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docs/features/trail-line-visualization.md

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+# Trail Line Visualization
+
+## Overview
+
+When tracking a specific callsign, the map displays a trail line (polyline) connecting all position points chronologically at zoom levels ≤ 8, instead of showing a heat map.
+
+## User Experience
+
+### Viewing Trail Lines
+
+1. Search for and track a callsign using the search box
+2. Zoom out to level 8 or below
+3. A blue trail line appears connecting all positions in chronological order
+4. The trail line respects the "Trail Duration" setting (1 hour, 6 hours, etc.)
+
+### Zoom Behavior
+
+- **Zoom ≤ 8 + Tracking:** Trail line visible
+- **Zoom > 8 + Tracking:** Individual markers visible
+- **Zoom ≤ 8 + No tracking:** Heat map visible
+- **Zoom > 8 + No tracking:** Individual markers visible
+
+## Technical Details
+
+### Backend
+
+- `DisplayManager.send_trail_line_for_tracked_callsign/1` - Generates trail line data
+- `DisplayManager.handle_zoom_threshold_crossing/2` - Handles mode switching
+- Trail line data filtered by `packet_age_threshold` (trail duration setting)
+- Points sorted chronologically by `received_at` timestamp
+
+### Frontend
+
+- `MapHook.trailLine` - Leaflet Polyline instance
+- Event: `show_trail_line` - Display trail line
+- Event: `clear_trail_line` - Remove trail line
+- Trail line cleared when switching to heat map or markers
+
+### Styling
+
+- Color: `#3B82F6` (blue-500)
+- Weight: 3 pixels
+- Opacity: 0.8
+- Line Cap/Join: round
+- Interactive: true
+
+## Continuous Path Connection
+
+Trail lines connect **all points regardless of distance** between them. This is important for:
+
+- High-altitude balloons traveling long distances
+- Aircraft on cross-country flights
+- Stations with intermittent coverage gaps
+- Showing complete journey paths
+
+No distance-based filtering or gap detection is applied. The trail always shows the complete chronological path within the trail duration.
+
+## Future Enhancements
+
+- Direction arrows along the trail
+- Time-based gradient coloring (old → new)
+- Click on trail to show packet details
+- Line simplification for very long trails
+- Multiple callsign tracking with different colors