geojson -> topojson : no output

Question

Trying to convert my geoJSON file into topoJSON so

• I downloaded topojson from npm and
• followed the instructions on gis.stackexchange Convert GeoJSON to TopoJSON
• basically the same as the topoJSON GitHub page

However, I got an error message:

> topojson -o prTopo.json prCounties.json
quantization: bounds NaN NaN NaN NaN (spherical)
quantization: maximum error 0.00m (0.00°)
prune: retained 0 / 0 arcs (NaN%)

The original output I have is all the municipalities of Puerto Rico which I collected on GitHub (Municipios). Here is an abridged version:

{"type": "FeatureCollection", 
"properties": {"kind": "state", "state": "PR"}, 
"features": [[
    {"geometry": { 
        "type": "MultiPolygon", 
        "coordinates": [[[[-65.4406, 18.1541], [-65.2763, 18.1322], [-65.4515, 18.0884], [-65.5775, 18.1048]]]]}, 
        "type": "Feature", "properties": {"kind": "county", "name": "Vieques", "state": "PR"}}]]
}

The output I got was

{"type":"Topology",
"transform":{"scale":[null,null],"translate":[null,null]},
"objects":{"prCounties":{"type":null,"properties":{"kind":"state","state":"PR"}}},
"arcs":[]}

I don't really understand the geoJSON or topoJSON specs, but it looks like the transform data comes out null and the arc data is null is empty.

Answer 1

Your input GeoJSON is invalid. Your feature collection has a doubly-nested features array:

{
  "type": "FeatureCollection",
  "properties": {
    "kind": "state",
    "state": "PR"
  },
  "features": [
    [
      {
        "type": "Feature",
        "properties": {
          "kind": "county",
          "name": "Vieques",
          "state": "PR"
        },
        "geometry": {
          "type": "MultiPolygon",
          "coordinates": [[[[-65.4406,18.1541],[-65.2763,18.1322],[-65.4515,18.0884],[-65.5775,18.1048]]]]
        }
      }
    ]
  ]
}

What you need is a singly-nested features array:

{
  "type": "FeatureCollection",
  "properties": {
    "kind": "state",
    "state": "PR"
  },
  "features": [
    {
      "type": "Feature",
      "properties": {
        "kind": "county",
        "name": "Vieques",
        "state": "PR"
      },
      "geometry": {
        "type": "MultiPolygon",
        "coordinates": [[[[-65.4406,18.1541],[-65.2763,18.1322],[-65.4515,18.0884],[-65.5775,18.1048]]]]
      }
    }
  ]
}

Which produces the following valid TopoJSON with default settings:

{
  "type": "Topology",
  "transform": {
    "scale": [0.000030123012301229557,0.000006570657065706536],
    "translate": [-65.5775,18.0884]
  },
  "objects": {
    "test": {
      "type": "GeometryCollection",
      "properties": {
        "kind": "state",
        "state": "PR"
      },
      "geometries": [{"type":"MultiPolygon","arcs":[[[0]]]}]
    }
  },
  "arcs": [[[4545,9999],[5454,-3333],[-5816,-6666],[362,9999]]]
}