Distortion Comparison Example

This example visualizes the difference in projected area of countries using two different map projections. Each country is abstracted to a circle, sized by the area of that country under a particular map projection. Country circles are positioned based on the centroid position of the country under a primary projection. A second set of circles are then overlaid, with projected areas based on a secondary projection. This example demonstrates Vega’s geoArea and geoCentroid expression language functions.

Vega JSON Specification <>

{
  "$schema": "https://vega.github.io/schema/vega/v3.0.json",
  "width": 900,
  "height": 500,
  "autosize": "none",

  "signals": [
    {
      "name": "baseProjection",
      "value": "azimuthalEqualArea",
      "bind": {
        "input": "select",
        "options": [
          "albers",
          "albersUsa",
          "azimuthalEqualArea",
          "azimuthalEquidistant",
          "conicConformal",
          "conicEqualArea",
          "conicEquidistant",
          "equirectangular",
          "gnomonic",
          "mercator",
          "orthographic",
          "stereographic",
          "transverseMercator"
        ]
      }
    },
    {
      "name": "altProjection",
      "value": "mercator",
      "bind": {
        "input": "select",
        "options": [
          "albers",
          "albersUsa",
          "azimuthalEqualArea",
          "azimuthalEquidistant",
          "conicConformal",
          "conicEqualArea",
          "conicEquidistant",
          "equirectangular",
          "gnomonic",
          "mercator",
          "orthographic",
          "stereographic",
          "transverseMercator"
        ]
      }
    },
    {
      "name": "baseColor",
      "value": "#bb8800",
      "bind": {"input": "color"}
    },
    {
      "name": "altColor",
      "value": "#0088bb",
      "bind": {"input": "color"}
    },
    {
      "name": "opacity",
      "value": 0.15,
      "bind": {"input": "range", "min": 0, "max": 1, "step": 0.05}
    },
    {
      "name": "scaleFactor",
      "value": 1,
      "bind": {"input": "range", "min": 0.05, "max": 2, "step": 0.05}
    }
  ],

  "projections": [
    {
      "name": "projection1",
      "type": {"signal": "baseProjection"},
      "scale": 150,
      "rotate": [0, 0, 0],
      "center": [0, 0],
      "translate": [
        {"signal": "width / 2"},
        {"signal": "height / 2"}
      ]
    },
    {
      "name": "projection2",
      "type": {"signal": "altProjection"},
      "scale": 150,
      "rotate": [0, 0, 0],
      "center": [0, 0],
      "translate": [
        {"signal": "width / 2"},
        {"signal": "height / 2"}
      ]
    }
  ],

  "data": [
    {
      "name": "world",
      "url": "data/world-110m.json",
      "format": {
        "type": "topojson",
        "feature": "countries"
      },
      "transform": [
        {
          "type": "formula",
          "expr": "geoCentroid('projection1', datum)",
          "as": "centroid"
        },
        {
          "type": "formula",
          "expr": "geoArea('projection1', datum)",
          "as": "area1"
        },
        {
          "type": "formula",
          "expr": "geoArea('projection2', datum)",
          "as": "area2"
        }
      ]
    },
    {
      "name": "graticule",
      "transform": [
        { "type": "graticule" }
      ]
    }
  ],

  "marks": [
    {
      "type": "shape",
      "from": {"data": "graticule"},
      "encode": {
        "update": {
          "strokeWidth": {"value": 1},
          "stroke": {"value": "#ddd"},
          "fill": {"value": null}
        }
      },
      "transform": [
        { "type": "geoshape", "projection": "projection1" }
      ]
    },
    {
      "type": "symbol",
      "from": {"data": "world"},
      "encode": {
        "update": {
          "strokeWidth": {"value": 1},
          "stroke": {"value": "#bbb"},
          "fill": {"signal": "altColor"},
          "fillOpacity": {"signal": "opacity"},
          "zindex": {"value": 0},
          "x": {"field": "centroid[0]"},
          "y": {"field": "centroid[1]"},
          "size": {"field": "area2", "mult": {"signal": "scaleFactor"}}
        },
        "hover": {
          "strokeWidth": {"value": 2},
          "stroke": {"value": "firebrick"},
          "zindex": {"value": 1}
        }
      }
    },
    {
      "type": "symbol",
      "from": {"data": "world"},
      "encode": {
        "update": {
          "strokeWidth": {"value": 1},
          "stroke": {"value": "#bbb"},
          "fill": {"signal": "baseColor"},
          "fillOpacity": {"signal": "opacity"},
          "zindex": {"value": 0},
          "x": {"field": "centroid[0]"},
          "y": {"field": "centroid[1]"},
          "size": {"field": "area1", "mult": {"signal": "scaleFactor"}}
        },
        "hover": {
          "strokeWidth": {"value": 2},
          "stroke": {"value": "firebrick"},
          "zindex": {"value": 1}
        }
      }
    }
  ]
}