Index: node_modules/d3-geo/src/centroid.js
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--- node_modules/d3-geo/src/centroid.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
+++ node_modules/d3-geo/src/centroid.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
@@ -0,0 +1,143 @@
+import {Adder} from "d3-array";
+import {asin, atan2, cos, degrees, epsilon, epsilon2, hypot, radians, sin, sqrt} from "./math.js";
+import noop from "./noop.js";
+import stream from "./stream.js";
+
+var W0, W1,
+    X0, Y0, Z0,
+    X1, Y1, Z1,
+    X2, Y2, Z2,
+    lambda00, phi00, // first point
+    x0, y0, z0; // previous point
+
+var centroidStream = {
+  sphere: noop,
+  point: centroidPoint,
+  lineStart: centroidLineStart,
+  lineEnd: centroidLineEnd,
+  polygonStart: function() {
+    centroidStream.lineStart = centroidRingStart;
+    centroidStream.lineEnd = centroidRingEnd;
+  },
+  polygonEnd: function() {
+    centroidStream.lineStart = centroidLineStart;
+    centroidStream.lineEnd = centroidLineEnd;
+  }
+};
+
+// Arithmetic mean of Cartesian vectors.
+function centroidPoint(lambda, phi) {
+  lambda *= radians, phi *= radians;
+  var cosPhi = cos(phi);
+  centroidPointCartesian(cosPhi * cos(lambda), cosPhi * sin(lambda), sin(phi));
+}
+
+function centroidPointCartesian(x, y, z) {
+  ++W0;
+  X0 += (x - X0) / W0;
+  Y0 += (y - Y0) / W0;
+  Z0 += (z - Z0) / W0;
+}
+
+function centroidLineStart() {
+  centroidStream.point = centroidLinePointFirst;
+}
+
+function centroidLinePointFirst(lambda, phi) {
+  lambda *= radians, phi *= radians;
+  var cosPhi = cos(phi);
+  x0 = cosPhi * cos(lambda);
+  y0 = cosPhi * sin(lambda);
+  z0 = sin(phi);
+  centroidStream.point = centroidLinePoint;
+  centroidPointCartesian(x0, y0, z0);
+}
+
+function centroidLinePoint(lambda, phi) {
+  lambda *= radians, phi *= radians;
+  var cosPhi = cos(phi),
+      x = cosPhi * cos(lambda),
+      y = cosPhi * sin(lambda),
+      z = sin(phi),
+      w = atan2(sqrt((w = y0 * z - z0 * y) * w + (w = z0 * x - x0 * z) * w + (w = x0 * y - y0 * x) * w), x0 * x + y0 * y + z0 * z);
+  W1 += w;
+  X1 += w * (x0 + (x0 = x));
+  Y1 += w * (y0 + (y0 = y));
+  Z1 += w * (z0 + (z0 = z));
+  centroidPointCartesian(x0, y0, z0);
+}
+
+function centroidLineEnd() {
+  centroidStream.point = centroidPoint;
+}
+
+// See J. E. Brock, The Inertia Tensor for a Spherical Triangle,
+// J. Applied Mechanics 42, 239 (1975).
+function centroidRingStart() {
+  centroidStream.point = centroidRingPointFirst;
+}
+
+function centroidRingEnd() {
+  centroidRingPoint(lambda00, phi00);
+  centroidStream.point = centroidPoint;
+}
+
+function centroidRingPointFirst(lambda, phi) {
+  lambda00 = lambda, phi00 = phi;
+  lambda *= radians, phi *= radians;
+  centroidStream.point = centroidRingPoint;
+  var cosPhi = cos(phi);
+  x0 = cosPhi * cos(lambda);
+  y0 = cosPhi * sin(lambda);
+  z0 = sin(phi);
+  centroidPointCartesian(x0, y0, z0);
+}
+
+function centroidRingPoint(lambda, phi) {
+  lambda *= radians, phi *= radians;
+  var cosPhi = cos(phi),
+      x = cosPhi * cos(lambda),
+      y = cosPhi * sin(lambda),
+      z = sin(phi),
+      cx = y0 * z - z0 * y,
+      cy = z0 * x - x0 * z,
+      cz = x0 * y - y0 * x,
+      m = hypot(cx, cy, cz),
+      w = asin(m), // line weight = angle
+      v = m && -w / m; // area weight multiplier
+  X2.add(v * cx);
+  Y2.add(v * cy);
+  Z2.add(v * cz);
+  W1 += w;
+  X1 += w * (x0 + (x0 = x));
+  Y1 += w * (y0 + (y0 = y));
+  Z1 += w * (z0 + (z0 = z));
+  centroidPointCartesian(x0, y0, z0);
+}
+
+export default function(object) {
+  W0 = W1 =
+  X0 = Y0 = Z0 =
+  X1 = Y1 = Z1 = 0;
+  X2 = new Adder();
+  Y2 = new Adder();
+  Z2 = new Adder();
+  stream(object, centroidStream);
+
+  var x = +X2,
+      y = +Y2,
+      z = +Z2,
+      m = hypot(x, y, z);
+
+  // If the area-weighted ccentroid is undefined, fall back to length-weighted ccentroid.
+  if (m < epsilon2) {
+    x = X1, y = Y1, z = Z1;
+    // If the feature has zero length, fall back to arithmetic mean of point vectors.
+    if (W1 < epsilon) x = X0, y = Y0, z = Z0;
+    m = hypot(x, y, z);
+    // If the feature still has an undefined ccentroid, then return.
+    if (m < epsilon2) return [NaN, NaN];
+  }
+
+  return [atan2(y, x) * degrees, asin(z / m) * degrees];
+}