d3 v4 geo draws boundary inverted

Question

When I draw the Bermuda Triangle in an SVG element the scale is not what I expect (triangle should extend to edges of box) and the fill is backward (instead of drawing a triangle, it draws a square with a triangle cut out).

var geojson = {
  "features": [
    {
      "type": "Feature",
      "properties": {
        "name": "Bermuda Triangle",
        "area": 1150180
      },
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [-64.73, 32.31],
            [-80.19, 25.76],
            [-66.09, 18.43],
            [-64.73, 32.31]
          ]
        ]
      }
    }
  ],
  "type":"FeatureCollection"
};

var width = 480;
var height = 480;

var svg = d3.select("body").append("svg")
  .attr("width", width)
  .attr("height", height)
  .attr("style", "border: 2px solid black");

var projection = d3.geoMercator().fitSize([width, height], geojson);
var path = d3.geoPath().projection(projection);

svg.selectAll('path')
  .data(geojson.features)
  .enter()
  .append('path')
  .attr('d', path)
  .style("fill", "red")
  .style("stroke-width", "1")
  .style("stroke", "black");

<script src="//d3js.org/d3.v4.js"></script>

What am I doing wrong?

Answer 1

Let's change this:

[
    [-64.73, 32.31],
    [-80.19, 25.76],
    [-66.09, 18.43],
    [-64.73, 32.31]
]

To this:

[
    [-64.73, 32.31],
    [-66.09, 18.43],
    [-80.19, 25.76],
    [-64.73, 32.31]
]

It seems like a small change, but it is an important one: D3 expects the polygon vertices in a clockwise order.

According to the API:

Spherical polygons also require a winding order convention to determine which side of the polygon is the inside: the exterior ring for polygons smaller than a hemisphere must be clockwise, while the exterior ring for polygons larger than a hemisphere must be anticlockwise. (emphasis mine)

Also, this is an interesting bl.ocks made by Bostock (D3 creator), didactically explaining your issue: https://bl.ocks.org/mbostock/a7bdfeb041e850799a8d3dce4d8c50c8

Here is your code with that change (and removing the fitSize):

var geojson = {
  "features": [{
    "type": "Feature",
    "properties": {
      "name": "Bermuda Triangle",
      "area": 1150180
    },
    "geometry": {
      "type": "Polygon",
      "coordinates": [
        [
          [-64.73, 32.31],
          [-66.09, 18.43],
          [-80.19, 25.76],
          [-64.73, 32.31]
        ]
      ]
    }
  }],
  "type": "FeatureCollection"
};

var width = 480;
var height = 480;

var svg = d3.select("body").append("svg")
  .attr("width", width)
  .attr("height", height)
  .attr("style", "border: 2px solid black");

var projection = d3.geoMercator();
var path = d3.geoPath().projection(projection);

svg.selectAll('path')
  .data(geojson.features)
  .enter()
  .append('path')
  .attr('d', path)
  .style("fill", "red")
  .style("stroke-width", "1")
  .style("stroke", "black");

<script src="//d3js.org/d3.v4.js"></script>

Answer 2

If someone will see a similar problem, I created a tool which will help you to rewind or reverse geojson

It helped me to make changes in some geoJson files, without much hassle

https://observablehq.com/@bumbeishvili/rewind-geojson

You can run it as a snippet just bellow

<div class="notebook-content">
  
</div>

<script type="module"> 

import notebook from "https://api.observablehq.com/@bumbeishvili/rewind-geojson.js";  //  "download code" url

document.querySelector('.notebook-content').innerHTML =notebook.modules[0].variables
.filter(d=>d)
.map((d,i)=>` <div class=" observable-wrapper div-number-${i}"></div>`)
.join('')
.concat('<div style="display:none" class="hidden"></div>')


import {Inspector, Runtime} from "https://unpkg.com/@observablehq/runtime@3/dist/runtime.js"; 
let i=1;
Runtime.load(notebook, (variable) => { 
if(i==4 ){i++;  return new Inspector(document.querySelector(`.hidden`));}
if(i==13)return;
return new Inspector(document.querySelector(`.observable-wrapper:nth-child(${i++})`));
 }); 


</script>