fix(@turf/midpoint): interpolate altitude (z-coordinate) for 3D points

packages/turf-midpoint/index.ts

@@ -3,6 +3,7 @@ import { bearing } from "@turf/bearing";
 import { destination } from "@turf/destination";
 import { distance } from "@turf/distance";
 import { Coord } from "@turf/helpers";
+import { getCoord } from "@turf/invariant";
 
 /**
  * Takes two points and returns a point midway between them. The midpoint is
@@ -28,6 +29,14 @@ function midpoint(point1: Coord, point2: Coord): Feature<Point> {
   const heading = bearing(point1, point2);
   const midpoint = destination(point1, dist / 2, heading);
 
+  // Interpolate altitude (z-coordinate) when both input points carry one.
+  // destination() propagates only the origin's z unchanged; average them instead.
+  const coord1 = getCoord(point1);
+  const coord2 = getCoord(point2);
+  if (coord1[2] !== undefined && coord2[2] !== undefined) {
+    midpoint.geometry.coordinates[2] = (coord1[2] + coord2[2]) / 2;
+  }
+
   return midpoint;
 }
 

packages/turf-midpoint/package.json

@@ -64,6 +64,7 @@
     "@turf/destination": "workspace:*",
     "@turf/distance": "workspace:*",
     "@turf/helpers": "workspace:*",
+    "@turf/invariant": "workspace:*",
     "@types/geojson": "catalog:",
     "tslib": "catalog:"
   }

packages/turf-midpoint/test.ts

@@ -68,3 +68,34 @@ test("midpoint -- long distance", function (t) {
 
   t.end();
 });
+test("midpoint -- interpolates altitude (z-coordinate) from 3D points", function (t) {
+  var pt1 = point([0, 0, 100]);
+  var pt2 = point([10, 0, 200]);
+
+  var mid = midpoint(pt1, pt2);
+  var coords = mid.geometry.coordinates;
+
+  // The midpoint of two 3D points must interpolate z as the average
+  t.equal(
+    coords[2],
+    150,
+    "z should be the average of the two input altitudes (100+200)/2 = 150"
+  );
+
+  t.end();
+});
+
+test("midpoint -- returns undefined z when inputs have no altitude", function (t) {
+  var pt1 = point([0, 0]);
+  var pt2 = point([10, 0]);
+
+  var mid = midpoint(pt1, pt2);
+
+  t.equal(
+    mid.geometry.coordinates[2],
+    undefined,
+    "z should be undefined when inputs have no altitude"
+  );
+
+  t.end();
+});