something they call a polygon that's actually a multipolygon, with holes mixed randomly with exteriors. Its wonderful.

Either you can just wrap in MultiPolygon before writing, or you can PR here so the writer accepts polygons and LinearRing and writes them as MultiPolygonTrait.

GeoInterface.getring will give what you need to write for both Polygon and MultiPolygon, but you may need to special case LinearRing as it is itself the ring you need to write.

Edit: some of that will already work internally. Its just where look for types in TRAITSHAPE that the problem occurs. It should be too much work to have this working.

Just note that it will still always return a MultiPolygon when you read it again no matter what we do.

[ErickChacon]

@rafaqz Thank you for your explanation. I will see if I can help here, the idea would be to convert to Polyline and MultiPolygon before writing.

With respect to reading no problem with obtaining MultiPolygon.

[rafaqz]

GeoInterface has wrapper types for things like this now - MultiPolygon will wrap a vector of any PolygonTrait object, for example.

[rafaqz]

I it would be good to fix this here rather than special casing packages.

[ErickChacon]

I it would be good to fix this here rather than special casing packages.

Yes, I am already working on this. I have this working now, I just need to check test are passing.