made poly arrows via svg markers

src/utils/geom.js

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+export const middleCoord = (l1, l2) => ({
+  lat: (l2.lat + ((l1.lat - l2.lat) / 2)),
+  lng: (l2.lng + ((l1.lng - l2.lng) / 2))
+});
+
+export const deg2rad = deg => deg * Math.PI / 180;
+
+export const findDistance = (t1, n1, t2, n2) => {
+  // convert coordinates to radians
+  const lat1 = deg2rad(t1);
+  const lon1 = deg2rad(n1);
+  const lat2 = deg2rad(t2);
+  const lon2 = deg2rad(n2);
+
+  // find the differences between the coordinates
+  const dlat = lat2 - lat1;
+  const dlon = lon2 - lon1;
+
+  // here's the heavy lifting
+  const a = (Math.sin(dlat / 2) ** 2) +
+    (Math.cos(lat1) * Math.cos(lat2) * (Math.sin(dlon / 2) ** 2));
+  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a)); // great circle distance in radians
+  // const dm = c * 3961; // great circle distance in miles
+  const dk = c * 6373; // great circle distance in km
+
+  // round the results down to the nearest 1/1000
+  // const mi = round(dm);
+  return (Math.round(dk * 1000) / 1000);
+};