This gist contains GeoJSON to illustrate center and centroid differences on a rectangular polygon

centroid-and-center-difference-on-a-rectangular-polygon

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {
        "stroke": "#555555",
        "stroke-width": 2,
        "stroke-opacity": 1,
        "fill": "#555555",
        "fill-opacity": 0.3
      },
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [
              -116.89764209999407,
              34.43966909255738
            ],
            [
              -113.38201709999477,
              34.43966909255738
            ],
            [
              -113.38201709999477,
              37.862840316787185
            ],
            [
              -116.89764209999407,
              37.862840316787185
            ],
            [
              -116.89764209999407,
              34.43966909255738
            ]
          ]
        ]
      }
    },
    {
      "type": "Feature",
      "properties": {
        "marker-color": "#ff0000",
        "marker-size": "small",
        "marker-symbol": "circle",
        "name": "centroid"
      },
      "geometry": {
        "type": "Point",
        "coordinates": [
          -115.49139209999434,
          35.8089375822493
        ]
      }
    },
    {
      "type": "Feature",
      "properties": {
        "marker-color": "#00ff00",
        "marker-size": "small",
        "marker-symbol": "star",
        "name": "center-of-request"
      },
      "geometry": {
        "type": "Point",
        "coordinates": [
          -115.13982959999441,
          36.15125470467228
        ]
      }
    },
    {
      "type": "Feature",
      "properties": {
        "name": "center-of-mass",
        "marker-color": "#0000ff",
        "marker-size": "large",
        "marker-symbol": "cross"
      },
      "geometry": {
        "type": "Point",
        "coordinates": [
          -115.13982959999441,
          36.15125470467228
        ]
      }
    }
  ]
}