diff --git a/src/rf_mapper/database.py b/src/rf_mapper/database.py
index 5a36803..0b9bba2 100644
--- a/src/rf_mapper/database.py
+++ b/src/rf_mapper/database.py
@@ -496,6 +496,56 @@ class DeviceDatabase:
             max_distance_m=round(stats['max_distance_m'], 2) if stats['max_distance_m'] else 0
         )
 
+    def get_device_multi_scanner_rssi(self, device_id: str, seconds: int = 60) -> list[dict]:
+        """Get recent RSSI readings per scanner for a device.
+
+        Args:
+            device_id: The device MAC address
+            seconds: Time window in seconds (default 60)
+
+        Returns:
+            List of dicts with scanner_id, avg_rssi, sample_count, last_seen
+        """
+        conn = self._get_connection()
+        cursor = conn.cursor()
+
+        query = """
+            SELECT scanner_id,
+                   AVG(rssi) as avg_rssi,
+                   COUNT(*) as sample_count,
+                   MAX(timestamp) as last_seen
+            FROM rssi_history
+            WHERE device_id = ?
+              AND scanner_id IS NOT NULL
+              AND timestamp >= datetime('now', '-' || ? || ' seconds')
+            GROUP BY scanner_id
+        """
+        cursor.execute(query, (device_id, seconds))
+        return [dict(r) for r in cursor.fetchall()]
+
+    def get_devices_seen_by_multiple_scanners(self, seconds: int = 60) -> list[str]:
+        """Get device IDs seen by 2+ scanners recently.
+
+        Args:
+            seconds: Time window in seconds (default 60)
+
+        Returns:
+            List of device IDs seen by multiple scanners
+        """
+        conn = self._get_connection()
+        cursor = conn.cursor()
+
+        query = """
+            SELECT device_id
+            FROM rssi_history
+            WHERE scanner_id IS NOT NULL
+              AND timestamp >= datetime('now', '-' || ? || ' seconds')
+            GROUP BY device_id
+            HAVING COUNT(DISTINCT scanner_id) >= 2
+        """
+        cursor.execute(query, (seconds,))
+        return [r['device_id'] for r in cursor.fetchall()]
+
     def get_movement_events(self, device_id: Optional[str] = None,
                             since: Optional[str] = None,
                             limit: int = 100) -> list[dict]:
diff --git a/src/rf_mapper/distance.py b/src/rf_mapper/distance.py
index b65fba3..d5330cb 100644
--- a/src/rf_mapper/distance.py
+++ b/src/rf_mapper/distance.py
@@ -118,3 +118,85 @@ def trilaterate_2d(
     y = (A * F - D * C) / denom
 
     return (x, y)
+
+
+def trilaterate_weighted_latlon(
+    scanner_data: list[dict]
+) -> tuple[float, float, float] | None:
+    """
+    Weighted trilateration using lat/lon coordinates.
+
+    Args:
+        scanner_data: List of dicts with:
+            - lat: Scanner latitude
+            - lon: Scanner longitude
+            - distance: Estimated distance in meters
+            - rssi: Signal strength (for weighting)
+
+    Returns:
+        (lat, lon, confidence) or None if insufficient data
+    """
+    if len(scanner_data) < 2:
+        return None
+
+    # Calculate weights from RSSI (higher = better)
+    for s in scanner_data:
+        s['weight'] = 10 ** ((s['rssi'] + 100) / 40)  # Normalize -100 to 0 range
+
+    total_weight = sum(s['weight'] for s in scanner_data)
+
+    if len(scanner_data) == 2:
+        # Two scanners: weighted average along line between them
+        s1, s2 = scanner_data[0], scanner_data[1]
+
+        # Calculate position along line based on distance ratio
+        total_dist = s1['distance'] + s2['distance']
+        if total_dist == 0:
+            ratio = 0.5
+        else:
+            ratio = s1['distance'] / total_dist
+
+        # Weighted interpolation
+        lat = s1['lat'] + ratio * (s2['lat'] - s1['lat'])
+        lon = s1['lon'] + ratio * (s2['lon'] - s1['lon'])
+
+        # Lower confidence for 2-scanner solution
+        confidence = 0.5
+        return (lat, lon, confidence)
+
+    # 3+ scanners: convert to local XY and trilaterate
+    # Use centroid as origin
+    center_lat = sum(s['lat'] for s in scanner_data) / len(scanner_data)
+    center_lon = sum(s['lon'] for s in scanner_data) / len(scanner_data)
+
+    # Convert to meters from centroid
+    positions = []
+    distances = []
+    for s in scanner_data:
+        x = (s['lon'] - center_lon) * 111000 * math.cos(math.radians(center_lat))
+        y = (s['lat'] - center_lat) * 111000
+        positions.append((x, y))
+        distances.append(s['distance'])
+
+    # Use existing trilaterate_2d
+    result = trilaterate_2d(positions, distances)
+    if result is None:
+        return None
+
+    x, y = result
+
+    # Convert back to lat/lon
+    lat = center_lat + (y / 111000)
+    lon = center_lon + (x / (111000 * math.cos(math.radians(center_lat))))
+
+    # Calculate confidence based on residuals
+    residuals = []
+    for i, s in enumerate(scanner_data):
+        calc_dist = math.sqrt((x - positions[i][0])**2 + (y - positions[i][1])**2)
+        residuals.append(abs(calc_dist - distances[i]))
+
+    avg_residual = sum(residuals) / len(residuals)
+    avg_distance = sum(distances) / len(distances)
+    confidence = max(0, min(1, 1 - (avg_residual / max(avg_distance, 1))))
+
+    return (lat, lon, confidence)
diff --git a/src/rf_mapper/web/app.py b/src/rf_mapper/web/app.py
index 3475fec..6d79a8e 100644
--- a/src/rf_mapper/web/app.py
+++ b/src/rf_mapper/web/app.py
@@ -1294,6 +1294,132 @@ def create_app(config: Config | None = None) -> Flask:
 
         return jsonify(activity)
 
+    # ==================== Trilateration API ====================
+
+    @app.route("/api/positions/trilaterated")
+    def api_trilaterated_positions():
+        """Get trilaterated positions for devices seen by multiple scanners."""
+        from ..distance import trilaterate_weighted_latlon, estimate_distance
+
+        db = app.config.get("DATABASE")
+        if not db:
+            return jsonify({"error": "Database not enabled"}), 503
+
+        seconds = int(request.args.get("seconds", 60))
+
+        # Get devices seen by multiple scanners
+        device_ids = db.get_devices_seen_by_multiple_scanners(seconds=seconds)
+
+        # Get all scanner positions (local + peers)
+        scanner_positions = {}
+
+        # Local scanner
+        local = app.config.get("SCANNER_IDENTITY", {})
+        if local.get("id"):
+            scanner_positions[local["id"]] = {
+                "lat": local.get("latitude"),
+                "lon": local.get("longitude")
+            }
+
+        # Peer scanners
+        peers = db.get_peers()
+        for peer in peers:
+            scanner_positions[peer["scanner_id"]] = {
+                "lat": peer.get("latitude"),
+                "lon": peer.get("longitude")
+            }
+
+        results = []
+        for device_id in device_ids:
+            scanner_rssi = db.get_device_multi_scanner_rssi(device_id, seconds=seconds)
+
+            # Build scanner data for trilateration
+            scanner_data = []
+            for sr in scanner_rssi:
+                sid = sr["scanner_id"]
+                if sid in scanner_positions and scanner_positions[sid].get("lat"):
+                    pos = scanner_positions[sid]
+                    dist = estimate_distance(int(sr["avg_rssi"]), tx_power=-65)
+                    scanner_data.append({
+                        "scanner_id": sid,
+                        "lat": pos["lat"],
+                        "lon": pos["lon"],
+                        "distance": dist,
+                        "rssi": sr["avg_rssi"]
+                    })
+
+            if len(scanner_data) < 2:
+                continue
+
+            # Calculate trilaterated position
+            result = trilaterate_weighted_latlon(scanner_data)
+            if result:
+                lat, lon, confidence = result
+                results.append({
+                    "device_id": device_id,
+                    "position": {"lat": lat, "lon": lon},
+                    "confidence": round(confidence, 2),
+                    "scanners": [s["scanner_id"] for s in scanner_data],
+                    "method": "trilateration" if len(scanner_data) >= 3 else "weighted_average"
+                })
+
+        return jsonify({"devices": results, "count": len(results)})
+
+    @app.route("/api/heatmap/signal")
+    def api_signal_heatmap():
+        """Get signal strength data for heat map visualization."""
+        db = app.config.get("DATABASE")
+        if not db:
+            return jsonify({"error": "Database not enabled"}), 503
+
+        floor = request.args.get("floor")
+
+        # Get scanner positions as anchor points (max signal)
+        points = []
+
+        # Local scanner
+        local = app.config.get("SCANNER_IDENTITY", {})
+        if local.get("latitude"):
+            points.append({
+                "lat": local["latitude"],
+                "lon": local["longitude"],
+                "signal": -30,  # Scanner location = max signal
+                "weight": 1.0
+            })
+
+        # Peer scanners
+        peers = db.get_peers()
+        for peer in peers:
+            if peer.get("latitude"):
+                points.append({
+                    "lat": peer["latitude"],
+                    "lon": peer["longitude"],
+                    "signal": -30,
+                    "weight": 1.0
+                })
+
+        # Get recent device observations with positions
+        # Use trilaterated positions if available, otherwise source scanner positions
+        devices = db.get_all_devices(device_type="bluetooth", limit=100)
+        for dev in devices:
+            # Get most recent RSSI for this device
+            rssi_history = db.get_device_rssi_history(dev["device_id"], limit=1)
+            if rssi_history:
+                last_rssi = rssi_history[0].rssi
+                # Get device source info
+                if dev.get("source_scanner_lat") and dev.get("source_scanner_lon"):
+                    # Calculate rough position from source scanner
+                    from ..distance import estimate_distance
+                    dist = estimate_distance(last_rssi, tx_power=-65)
+                    points.append({
+                        "lat": dev["source_scanner_lat"],
+                        "lon": dev["source_scanner_lon"],
+                        "signal": last_rssi,
+                        "weight": max(0.1, min(0.8, 1 - (dist / 50)))  # Weight by distance
+                    })
+
+        return jsonify({"points": points, "count": len(points)})
+
     @app.route("/api/history/stats")
     def api_history_stats():
         """Get database statistics"""
diff --git a/src/rf_mapper/web/static/css/style.css b/src/rf_mapper/web/static/css/style.css
index 5acbdff..bc75d48 100644
--- a/src/rf_mapper/web/static/css/style.css
+++ b/src/rf_mapper/web/static/css/style.css
@@ -832,6 +832,10 @@ body {
     color: var(--color-primary);
 }
 
+.popup-position-status .status-value.trilaterated {
+    color: #ffd700;
+}
+
 .popup-source-info {
     margin-top: 6px;
     padding: 4px 8px;
@@ -974,6 +978,39 @@ body {
     background: rgba(0, 200, 255, 0.9);
 }
 
+/* Trilaterated device markers - gold border */
+.marker-3d.trilaterated .marker-icon {
+    border: 2px dashed #ffd700 !important;
+}
+
+.marker-3d.trilaterated.high-confidence .marker-icon {
+    border-style: solid !important;
+    box-shadow: 0 0 10px rgba(255, 215, 0, 0.7), 0 2px 8px rgba(0, 0, 0, 0.5) !important;
+}
+
+.trilat-badge {
+    position: absolute;
+    top: -6px;
+    right: -6px;
+    background: #ffd700;
+    color: #000;
+    font-size: 7px;
+    padding: 1px 3px;
+    border-radius: 3px;
+    font-weight: bold;
+}
+
+/* Heat map toggle button */
+.filter-btn.heatmap {
+    color: #ff6b6b;
+    border-color: #ff6b6b;
+}
+
+.filter-btn.heatmap.active {
+    background: rgba(255, 107, 107, 0.2);
+    color: #ff6b6b;
+}
+
 /* Floor Controls */
 .floor-section {
     display: block;
diff --git a/src/rf_mapper/web/static/js/app.js b/src/rf_mapper/web/static/js/app.js
index 53fe68a..cc09713 100644
--- a/src/rf_mapper/web/static/js/app.js
+++ b/src/rf_mapper/web/static/js/app.js
@@ -24,6 +24,13 @@ let deviceSources = {}; // { deviceId: { scanner_id, lat, lon } }
 // Peer scanner positions - loaded from /api/peers (live positions)
 let peerScanners = {}; // { scanner_id: { lat, lon, floor, name } }
 
+// Trilateration state - positions calculated from multiple scanner RSSI
+let trilateratedPositions = {};  // { deviceId: { lat, lon, confidence, scanners, method } }
+let trilaterationEnabled = true;
+
+// Heat map state
+let heatMapEnabled = false;
+
 // Auto-scan state
 let autoScanEnabled = false;
 let autoScanPollInterval = null;
@@ -131,6 +138,7 @@ document.addEventListener('DOMContentLoaded', () => {
     loadLatestScan();
     loadAutoScanStatus();
     loadDevicePositions();  // Load saved manual positions
+    loadTrilateratedPositions();  // Load multi-scanner trilaterated positions
 
     // Initialize 3D map as default view
     setTimeout(() => {
@@ -287,6 +295,9 @@ function handleWebSocketScanUpdate(data) {
         document.getElementById('bt-count').textContent = scanData.bluetooth_devices.length;
         document.getElementById('bt-list-count').textContent = scanData.bluetooth_devices.length;
 
+        // Refresh trilaterated positions (non-blocking)
+        loadTrilateratedPositions();
+
         // Refresh views
         drawRadar();
         update3DMarkers();
@@ -1256,13 +1267,53 @@ async function loadDevicePositions() {
     }
 }
 
-// Get device position (manual or RSSI-based)
+// Load trilaterated positions from multi-scanner RSSI data
+async function loadTrilateratedPositions() {
+    if (!trilaterationEnabled) return;
+
+    try {
+        const resp = await fetch('/api/positions/trilaterated');
+        if (!resp.ok) return;
+
+        const data = await resp.json();
+        trilateratedPositions = {};
+        (data.devices || []).forEach(d => {
+            trilateratedPositions[d.device_id] = {
+                lat: d.position.lat,
+                lon: d.position.lon,
+                confidence: d.confidence,
+                scanners: d.scanners,
+                method: d.method
+            };
+        });
+        console.log(`[Trilateration] Loaded ${data.count} positions`);
+    } catch (e) {
+        console.error('[Trilateration] Error:', e);
+    }
+}
+
+// Get device position (manual, trilaterated, or RSSI-based)
+// Priority: 1. Manual position, 2. Trilaterated (if confidence > 0.3), 3. RSSI-based
 // Uses source scanner position for synced devices so they don't move when local scanner moves
 function getDevicePosition(device, scannerLat, scannerLon, minDistanceM) {
     const deviceId = device.bssid || device.address;
     const customPos = manualPositions[deviceId];
     const sourceInfo = deviceSources[deviceId];
 
+    // Check for trilaterated position first (if enabled and confident)
+    const triData = trilateratedPositions[deviceId];
+    if (trilaterationEnabled && triData && triData.confidence > 0.3) {
+        return {
+            lat: triData.lat,
+            lon: triData.lon,
+            isManual: false,
+            isTrilaterated: true,
+            confidence: triData.confidence,
+            scannerCount: triData.scanners.length,
+            method: triData.method
+        };
+    }
+
     // Determine which scanner position to use for this device
     // If device was synced from another scanner, use that scanner's CURRENT position
     let baseLat = scannerLat;
@@ -1780,26 +1831,33 @@ function update3DMarkers() {
         const missCount = dev.miss_count || 0;
         // Calculate opacity: 1.0 -> 0.6 -> 0.3 based on miss count
         const opacity = missCount === 0 ? 1.0 : (missCount === 1 ? 0.6 : 0.3);
+        const isTrilaterated = pos.isTrilaterated === true;
         const el = document.createElement('div');
         el.className = `marker-3d bluetooth${isMoving ? ' moving' : ''}`;
         if (isDraggable) el.classList.add('draggable');
         if (hasManualPosition) el.classList.add('has-manual-position');
+        if (isTrilaterated) {
+            el.classList.add('trilaterated');
+            if (pos.confidence >= 0.7) el.classList.add('high-confidence');
+        }
         el.style.opacity = opacity;
         el.style.transition = 'opacity 0.5s ease';
-        el.innerHTML = `<div class="marker-icon">${isMoving ? '🟣' : '🔵'}</div><div class="marker-floor">${btFloorLabel}</div>`;
-        el.title = `${dev.name} - ${deviceFloor !== null ? 'Floor ' + deviceFloor : 'Unknown floor'}${isMoving ? ' (MOVING)' : ''}${hasManualPosition ? ' (Manual position)' : ''}${missCount > 0 ? ` (fading: ${missCount}/${MAX_MISSED_SCANS})` : ''}`;
+        const trilatBadge = isTrilaterated ? `<span class="trilat-badge">${pos.scannerCount}📡</span>` : '';
+        el.innerHTML = `<div class="marker-icon">${isMoving ? '🟣' : '🔵'}${trilatBadge}</div><div class="marker-floor">${btFloorLabel}</div>`;
+        el.title = `${dev.name} - ${deviceFloor !== null ? 'Floor ' + deviceFloor : 'Unknown floor'}${isMoving ? ' (MOVING)' : ''}${isTrilaterated ? ` (Trilaterated: ${pos.scannerCount} scanners, ${Math.round(pos.confidence*100)}% conf)` : ''}${hasManualPosition ? ' (Manual position)' : ''}${missCount > 0 ? ` (fading: ${missCount}/${MAX_MISSED_SCANS})` : ''}`;
 
         const btDistLabel = hasCustomDist ? `${effectiveDist}m (custom)` : `~${dev.estimated_distance_m}m`;
-        const positionStatus = hasManualPosition ? 'Manual' : 'Auto';
-        const positionClass = hasManualPosition ? 'manual' : 'auto';
+        const positionStatus = isTrilaterated ? `Trilaterated (${pos.scannerCount} scanners)` : (hasManualPosition ? 'Manual' : 'Auto');
+        const positionClass = isTrilaterated ? 'trilaterated' : (hasManualPosition ? 'manual' : 'auto');
         const resetBtn = hasManualPosition ? `<button class="popup-reset-btn" onclick="resetDevicePosition('${btDeviceId}')">Reset to Auto</button>` : '';
-        const dragHint = isDraggable && !hasManualPosition ? '<div style="font-size:0.7rem;color:#888;margin-top:4px;">Drag marker to set position</div>' : '';
+        const dragHint = isDraggable && !hasManualPosition && !isTrilaterated ? '<div style="font-size:0.7rem;color:#888;margin-top:4px;">Drag marker to set position</div>' : '';
         const trailBtnHtml = isMoving ? `
             <button class="popup-trail-btn" id="trail-btn-${btDeviceId.replace(/:/g, '')}"
                     onclick="toggleDeviceTrail('${btDeviceId}', '${escapeHtml(dev.name)}', 'bluetooth')">
                 Show Trail
             </button>` : '';
         const btSourceInfo = pos.isRemoteSource ? `<div class="popup-source-info">📡 Source: ${pos.sourceScanner}</div>` : '';
+        const trilatInfo = isTrilaterated ? `<div class="popup-source-info" style="background:rgba(255,215,0,0.1);border-color:rgba(255,215,0,0.3);color:#ffd700;">📐 ${pos.method}: ${Math.round(pos.confidence*100)}% confidence</div>` : '';
 
         const popup = new maplibregl.Popup({ offset: 25 }).setHTML(`
             <strong>${isMoving ? '🟣' : '🔵'} ${escapeHtml(dev.name)}</strong>${isMoving ? ' <span style="color:#9b59b6;font-size:0.8em;">(Moving)</span>' : ''}<br>
@@ -1808,6 +1866,7 @@ function update3DMarkers() {
             Type: ${escapeHtml(dev.device_type)}<br>
             ${escapeHtml(dev.manufacturer)}<br>
             ${btSourceInfo}
+            ${trilatInfo}
             <div class="popup-floor-control">
                 <label>Floor:</label>
                 <select onchange="updateDeviceFloor('${btDeviceId}', this.value)">
@@ -2221,6 +2280,9 @@ async function performLiveBTScan() {
             document.getElementById('bt-count').textContent = scanData.bluetooth_devices.length;
             document.getElementById('bt-list-count').textContent = scanData.bluetooth_devices.length;
 
+            // Refresh trilaterated positions (non-blocking)
+            loadTrilateratedPositions();
+
             // Refresh views
             drawRadar();
             update3DMarkers();
@@ -2477,3 +2539,85 @@ function clearAllTrails() {
 
     deviceTrails = {};
 }
+
+// ========== Heat Map Functions ==========
+
+// Toggle heat map layer
+function toggleHeatMap() {
+    heatMapEnabled = !heatMapEnabled;
+    const btn = document.getElementById('btn-heatmap');
+    if (btn) btn.classList.toggle('active', heatMapEnabled);
+
+    if (heatMapEnabled) {
+        renderHeatMap();
+    } else {
+        removeHeatMap();
+    }
+}
+
+// Render heat map layer on 3D map
+async function renderHeatMap() {
+    if (!map3d || !map3dLoaded) {
+        console.log('[HeatMap] 3D map not ready');
+        return;
+    }
+
+    try {
+        const resp = await fetch('/api/heatmap/signal');
+        if (!resp.ok) return;
+
+        const data = await resp.json();
+        console.log(`[HeatMap] Loaded ${data.count} points`);
+
+        // Remove existing heat map if present
+        removeHeatMap();
+
+        // Add heat map source
+        map3d.addSource('signal-heatmap', {
+            type: 'geojson',
+            data: {
+                type: 'FeatureCollection',
+                features: data.points.map(p => ({
+                    type: 'Feature',
+                    geometry: { type: 'Point', coordinates: [p.lon, p.lat] },
+                    properties: { weight: p.weight }
+                }))
+            }
+        });
+
+        // Add heat map layer
+        map3d.addLayer({
+            id: 'signal-heatmap-layer',
+            type: 'heatmap',
+            source: 'signal-heatmap',
+            paint: {
+                'heatmap-weight': ['get', 'weight'],
+                'heatmap-intensity': 0.6,
+                'heatmap-radius': 40,
+                'heatmap-opacity': 0.6,
+                'heatmap-color': [
+                    'interpolate', ['linear'], ['heatmap-density'],
+                    0, 'rgba(0,0,255,0)',
+                    0.3, 'rgba(0,255,255,0.5)',
+                    0.5, 'rgba(0,255,0,0.6)',
+                    0.7, 'rgba(255,255,0,0.7)',
+                    1, 'rgba(255,0,0,0.8)'
+                ]
+            }
+        });
+    } catch (e) {
+        console.error('[HeatMap] Error:', e);
+    }
+}
+
+// Remove heat map layer from 3D map
+function removeHeatMap() {
+    if (!map3d) return;
+
+    if (map3d.getLayer('signal-heatmap-layer')) {
+        map3d.removeLayer('signal-heatmap-layer');
+    }
+    if (map3d.getSource('signal-heatmap')) {
+        map3d.removeSource('signal-heatmap');
+    }
+}
diff --git a/src/rf_mapper/web/templates/index.html b/src/rf_mapper/web/templates/index.html
index 9f7954e..e8802f0 100644
--- a/src/rf_mapper/web/templates/index.html
+++ b/src/rf_mapper/web/templates/index.html
@@ -33,6 +33,9 @@
                     <span class="filter-indicator"></span>
                     <span>Bluetooth</span>
                 </button>
+                <button id="btn-heatmap" class="filter-btn heatmap" onclick="toggleHeatMap()">
+                    <span>🌡️ Heat Map</span>
+                </button>
             </div>
 
             <canvas id="radar-canvas" class="radar-canvas"></canvas>