From 0c97ad38d330e22efae796c38207e4df35b1ef35 Mon Sep 17 00:00:00 2001
From: Peter Johnson <738069+missinglink@users.noreply.github.com>
Date: Tue, 11 Nov 2025 17:39:15 +0100
Subject: [PATCH] feat(geometry): convert 3d and 4d geometries to 2d

---
 import/format.js                              | 21 +++++++++-
 .../source/whosonfirst/map/geometries.test.js | 41 +++++++++++++++++++
 package.json                                  |  2 +-
 3 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/import/format.js b/import/format.js
index 0bf906e..e532898 100644
--- a/import/format.js
+++ b/import/format.js
@@ -48,7 +48,26 @@ function from (type, format, geom) {
     throw new Error(`invalid format: ${format}`)
   }
 
-  return func(geom)
+  return castToXY(func(geom))
+}
+
+function to2D (coords) {
+  if (typeof coords[0] === 'number') {
+    return coords.slice(0, 2)
+  }
+  return coords.map(to2D)
+}
+
+// Convert a geometry to 2D by stripping Z and M values
+function castToXY (geom) {
+  if (!geom.hasZ && !geom.hasM) {
+    return geom
+  }
+  const gj = geom.toGeoJSON()
+  return wkx.Geometry.parseGeoJSON({
+    type: gj.type,
+    coordinates: to2D(gj.coordinates)
+  })
 }
 
 function to (geom, format) {
diff --git a/import/source/whosonfirst/map/geometries.test.js b/import/source/whosonfirst/map/geometries.test.js
index c1e192f..3b684ad 100644
--- a/import/source/whosonfirst/map/geometries.test.js
+++ b/import/source/whosonfirst/map/geometries.test.js
@@ -139,3 +139,44 @@ tap.test('mapper: mapshaper position', (t) => {
   t.equal(p.geometry[1].role, 'mapshaper_position')
   t.end()
 })
+tap.test('mapper: converts 3D geometries to 2D', (t) => {
+  let p = new Place()
+  map(p, {
+    geometry: {
+      'type': 'Polygon',
+      'coordinates': [
+        [
+          [
+            39.0234375,
+            48.922499263758255,
+            10
+          ],
+          [
+            47.8125,
+            39.639537564366684,
+            20
+          ],
+          [
+            61.17187499999999,
+            49.83798245308484,
+            30
+          ],
+          [
+            39.0234375,
+            48.922499263758255,
+            40
+          ]
+        ]
+      ]
+    }
+  })
+  t.equal(p.geometry.length, 1)
+  t.ok(p.geometry[0] instanceof Geometry)
+  t.equal(p.geometry[0].geometry.constructor.name.toUpperCase(), 'POLYGON')
+  t.equal(p.geometry[0].role, 'boundary')
+  t.equal(p.geometry[0].geometry.hasZ, false)
+  t.equal(p.geometry[0].geometry.hasM, false)
+  t.same(p.geometry[0].geometry.toGeoJSON().coordinates[0][0], [39.0234375, 48.922499263758255])
+  t.equal(p.geometry[0].geometry.toGeoJSON().coordinates[0].every(c => c.length === 2), true)
+  t.end()
+})
diff --git a/package.json b/package.json
index 92c05fe..f8ab782 100644
--- a/package.json
+++ b/package.json
@@ -50,7 +50,7 @@
     "split2": "^3.1.1",
     "through2": "^3.0.1",
     "turf-point": "^2.0.1",
-    "wkx": "^0.4.8",
+    "wkx": "^0.5.0",
     "yargs": "^13.3.0"
   },
   "devDependencies": {