FeatureCollection decoded into GeometryCollection

[evancloutier]

Hiya!

I started using this library to transform results from GDAL ogr2ogr CLI executions into structured GeoJSON.

I noticed that if a FeatureCollection is passed into Geo.JSON.decode!/1, it is returned as a %Geo.GeometryCollection{}.

As an example, consider the following GeoJSON:

%{
  "features" => [
    %{
      "geometry" => %{
        "coordinates" => [
          [
            [-76.34699604768392, 36.33170421172728],
            [-76.33589514587374, 36.332348509701035],
            [-76.33512736015777, 36.32399799501997],
            [-76.34610029768193, 36.32337940277699],
            [-76.34699604768392, 36.33170421172728]
          ]
        ],
        "type" => "Polygon"
      },
      "properties" => %{"id" => "567"},
      "type" => "Feature"
    },
    %{
      "geometry" => %{
        "coordinates" => [
          [
            [-76.33554324408723, 36.3323227378844],
            [-76.32210699405763, 36.33325051791261],
            [-76.3202515119107, 36.325080519630006],
            [-76.33487143158575, 36.32410109324975],
            [-76.33554324408723, 36.3323227378844]
          ]
        ],
        "type" => "Polygon"
      },
      "properties" => %{"id" => "456"},
      "type" => "Feature"
    },
    %{
      "geometry" => %{
        "coordinates" => [
          [
            [-76.36231977093198, 36.33703883835424],
            [-76.35771305663607, 36.337296540535085],
            [-76.3571372173491, 36.331420718930794],
            [-76.36174393164497, 36.331420718930794],
            [-76.36231977093198, 36.33703883835424]
          ]
        ],
        "type" => "Polygon"
      },
      "properties" => %{"id" => "234"},
      "type" => "Feature"
    },
    %{
      "geometry" => %{
        "coordinates" => [
          [
            [-76.31868394940722, 36.31925532901004],
            [-76.30972644938747, 36.32023481628918],
            [-76.30793494938354, 36.3127594703222],
            [-76.31702041368928, 36.31162521726675],
            [-76.31868394940722, 36.31925532901004]
          ]
        ],
        "type" => "Polygon"
      },
      "properties" => %{"id" => "123"},
      "type" => "Feature"
    }
  ],
  "name" => "farm_fields",
  "type" => "FeatureCollection"
}

Calling Geo.JSON.decode! on this data will result in a %Geo.GeometryCollection{} to be returned. The properties are preserved in the Geo structs – which is great – but won't be interpreted by other GeoJSON consumers (unless I'm misunderstanding here) because they're not technically part of the spec.

Is there a reason why there isn't first class FeatureCollection support in Geo?

[aseigo]

Based on reading the code, the JSON encode/decode was designed to move between geometry objects (e.g. GeometryCollection) and GeoJSON representations.

The map in your comment above is on its own valid GeoJSON and already is a FeatureCollection. See https://datatracker.ietf.org/doc/html/rfc7946#section-3.3

What is your use case with processing/handling this GeoJSON? That may help with understanding the motivation and needs behind yoru question.