Add a distance/area measuring tool

A ruler button in the top-left control stack, matching NPS's own
Area/Distance ruler. Click to add vertices; distance sums haversine
segment lengths, area uses the spherical-excess polygon formula
(same one turf.js's area() uses) rather than a flat shoelace, since a
flat approximation drifts at park-boundary scale. SI units only
(m/km, m²/km²) — no imperial toggle.

public/css/style.css

@@ -472,6 +472,82 @@ html, body {
   word-break: break-word;
 }
 
+.measure-ctrl {
+  position: relative;
+}
+
+.measure-ctrl__button svg {
+  display: block;
+  margin: 0 auto;
+  width: 18px;
+  height: 18px;
+}
+
+.measure-ctrl__button.active {
+  background: #dbe9ff;
+}
+
+.measure-panel {
+  position: absolute;
+  top: 0;
+  left: 100%;
+  margin-left: 8px;
+  z-index: 20;
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  min-width: max-content;
+  background: #fff;
+  border: 1px solid #ddd;
+  border-radius: 6px;
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
+  padding: 6px 8px;
+  font-size: 13px;
+}
+
+.measure-panel[hidden] {
+  display: none;
+}
+
+.measure-panel__mode {
+  padding: 4px 8px;
+  border: none;
+  border-radius: 4px;
+  background: none;
+  font: inherit;
+  font-size: 12px;
+  cursor: pointer;
+}
+
+.measure-panel__mode:hover {
+  background: #f0f0f0;
+}
+
+.measure-panel__mode.active {
+  background: #dbe9ff;
+  font-weight: 600;
+}
+
+.measure-panel__result {
+  min-width: 4em;
+  color: #333;
+  font-variant-numeric: tabular-nums;
+}
+
+.measure-panel__clear {
+  border: none;
+  background: none;
+  font-size: 16px;
+  line-height: 1;
+  color: #888;
+  cursor: pointer;
+  padding: 0 2px;
+}
+
+.measure-panel__clear:hover {
+  color: #333;
+}
+
 .zoom-level-ctrl__value {
   width: 29px;
   height: 29px;

public/js/icons.js

@@ -26,6 +26,12 @@ export const GNSS_ICON = `
   <path d="M12 2v3M12 19v3M2 12h3M19 12h3"/>
 </svg>`;
 
+export const RULER_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linejoin="round" stroke-linecap="round">
+  <path d="M4 15 15 4l5 5-11 11-5-5z"/>
+  <path d="M9 10l1.5 1.5M12 7l1.5 1.5M15 4l1.5 1.5M6 13l1.5 1.5"/>
+</svg>`;
+
 export const NEW_FOLDER_ICON = `
 <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
   <path d="M3 7a2 2 0 0 1 2-2h4l2 2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V7z"/>

public/js/map.js

@@ -10,6 +10,7 @@ import { GpxPanelControl } from "/js/gpx.js";
 import { GnssControl } from "/js/gnss.js";
 import { LiveTrackPanelControl } from "/js/livetrack.js";
 import { TrackProfilePanel } from "/js/track_profile.js";
+import { MeasureControl } from "/js/measure.js";
 import { LAYERS_ICON } from "/js/icons.js";
 import { notifySidePanelOpen, onlyOneSidePanelOpen } from "/js/panels.js";
 import HomeButton from "/vendor/npmap-home-button/home-button.esm.js";
@@ -228,8 +229,9 @@ const map = new MaplibreMap({
   hash: "map",
 });
 
-// A compact top-left stack — Home, zoom, zoom-level readout, locate-me,
+// A compact top-left stack — Home, zoom, zoom-level readout, measure,
-// base map — matching the National Park Service's own map control layout,
+// locate-me, base map — matching the National Park Service's own map
+// control layout,
 // all grouped together rather than split across corners. Added in this
 // order since MapLibre stacks same-position controls in add order.
 //
@@ -245,6 +247,7 @@ map.addControl(new HomeButton({ homePosition: CENTER, zoom: ZOOM }), "top-left")
 // (NavigationControl's default third button) would just be dead weight.
 map.addControl(new NavigationControl({ showCompass: false }), "top-left");
 map.addControl(new ZoomLevelControl(), "top-left");
+map.addControl(new MeasureControl(), "top-left");
 map.addControl(
   new GeolocateControl({
     positionOptions: { enableHighAccuracy: true },

public/js/measure.js

@@ -0,0 +1,194 @@
+import { RULER_ICON } from "/js/icons.js";
+
+const SOURCE_ID = "measure";
+const LINE_LAYER_ID = "measure-line";
+const FILL_LAYER_ID = "measure-fill";
+const POINT_LAYER_ID = "measure-points";
+const EARTH_RADIUS_M = 6378137; // WGS84 equatorial radius, matches turf.js's area formula
+
+// A distance/area measuring tool, matching the ruler button in NPS's own
+// map controls (Home/zoom/ruler, screenshot). Click to add vertices,
+// distance sums the haversine length of the resulting line; area uses the
+// standard spherical-excess polygon formula (Chamberlain & Duquette, the
+// same one turf.js's `area()` uses) rather than a flat-plane shoelace,
+// since a flat approximation drifts at the scale this app's park-sized
+// areas can reach. SI units throughout (m/km, m²/km²) — no imperial toggle.
+export class MeasureControl {
+  constructor() {
+    this._mode = "distance";
+    this._active = false;
+    this._points = [];
+  }
+
+  onAdd(map) {
+    this._map = map;
+
+    this._container = document.createElement("div");
+    this._container.className = "maplibregl-ctrl maplibregl-ctrl-group measure-ctrl";
+    this._container.innerHTML = `
+      <button type="button" class="measure-ctrl__button" title="Measure" aria-label="Measure">${RULER_ICON}</button>
+      <div class="measure-panel" hidden>
+        <button type="button" class="measure-panel__mode active" data-mode="distance">Distance</button>
+        <button type="button" class="measure-panel__mode" data-mode="area">Area</button>
+        <span class="measure-panel__result"></span>
+        <button type="button" class="measure-panel__clear" title="Clear" aria-label="Clear">×</button>
+      </div>
+    `;
+    this._button = this._container.querySelector(".measure-ctrl__button");
+    this._panel = this._container.querySelector(".measure-panel");
+    this._result = this._container.querySelector(".measure-panel__result");
+
+    this._button.addEventListener("click", () => this._setActive(!this._active));
+    this._panel.querySelectorAll(".measure-panel__mode").forEach((button) => {
+      button.addEventListener("click", () => this._setMode(button.dataset.mode));
+    });
+    this._panel.querySelector(".measure-panel__clear").addEventListener("click", () => this._clear());
+
+    this._onClick = (event) => {
+      if (!this._active) return;
+      this._points.push([event.lngLat.lng, event.lngLat.lat]);
+      this._render();
+    };
+    map.on("click", this._onClick);
+
+    // setStyle() (switching Online/Offline) tears down all sources/layers —
+    // same re-add-on-reload pattern as radar.js/gpx.js.
+    map.on("style.load", () => {
+      if (this._active) this._addLayers();
+    });
+
+    return this._container;
+  }
+
+  onRemove() {
+    this._map.off("click", this._onClick);
+    this._removeLayers();
+    this._container.remove();
+  }
+
+  getDefaultPosition() {
+    return "top-left";
+  }
+
+  _setActive(active) {
+    this._active = active;
+    this._button.classList.toggle("active", active);
+    this._panel.hidden = !active;
+    this._map.getCanvas().style.cursor = active ? "crosshair" : "";
+    if (active) {
+      this._addLayers();
+    } else {
+      this._clear();
+    }
+  }
+
+  _setMode(mode) {
+    this._mode = mode;
+    this._panel.querySelectorAll(".measure-panel__mode").forEach((button) => {
+      button.classList.toggle("active", button.dataset.mode === mode);
+    });
+    this._points = [];
+    this._render();
+  }
+
+  _clear() {
+    this._points = [];
+    this._render();
+  }
+
+  _addLayers() {
+    if (this._map.getSource(SOURCE_ID)) return;
+
+    this._map.addSource(SOURCE_ID, { type: "geojson", data: emptyCollection() });
+    this._map.addLayer({
+      id: FILL_LAYER_ID,
+      type: "fill",
+      source: SOURCE_ID,
+      filter: ["==", "$type", "Polygon"],
+      paint: { "fill-color": "#1e5fbf", "fill-opacity": 0.15 },
+    });
+    this._map.addLayer({
+      id: LINE_LAYER_ID,
+      type: "line",
+      source: SOURCE_ID,
+      filter: ["!=", "$type", "Point"],
+      paint: { "line-color": "#1e5fbf", "line-width": 2 },
+    });
+    this._map.addLayer({
+      id: POINT_LAYER_ID,
+      type: "circle",
+      source: SOURCE_ID,
+      filter: ["==", "$type", "Point"],
+      paint: { "circle-radius": 4, "circle-color": "#fff", "circle-stroke-color": "#1e5fbf", "circle-stroke-width": 2 },
+    });
+
+    this._render();
+  }
+
+  _removeLayers() {
+    [FILL_LAYER_ID, LINE_LAYER_ID, POINT_LAYER_ID].forEach((id) => {
+      if (this._map.getLayer(id)) this._map.removeLayer(id);
+    });
+    if (this._map.getSource(SOURCE_ID)) this._map.removeSource(SOURCE_ID);
+  }
+
+  _render() {
+    const source = this._map.getSource(SOURCE_ID);
+    if (!source) return;
+
+    const pointFeatures = this._points.map((coord) => ({ type: "Feature", geometry: { type: "Point", coordinates: coord } }));
+    const shapeFeature =
+      this._mode === "area"
+        ? this._points.length >= 3 && { type: "Feature", geometry: { type: "Polygon", coordinates: [[...this._points, this._points[0]]] } }
+        : this._points.length >= 2 && { type: "Feature", geometry: { type: "LineString", coordinates: this._points } };
+
+    source.setData({ type: "FeatureCollection", features: shapeFeature ? [shapeFeature, ...pointFeatures] : pointFeatures });
+
+    this._result.textContent = this._mode === "area" ? formatArea(polygonArea(this._points)) : formatDistance(lineLength(this._points));
+  }
+}
+
+function emptyCollection() {
+  return { type: "FeatureCollection", features: [] };
+}
+
+function haversine([lng1, lat1], [lng2, lat2]) {
+  const rad = Math.PI / 180;
+  const dLat = (lat2 - lat1) * rad;
+  const dLng = (lng2 - lng1) * rad;
+  const a = Math.sin(dLat / 2) ** 2 + Math.cos(lat1 * rad) * Math.cos(lat2 * rad) * Math.sin(dLng / 2) ** 2;
+  return 2 * EARTH_RADIUS_M * Math.asin(Math.sqrt(a));
+}
+
+function lineLength(points) {
+  let total = 0;
+  for (let i = 1; i < points.length; i++) total += haversine(points[i - 1], points[i]);
+  return total;
+}
+
+// Spherical polygon area (Chamberlain & Duquette, 2007) — the same formula
+// turf.js's area() uses. A flat shoelace formula would drift noticeably at
+// the scale a park boundary can reach; this accounts for the sphere.
+function polygonArea(points) {
+  if (points.length < 3) return 0;
+  const rad = (deg) => (deg * Math.PI) / 180;
+  let area = 0;
+  const n = points.length;
+  for (let i = 0; i < n; i++) {
+    const [, lat] = points[i];
+    const [lng1] = points[(i + n - 1) % n];
+    const [lng2] = points[(i + 1) % n];
+    area += (rad(lng2) - rad(lng1)) * Math.sin(rad(lat));
+  }
+  return Math.abs((area * EARTH_RADIUS_M * EARTH_RADIUS_M) / 2);
+}
+
+function formatDistance(meters) {
+  if (meters === 0) return "";
+  return meters >= 1000 ? `${(meters / 1000).toFixed(2)} km` : `${Math.round(meters)} m`;
+}
+
+function formatArea(squareMeters) {
+  if (squareMeters === 0) return "";
+  return squareMeters >= 1e6 ? `${(squareMeters / 1e6).toFixed(2)} km²` : `${Math.round(squareMeters)} m²`;
+}