Emergency tab — navigating without GPS
Recover your heading and position using the sun, the stars and the horizon when GPS and instruments have failed, following David Burch's techniques.
Where to find it
Sidebar › Emergency (alert-triangle icon).
All "real-time" figures (sun azimuth, local apparent noon, sunrise/sunset, etc.) are computed from the map center. Center the map on your estimated position before reading the values.
Hierarchy
Emergency
├── Expand all / Collapse all button (header)
├── 1. Sun Compass (real-time sun data)
├── 2. Compass Check (compass error via sun azimuth)
├── 3. Latitude Calculator (Noon Sun / Polaris / Day Length)
├── 4. Longitude Calculator (UTC time of local apparent noon)
├── 5. Visible Range (distance an object becomes visible)
├── 6. DR Errors & Leeway (course error, leeway, beam current)
└── 7. Emergency AI Assistant (RAG-assisted Burch chat)
Each section has a chevron to open/close, a How to use button (expands help text with figures from the book), and the Expand all / Collapse all button at the top acts on all seven sections at once.
1. Sun Compass
What it's for: continuously shows the true direction of the sun and the local solar time markers, so you can orient yourself without an electronic compass.
How to use it: 1. Center the map on your estimated position. 2. Read the Sun azimuth: the true direction (in degrees) of the sun right now. Point at the sun and this bearing gives you true north. 3. At Local Apparent Noon (LAN), the sun is due south (N hemisphere): your best heading reference. 4. Sunrise/sunset bearings let you calibrate wind or swell direction.
Options & settings: this section is read-only. Values shown:
| Value | Meaning |
|---|---|
| Sun azimuth (°) | True direction of the sun now |
| Sun altitude (°) | Sun's elevation above the horizon |
| Solar declination (°) | Sun's N/S position (used by latitude calcs) |
| Equation of Time (min) | Gap between true and mean solar time |
| LAN (UTC) / (local) | Time the sun reaches its highest point |
| Sunrise / sunset bearing (°) | Direction the sun rises / sets |
| Amplitude (from E/W) (°) | Offset of sunrise/sunset from due east/west |
| Sunrise / Sunset | Local times |
| Day length (h) | Sunrise→sunset (used by the Day Length latitude method) |
| Horizon (NM) | Distance to the horizon from your eye |
Tips & limits: at LAN exactly, the sun is due south (northern hemisphere) — the most reliable heading reference. Accuracy depends entirely on the estimated position you set via the map center.
2. Compass Check
What it's for: measure your magnetic compass error by comparing the compass bearing of the sun with its computed true bearing.
How to use it: 1. Point your bow at the sun (or take a compass bearing of the sun). 2. Enter that reading in Compass bearing of sun. 3. The Magnetic variation is auto-filled by the WMM2025 model for the map-center position (today's date). Adjust it if needed; the Use button re-applies the model value. 4. Read the computed Total compass error and Deviation.
Options & settings:
| Field | Role |
|---|---|
| Compass bearing of sun (°) | 0–360, step 1° — your compass reading of the sun |
| Magnetic variation (°) | -30 to +30, step 0.5° — pre-filled by WMM2025, editable |
| WMM2025 (+ Use) | World Magnetic Model value for the current position (e.g. "+2.3° E", with the magnetic dip). Use writes it into the field. |
| Sun true bearing (°) | Computed — true direction of the sun |
| Total compass error (°) | Total gap, compass vs true |
| Deviation (this heading) (°) | Boat-specific part, excluding variation |
Tips & limits: if the error is +8°, the compass reads 8° too high: subtract 8° from all readings. Deviation depends on the heading you are on; repeat the check on several headings. The WMM2025 variation is computed offline (no connection) and updates as the map center moves; convention East positive (+), West negative (−), valid within the model window (2025 epoch + 5 years). As a last resort, trust the variation printed on your chart.
3. Latitude Calculator
What it's for: estimate your latitude with three selectable methods, depending on what you can observe (sun by day or stars by night).
How to use it: 1. Choose a method via the three tabs: Noon Sun, Polaris or Day Length. 2. Enter the measurement the method asks for. 3. Read the Calculated latitude shown at the bottom (degrees + N/S).
Options & settings:
| Tab | Input | Principle | Typical accuracy |
|---|---|---|---|
| Noon Sun | Noon altitude (°) + hemisphere N/S button ("Sun is to") | Lat = 90° − altitude ± today's declination (displayed) | ±30 NM |
| Polaris | Polaris altitude (°) | Latitude ≈ height of Polaris above the horizon | ±60 NM, Northern Hemisphere only |
| Day Length | Observed day length (h) | Sunrise→sunset + declination → approximate latitude | ±100 NM |
Tips & limits: measure the sun's altitude by hand if you have no sextant (fist ≈ 10°, open hand ≈ 20°). For the noon sight, take the altitude exactly at the LAN time shown in Sun Compass. Polaris works in the Northern Hemisphere only.
4. Longitude Calculator
What it's for: estimate your longitude from the UTC time at which the sun reaches its highest point (Local Apparent Noon, LAN).
How to use it: 1. Note the exact UTC time when the sun is at its highest (tip: average the UTC times of sunrise and sunset). 2. Enter that time in the two Observed LAN fields: hours (0–23) and minutes (0–59), in UTC. 3. Read the Calculated longitude at the bottom (degrees + E/W). Today's Equation of Time is shown for reference.
Options & settings:
| Field | Role |
|---|---|
| Observed LAN — hours (UTC) | 0 to 23 |
| Observed LAN — minutes (UTC) | 0 to 59 |
| EqT today (min) | Equation of Time, applied to the calculation |
| Calculated longitude | Lng = (Greenwich noon − observed LAN) × 15°/h |
Tips & limits: accuracy depends on your watch: ±1 min error on the time = ±15 NM on longitude. A good watch gives about ±30 NM.
5. Visible Range
What it's for: compute at what distance an object (lighthouse, island peak) becomes visible above the horizon, and the distance to your horizon from eye height.
How to use it: 1. Enter your Eye height (standing in the cockpit ~2.5 m, top of mast ~15 m). 2. Enter the Object height (lighthouse, island peak — read from charts). 3. Read the Horizon distance and the Geographic range.
Options & settings:
| Field | Range | Role |
|---|---|---|
| Eye height (m) | 0.5–30, step 0.5 | Your eye height above the water |
| Object height (m) | 0–500, step 1 | Height of the observed object |
| Horizon distance (NM) | Computed | Distance to your visible horizon |
| Geographic range (NM) | Computed | Max distance to see the top of the object |
Formula shown: D = 1.17 × (√h_eye + √h_obj) NM (heights in feet).
Tips & limits: if a lighthouse's charted light range exceeds the geographic range, the geographic range is the limit. A cloud cap over a distant island reveals land beyond the horizon.
6. DR Errors & Leeway
What it's for: estimate your off-track distance from a course error, your leeway (wind push) and the effect of a beam current.
How to use it: 1. Off-track: enter the Course error and Distance traveled; read the Off-track distance. 2. Leeway estimation: enter Wind speed and Boat speed; read the Estimated leeway. 3. Current set: enter the Beam current; read the Set angle.
Options & settings:
| Field | Range | Role |
|---|---|---|
| Course error (°) | 0–45, step 1 | Error on the steered course |
| Distance traveled (NM) | 1–5000, step 10 | Distance since the last sure position |
| Off-track distance (NM) | Computed | Resulting lateral offset |
| Wind speed (kts) | 0–60, step 1 | Wind strength |
| Boat speed (kts) | 0.5–20, step 0.5 | Speed through water |
| Estimated leeway (°) | Computed | Angle the wind pushes you to leeward |
| Beam current (kts) | 0–5, step 0.1 | Current across your track |
| Set angle (°) | Computed | Offset caused by the current |
Tips & limits: rule of thumb — every 6° of course error = ~10% off-track (100 NM at 6° → 10 NM sideways). Typical upwind leeway: 5 to 15°. A 1 kt beam current at 6 kts boat speed sets you ~10° off course; add it to leeway if both come from the same side.
7. Emergency AI Assistant
What it's for: an AI chat expert in emergency navigation that knows your position and current sun data, and whose answers are enriched with the actual passages of David Burch's Emergency Navigation book (RAG system).
How to use it: 1. Click one of the suggested questions shown before the first message, or type your situation in the input box. 2. Send with the Send button (paper plane) or the Enter key (Shift+Enter for a line break). 3. The reply streams in. To interrupt it, click the Stop button (red cross) while it is generating. 4. The second cross button clears the conversation.
Options & settings:
| Element | Role |
|---|---|
| RAG indicator (dot) | Green = Book RAG active (answers enriched with Burch passages); grey = Book RAG offline; pulsing amber = checking the RAG server |
| Suggested questions | 4 starters (GPS is dead, check the compass, how far is an island, course to the nearest port) |
| Input box | "Describe your situation…" |
| Send / Stop button | Sends the question / interrupts generation |
| Clear button | Empties the conversation |
Tips & limits: the AI is constrained to rely only on the book passages and the computed context data (position, azimuth, declination…); if information is missing from the references it says so rather than inventing it. If the indicator shows offline, the chat still works but without book enrichment — in web mode, start the RAG server (python3 scripts/ragServer.py); in the desktop app it starts automatically. For immediate danger, the assistant reminds you of distress basics: VHF channel 16, EPIRB, SART.