Navigation Instruments Study Guide
Master the compass, GPS, radar, AIS, depth sounder, sextant, barometer, and chart plotting tools tested on the USCG OUPV and Master captain's license exam. Detailed explanations, reference tables, and solved practice problems.
What This Guide Covers
1. Magnetic Compass
The magnetic compass is the oldest and most reliable navigation instrument. Every vessel is required to carry one as a backup to electronic systems. USCG exams test compass error calculation heavily.
Variation and Deviation
The angle between True North and Magnetic North at a given geographic location. Printed on the chart compass rose. Changes slowly over years (annual change noted on rose). Caused by the Earth's magnetic field; nothing on the vessel causes it.
The error caused by the vessel's own magnetic field — engines, wiring, metal structure, electronics — acting on the compass needle. It changes with the vessel's heading and is unique to each vessel. Recorded in a deviation table after swinging the compass.
Compass Correction — CDMVT Method
Remember: Can Dead Men Vote Twice (Compass, Deviation, Magnetic, Variation, True). East errors are added; West errors are subtracted when converting Compass to True. Reverse the process (and reverse the sign) when converting True to Compass.
| Letter | Term | What It Represents |
|---|---|---|
| C | Compass | What the compass reads |
| D | Deviation | Add East (+) or subtract West (-) deviation |
| M | Magnetic | Result after applying deviation |
| V | Variation | Add East (+) or subtract West (-) variation |
| T | True | Final true course or bearing |
Compass reads 135. Deviation 4E. Variation 12W. Find true course.
To convert True back to Compass: reverse the signs. True 127 + 12W variation = 139 Magnetic; 139 - 4E deviation = 135 Compass.
Compass Card and Swinging the Compass
The graduated disc inside the compass bowl that rotates with the magnetic field. On most marine compasses it is graduated in degrees 0-360 or in points (32-point system: N, NNE, NE, ENE, etc.). The card is fixed to the magnetic needle assembly and floats in damping fluid.
The process of comparing compass readings to known magnetic bearings on multiple headings (typically every 15 or 30 degrees around the full circle) to build a deviation table. Done at sea in calm conditions, or at a compass adjustment buoy. A compass adjuster uses compensating magnets to minimize deviation; residual errors go in the deviation table.
A card posted near the compass listing deviation for each 15 or 30 degree compass heading. Used to find the deviation to apply before correcting for variation. Interpolate between tabulated headings. USCG exams frequently give you a deviation table and ask you to solve a compass correction problem.
2. Gyrocompass
Required on SOLAS vessels. Not required on most recreational or small commercial vessels. Appears on USCG Master exams more than OUPV but basic principles can appear on any exam.
Operating Principle
A gyrocompass uses a rapidly spinning gyroscope (rotor) whose axis aligns with True North by exploiting the Earth's rotation. Unlike a magnetic compass, it points to True North, not Magnetic North — no variation correction is needed. It requires continuous electrical power and takes 1 to 4 hours to settle after startup.
Precession
Gyroscopic precession is the tendency of a spinning gyroscope to respond to an applied force 90 degrees later in the direction of rotation. In a gyrocompass, controlled precession is used to align the spin axis with True North. Uncontrolled precession from mechanical wear causes gyro error that must be monitored and corrected.
Gyrocompass Errors
Caused by the vessel's own motion over the Earth's surface. A vessel traveling east or west introduces a component that causes the gyro to deviate slightly from True North. Error is easterly when heading east and increases with speed. Significant on fast vessels at low latitudes.
The gyrocompass becomes unreliable at high latitudes (above approximately 75°) because the horizontal component of Earth's rotation — which the gyro uses to align — becomes very small near the poles. Accuracy degrades significantly.
A sudden acceleration (course change, speed change) imparts a force on the gyro's pendulous vane system, causing a temporary deflection. The compass will oscillate and then settle. This can cause momentary large errors during sharp turns.
Gyrocompass vs Magnetic Compass
| Feature | Gyrocompass | Magnetic Compass |
|---|---|---|
| Reference | True North (geographic) | Magnetic North |
| Error types | Speed error, latitude error, precession | Variation + deviation |
| Effect of ship's metal | None | Significant (deviation) |
| Accuracy in polar regions | Degrades near poles | Very unreliable near poles |
| Power requirement | Requires continuous power; ~1-4 hours to settle | None |
| Backup | Magnetic compass is backup | Primary backup when gyro fails |
| Typical error | 0.1 to 0.5 degrees when settled | Up to several degrees without compensation |
| Cost | Expensive; large ship equipment | Low cost; small vessels standard |
3. Hand Bearing Compass
A small, handheld compass used to take bearings of objects on shore or other vessels. Essential for visual fixes. Uses the same variation and deviation corrections as the ship's compass, though deviation is usually smaller when held away from the vessel's metalwork.
Taking Bearings
- Stand as far from metal and electronics as possible
- Hold compass level; sight through prism or lens at the object
- Read the bearing when the object is aligned with the lubber's line
- Record as compass bearing; apply deviation (small on handheld) and variation to get true bearing
- Plot on chart as a line of position (LOP) from the charted object
- Take at least two bearings simultaneously (or rapid succession) for a fix
Compass Rose Usage
The outer ring of a compass rose shows True bearings. The inner ring shows Magnetic bearings (already incorporating local variation). When plotting a magnetic bearing from a hand bearing compass, use the inner ring to lay off the bearing — no variation correction needed.
4. Depth Sounder (Echo Sounder)
The depth sounder measures water depth by timing the return of a sound pulse from the seabed. Critical for safe navigation in shoal water and for confirming chart soundings.
Transducer
The transducer is mounted through the hull or on the transom. It converts electrical energy into sound pulses (usually 200 kHz for shallow water) and converts returning echoes back into electrical signals. Mounting position affects accuracy — air bubbles under the hull at speed can cause false readings.
Principle of Operation
A pulse of sound travels to the bottom at approximately 4,800 feet per second (1,500 m/s) in seawater. The instrument times the round trip and divides by two to get depth. The display shows depth below the transducer — add the transducer depth below the waterline to get depth below the surface (keel clearance matters).
Reading Chart Soundings
US chart soundings are in feet (on large-scale charts) or fathoms, referenced to MLLW (Mean Lower Low Water). A sounding of 12 on the chart means 12 feet at mean lower low water — the lowest the tide is likely to reach on average. Actual depth = chart sounding plus tide height above MLLW.
Correcting for Tide
Example: Sounder reads 15 feet. Tide is 3.0 feet above MLLW. Chart depth = 15 - 3 = 12 feet. This matches a 12-foot charted sounding, confirming your position over that shoal.
5. GPS and Chartplotter
GPS (Global Positioning System) is the primary navigation system for virtually all mariners. Understanding its limitations and settings is critical for safe use — and for the USCG exam.
HDOP — Satellite Geometry
HDOP (Horizontal Dilution of Precision) measures the quality of satellite geometry affecting horizontal position accuracy. Lower is better.
Datum — WGS-84 vs NAD-83
A chart datum defines the mathematical model of the Earth used to create the chart. GPS uses WGS-84. Most US coastal charts use NAD-83. The difference is very small (less than 2 meters) but in other parts of the world datum mismatches can be hundreds of meters.
Key GPS Concepts for the Exam
The actual direction the vessel is moving relative to the seabed, derived from changes in GPS position. Includes the effect of current and leeway. This is not the same as heading. COG is what matters for making good a course to a waypoint.
The actual speed of the vessel relative to the seabed. Unlike knotmeter speed (speed through water), SOG accounts for current. In a following current SOG exceeds knotmeter speed; in a head current SOG is less.
The distance the vessel is off the planned track (rhumb line or great circle route between waypoints). Displayed in nautical miles. A positive XTE means you are to the right of track; negative means left. Used to steer back on track.
Saved geographic positions (latitude and longitude) stored in the GPS. Used to define a route. The GPS shows bearing and distance to the next waypoint (BRG and DTW — distance to waypoint). Always verify waypoints against the chart before use.
The true (or magnetic, depending on settings) bearing from current position to the next waypoint. Used to steer toward the waypoint. Differs from COG when current or leeway is present — must steer a slightly different heading than BTW to make good the direct route.
Speed made good toward the destination or waypoint. Accounts for the vessel's COG angle relative to the desired track. In a crosswind or cross-current, VMG may be less than SOG. Useful for optimizing sailing angles.
6. Radar
Radar (Radio Detection And Ranging) is essential for navigation in restricted visibility. Required on many commercial vessels; common on cruising yachts. USCG exams test radar operation, controls, and collision avoidance concepts heavily.
Radar Controls Reference
| Control | Purpose |
|---|---|
| Gain | Amplifies the return signal |
| Sea Clutter (STC) | Reduces echoes from wave tops near own vessel |
| Rain Clutter (FTC) | Reduces precipitation echoes |
| Range | Sets displayed area radius |
| VRM (Variable Range Marker) | Measures range to a target |
| EBL (Electronic Bearing Line) | Measures bearing to a target |
ARPA — Automatic Radar Plotting Aid
ARPA tracks targets automatically and computes CPA, TCPA, true course, and true speed for each tracked target. Required on vessels 10,000 GT and above. Smaller vessels may have mini-ARPA. Must be monitored — ARPA data is only as good as the tracking quality; it can lose targets in sea clutter or swap tracks.
Relative vs True Motion
Own vessel is fixed at center. Target vectors show movement relative to own ship. Default mode; excellent for collision avoidance. A target moving toward the center is closing; a constant relative bearing with decreasing range = collision risk.
Own vessel moves across the screen (periodically resets). Targets show actual courses and speeds. Better for situation awareness and chart overlay; own-vessel drift and current effects are visible. Less intuitive for CPA assessment.
Collision Avoidance with Radar (Rule 7 and 8)
A target at a constant bearing with decreasing range is on a collision course. This applies whether you use radar or visual observation. Take action early and boldly — Rules 16 and 8 require action that is apparent to the other vessel. Small course changes are dangerous because they may not register on the other vessel's radar.
7. AIS — Automatic Identification System
AIS transponders broadcast and receive vessel identity, position, and navigation status on VHF frequencies. Required on SOLAS vessels; increasingly common on smaller commercial and recreational vessels.
Class A Transponder
- Required on SOLAS vessels (300 GT+ international; 500 GT+ domestic; all passenger ships)
- Transmits every 2–10 seconds underway, 3 min at anchor
- 12.5 watts output
- Transmits voyage data: destination, ETA, draught
- SOTDMA (Self-Organized Time Division Multiple Access) protocol
Class B Transponder
- Voluntary on smaller commercial and recreational vessels
- Transmits every 30 seconds underway, 3 min at rest
- 2 watts output — shorter range
- Does not transmit destination, ETA, or draught
- CSTDMA protocol — lower priority than Class A
AIS Data Fields
| Field | Description |
|---|---|
| MMSI | 9-digit Maritime Mobile Service Identity — unique vessel identifier |
| Vessel Name | Ship name as registered |
| Call Sign | Radio call sign |
| Position | Lat/Lon from own GPS antenna |
| COG | Course Over Ground |
| SOG | Speed Over Ground |
| Heading | True heading from gyrocompass or GPS compass |
| ROT | Rate of Turn (Class A only) |
| Navigational Status | Underway, at anchor, moored, not under command, etc. |
| Ship Type | Cargo, tanker, passenger, fishing, sailing, etc. |
| Destination | Next port of call (Class A only, manually entered) |
| ETA | Estimated time of arrival (Class A only, manually entered) |
| Draught | Current draught (Class A only, manually entered) |
AIS Limitations
Fishing vessels, recreational boats, and many small craft do not carry AIS. Never assume a clear AIS display means clear water.
AIS data is manually entered for some fields (destination, draught). Vessels may not update navigational status. Position relies on vessel's own GPS.
VHF range is typically 20–40 nm line of sight. AIS does not work over the horizon without satellite AIS (S-AIS).
AIS supplements radar and visual watch — it does not replace them. A vessel may be AIS-equipped but transmitting bad data.
To transmit on AIS, a vessel must have a programmed MMSI. MMSI is registered with the FCC or BoatUS for US recreational vessels.
AIS provides identity; radar provides an independent position. Use both together. A radar target without AIS may be a small boat, rock, or debris.
8. Sextant
The sextant measures the angle between a celestial body and the horizon, allowing a line of position to be calculated. Required knowledge for Master Near Coastal and offshore endorsements. Appears on OUPV exams only at the conceptual level; full sight reduction appears on Master exams.
Parts of the Sextant
| Part | Function |
|---|---|
| Frame | Rigid arc-shaped structure; graduated in degrees |
| Index arm (alidade) | Pivoting arm that moves along the arc |
| Micrometer drum | Fine adjustment; reads minutes and tenths |
| Index mirror | Attached to index arm; reflects the body being measured |
| Horizon mirror | Fixed; shows both horizon (clear half) and reflected body (silvered half) |
| Telescope | Magnifies view; reduces parallax |
| Shade glasses | Filters for sun observation without eye damage |
| Vernier scale | On older sextants; provides arc-second precision |
Altitude Corrections — Order of Application
Caused by misalignment of the index mirror. Check by setting arc to zero and looking at the horizon — if not a straight line, read the error. If the error is 'on the arc' (arc reads a positive number), subtract IE from Hs. If 'off the arc' (reads negative), add IE. Mnemonic: on is off; off is on.
The observer's height above sea level causes the visible horizon to be depressed below the true horizontal plane. The higher the eye, the greater the dip. Dip is always subtracted. Found in the dip table using height of eye in feet or meters.
The atmosphere bends light, making celestial bodies appear slightly higher than they actually are. Refraction is always subtracted. It is greatest near the horizon (can be 34' of arc at the horizon) and zero at the zenith. Standard refraction tables assume standard atmosphere.
When shooting the lower or upper limb of the sun or moon, a semi-diameter correction is applied to get the altitude of the center. For lower limb (most common), SD is added. For upper limb, SD is subtracted. Not needed for stars or planets.
For nearby bodies (moon, Venus), parallax in altitude (PA) is applied. The moon's horizontal parallax is significant — up to 61' of arc. For stars, parallax is negligible (less than 0.001'). HP (horizontal parallax) is found in the Nautical Almanac daily pages.
9. Barometer
The barometer measures atmospheric pressure and is one of the most valuable weather prediction tools available to the mariner. Pressure trends over time are more important than the absolute value.
Aneroid Barometer
Contains a sealed metal capsule (aneroid cell) that expands and contracts with pressure changes. Movement is mechanically linked to a needle on a graduated dial. No liquid — safe at sea, not affected by motion. Most common type on boats. Must be calibrated against a known standard (weather service reading corrected to sea level).
Mercury Barometer
The original and most accurate barometer. Uses the weight of a mercury column to measure atmospheric pressure. Standard pressure = 760 mm Hg = 29.92 inches Hg = 1013.25 mb (hPa). Fragile, hazardous if broken, and sensitive to motion — not practical at sea. Used as the calibration standard ashore. Still referenced in USCG exam questions on units.
Pressure Units Reference
| Measurement | Standard Sea Level Pressure |
|---|---|
| Millibars (mb) | 1013.25 mb |
| Hectopascals (hPa) | 1013.25 hPa |
| Inches of Mercury (inHg) | 29.92 "Hg |
| Millimeters of Mercury (mmHg) | 760 mmHg |
Reading Pressure Trends
Approaching low — rain likely in 12–24 hours
Strong storm approaching — rough seas, strong winds
Severe storm or gale — seek shelter immediately
Clearing weather, high pressure building — improving
Strong gradient — winds may increase temporarily behind the front
Fair weather typical; sustained high = drought risk ashore
The 3-2-1 Rule
A practical rule of thumb for storm intensity based on pressure drop over 3-hour intervals:
11. Chart Plotting Tools
Chart plotting skills — using parallel rules, dividers, and the compass rose — are tested directly on the USCG exam with actual chart problems. Practice with a real nautical chart.
Transfer a bearing from compass rose to chart position (or vice versa)
Walk rules across chart by alternating; check each step or you will slip
Lay directly on a meridian or parallel to read course without a compass rose
Faster than parallel rules; less slippage error on small-scale charts
Measure distance by stepping off against the latitude scale (not longitude)
Always measure distance on the latitude scale at the same latitude as the area of interest
Outer ring = true directions; inner ring = magnetic directions (with variation noted)
Read variation from inner ring label; note year and annual change for corrections
Fix position from three charted objects by simultaneous bearings
Extremely accurate; not affected by compass errors
Alternative to parallel rules; rolls along a straight edge
Less common aboard; can slip if straight edge is not secured
Nautical Chart Symbols — Essential Exam Items
| Symbol / Abbreviation | Meaning |
|---|---|
| R (on buoy) | Red buoy (even-numbered; return from sea on starboard) |
| G (on buoy) | Green buoy (odd-numbered; return from sea on port) |
| RW (on buoy) | Safe water mark (fairway / mid-channel); red and white vertical stripes |
| fl R 4s | Flashing red light, 4-second period |
| fl (2+1) G 6s | Group flashing green — 2 flashes, then 1, every 6 seconds (preferred channel marker) |
| Wk | Wreck — potentially hazardous |
| Obstn | Obstruction — shallow or submerged hazard |
| PA | Position Approximate — not precisely located |
| ED | Existence Doubtful — charted feature may not exist |
| M, S, G, R, Sh, Co, Cy | Bottom composition: Mud, Sand, Gravel, Rock, Shells, Coral, Clay |
| Anchoring symbol | Recommended anchorage; circle with anchor symbol |
| DW | Deep Water (route or lane) |
| Subm piles | Submerged pilings — danger at low tide |
| 1013mb (on chart) | Not on nautical charts; barometric pressures appear on weather charts |
12. USCG Exam Tips — Navigation Instruments
The navigation instruments section tests both conceptual knowledge and calculation skills. Here are the areas where students most often lose points.
- →Memorize CDMVT (Can Dead Men Vote Twice) and the East = add, West = subtract rule
- →When converting True to Compass, reverse all signs: East becomes subtract, West becomes add
- →Always check: is the answer reasonable? A big deviation or variation change should move the answer significantly
- →Deviation comes before variation — always apply D before V in the mnemonic
- →USCG exam problems often give a deviation table — interpolate between entries
- →Know the difference between COG/SOG (over ground) and heading/knotmeter (through water)
- →Datum mismatch questions: GPS is WGS-84; charts may be NAD-83 or other datums
- →HDOP below 2 is good; above 5 is a warning
- →XTE tells you how far off track you are — positive = right of track, negative = left
- →Waypoints must be verified against the chart — GPS shows you the waypoint, not the hazard between you and it
- →Constant bearing + decreasing range = collision course (Rule 7)
- →Gain too high: noise and clutter obscure targets. Gain too low: weak targets disappear
- →Sea clutter suppresses close-range returns — small boats near you may disappear if FTC is over-applied
- →ARPA gives CPA and TCPA — know these abbreviations and what they mean
- →Relative motion: target moving toward center of screen is closing
- →Sounder reads depth below transducer — add transducer depth below waterline for true depth
- →Chart soundings are at MLLW — add tide height to get actual depth at that moment
- →Speed can cause aeration under hull, giving false shallow readings
- →A double trace or second bottom reading indicates a thermocline or density layer
- →Class A: required on SOLAS vessels, 12.5W, transmits every 2–10 seconds
- →Class B: voluntary, 2W, every 30 seconds — often misses fast-approaching contacts
- →MMSI is 9 digits — required to transmit on AIS
- →AIS does not replace radar — vessels without AIS (small boats, submarines) will not appear
- →AIS navigational status is manually set — it may be wrong (vessel may show 'at anchor' while underway)
- →Index error: on the arc = subtract, off the arc = add
- →Dip is always subtracted — height of eye table required
- →Refraction is always subtracted — atmospheric correction table required
- →Hs (sextant altitude) → apply IC → apply Dip → get Ha (apparent altitude) → apply altitude corrections → get Ho (observed altitude)
- →Ho (observed) is compared to Hc (computed) to plot an intercept (toward or away from the body's GP)
What to Memorize (Short List)
13. Practice Problems with Solutions
Work through these problems before your exam. Each mirrors actual USCG question formats. Try to solve before expanding the answer.
1Your compass reads 215. Deviation is 5W. Variation is 10E. What is the true course?Show Answer
- 1.Start: Compass = 215
- 2.Apply deviation (5W = subtract): 215 - 5 = 210 Magnetic
- 3.Apply variation (10E = add): 210 + 10 = 220 True
2You observe the sun and get a sextant altitude (Hs) of 42 degrees 18.4 minutes. Index error is 2.4 minutes off the arc. Height of eye is 10 feet. What is the apparent altitude (Ha)?Show Answer
- 1.IE off the arc = add: 42-18.4 + 2.4 = 42-20.8
- 2.Dip for 10 ft height of eye (from table) = -3.1 minutes: 42-20.8 - 3.1 = 42-17.7
- 3.Ha = 42 degrees 17.7 minutes
3A depth sounder reads 18 feet. The chart datum is MLLW. The current tide height is 4.2 feet above MLLW. What is the actual depth of water?Show Answer
- 1.Sounder reads actual water depth (not chart depth)
- 2.Chart depth = sounder reading - tide height = 18 - 4.2 = 13.8 feet (charted depth at MLLW)
- 3.Or: actual depth = charted depth + tide = 13.8 + 4.2 = 18 feet
- 4.The vessel has 18 feet of actual water beneath it right now
4Your GPS shows COG 090, SOG 6 knots. Your heading is 095 and knotmeter shows 6.5 knots. What can you infer?Show Answer
- 1.COG (090) differs from heading (095) by 5 degrees to port
- 2.SOG (6.0) is less than knotmeter (6.5)
- 3.A current is setting you slightly to port (north) and reducing your speed
- 4.You are experiencing a current from starboard that is pushing you left of heading
5On radar you observe a target at bearing 040 relative. Over 6 minutes the bearing has not changed but range has decreased from 5.0 nm to 3.5 nm. What action should you take?Show Answer
- 1.Constant bearing + decreasing range = collision risk (Rule 7)
- 2.CPA = 0 if nothing changes
- 3.You are the give-way vessel unless the other is overtaking or this is a crossing where you are stand-on
- 4.Take bold, early, and substantial action per Rule 8: alter course significantly to starboard or reduce speed
Frequently Asked Questions
What is the difference between variation and deviation on a compass?
Variation is the angle between true north and magnetic north at a given location — it is caused by the Earth's magnetic field and is printed on nautical charts. It changes with geographic location and slowly over time. Deviation is the error caused by the vessel's own magnetic influences (engines, electronics, metal) acting on the compass needle. It differs by heading. To convert compass to true: apply deviation first (Compass + East Dev = Magnetic), then variation (Magnetic + East Var = True). Use the phrase 'Can Dead Men Vote Twice' or 'CDMVT' to remember the order.
What does HDOP mean on a GPS and why does it matter?
HDOP stands for Horizontal Dilution of Precision. It is a measure of GPS satellite geometry quality. A low HDOP (ideally less than 2) means satellites are well-spread across the sky and the position fix is more accurate. A high HDOP (above 5) means satellites are clustered together, degrading accuracy. On coastal passages, always check HDOP before relying on a GPS position for a close approach. WGS-84 is the standard datum used by GPS — always verify your chartplotter datum matches your paper chart datum (NAD-83 on many US charts is very close but not identical to WGS-84).
What is the difference between COG and heading on a GPS chartplotter?
Heading is the direction the bow of the vessel is pointed, measured from north. COG (Course Over Ground) is the actual direction the vessel is traveling over the seabed, accounting for leeway and current. In a strong beam current, a vessel might be headed 090 degrees but making good a COG of 080 degrees — the current is setting it north. COG is what the GPS measures from actual position changes. For navigation, COG is what matters for arrival at a waypoint; heading matters for collision avoidance and radar bearings.
What is CPA and TCPA on radar?
CPA is Closest Point of Approach — the minimum distance that will exist between your vessel and a target if both vessels maintain their current courses and speeds. TCPA is Time to Closest Point of Approach — how many minutes until CPA is reached. ARPA (Automatic Radar Plotting Aid) calculates CPA and TCPA automatically for tracked targets. A CPA of zero means a collision. Mariners set a CPA alarm (commonly 0.5 to 1 nautical mile) so they are alerted when a target's projected path comes within the guard zone.
What is the difference between Class A and Class B AIS transponders?
Class A AIS transponders are required on SOLAS vessels (commercial ships 300 GT and above on international voyages, cargo ships 500 GT and above domestically, and all passenger ships). They transmit position every 2 to 10 seconds underway, with a 12.5-watt output. Class B transponders are used on smaller commercial and recreational vessels. They transmit less frequently (every 30 seconds underway), use 2 watts, and do not transmit voyage data or destination. MMSI (Maritime Mobile Service Identity) is the 9-digit vessel identifier broadcast by AIS — it links to vessel name, call sign, and flag state in databases.
How do you correct a sextant altitude for index error, dip, and refraction?
Sextant altitude corrections are applied in order: (1) Index Error (IE) — if the arc is on the arc, subtract; if off the arc, add. Mnemonic: 'If it's on, it's off; if it's off, it's on.' (2) Dip — always subtracted from sextant altitude (Hs) to get apparent altitude (Ha). Dip accounts for the observer's height of eye above sea level; higher eye = greater dip correction. (3) Refraction — always subtracted (atmosphere bends light upward, making objects appear higher than they are). Additional corrections apply for the sun (semi-diameter, parallax). The result after all corrections is the true altitude (Ho — Observed altitude).
What is the 3-2-1 barometric pressure rule?
The 3-2-1 rule helps mariners gauge storm intensity from barometric pressure drops. A drop of 3 millibars per 3 hours indicates a moderate storm approaching. A drop of 6 millibars per 3 hours indicates a strong storm. A drop of 9 or more millibars per 3 hours indicates a violent storm (hurricane or severe gale). Some sources state it as: 3 mb drop = caution; 6 mb drop = severe; 9+ mb drop = extreme danger. Standard sea-level pressure is 1013.25 mb (29.92 inches of mercury). A rising barometer after a low indicates improving weather; a rapid rise can itself herald unsettled conditions.
Drill navigation instruments until they're automatic
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