give a sensible location to those that have no location set, and put a meaningful image to it

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https://gist.github.com/tlhunter/0ea604b77775b3e7d7d25ea0f70a23eb

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use a different geojson group for those that have guessed locations: https://gist.github.com/geog4046instructor/80ee78db60862ede74eacba220809b64

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Basically,

for (node in nodeswithoutlocation) {
  node.location = averageGeolocation(node.links.map(l => nodeposition[l]);
  node.location.estimated = true;
}

where averageLocation is

function averageGeolocation(coords) {
  if (coords.length === 1) {
    return coords[0];
  }

  let x = 0.0;
  let y = 0.0;
  let z = 0.0;

  for (let coord of coords) {
    let latitude = coord.latitude * Math.PI / 180;
    let longitude = coord.longitude * Math.PI / 180;

    x += Math.cos(latitude) * Math.cos(longitude);
    y += Math.cos(latitude) * Math.sin(longitude);
    z += Math.sin(latitude);
  }

  let total = coords.length;

  x = x / total;
  y = y / total;
  z = z / total;

  let centralLongitude = Math.atan2(y, x);
  let centralSquareRoot = Math.sqrt(x * x + y * y);
  let centralLatitude = Math.atan2(z, centralSquareRoot);

  return {
    latitude: centralLatitude * 180 / Math.PI,
    longitude: centralLongitude * 180 / Math.PI
  };
}

and then

function estimateNodeIcon(feature, latlng) {
  let myIcon = L.icon({
    iconUrl: 'my-icon.png',
    shadowUrl: 'my-icon.png',
    iconSize:     [25, 25], // width and height of the image in pixels
    shadowSize:   [35, 20], // width, height of optional shadow image
    iconAnchor:   [12, 12], // point of the icon which will correspond to marker's location
    shadowAnchor: [12, 6],  // anchor point of the shadow. should be offset
    popupAnchor:  [0, 0] // point from which the popup should open relative to the iconAnchor
  })
  return L.marker(latlng, { icon: myIcon })
}

// create an options object that specifies which function will called on each feature
let estimateNodeLayerOpts = {
  pointToLayer: estimateNodeIcon
}

certainnodes = nodes.filter(n => !nodes.location.estimated)
estimatednodes = nodes.filter(n => nodes.location.estimated)

L.geoJSON(estimatednodes, estimateNodeLayerOpts).addTo(map)

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and we can use this icon as a marker: https://fontawesome.com/icons/question-circle?style=solid