{"id":51627,"date":"2026-04-17T21:42:47","date_gmt":"2026-04-17T20:42:47","guid":{"rendered":"https:\/\/maritimehub.co.uk\/?p=51627"},"modified":"2026-04-17T21:42:47","modified_gmt":"2026-04-17T20:42:47","slug":"compass-error-checks-in-practice","status":"publish","type":"post","link":"https:\/\/maritimehub.co.uk\/compass-error-checks-in-practice\/","title":{"rendered":"Compass error checks in practice"},"content":{"rendered":"
\n

BRIDGE \u2192 Compasses<\/strong><\/p>\n

Position on the Bridge<\/strong><\/p>\n

System Group:<\/strong> Navigation<\/p>\n

Primary Role:<\/strong> Continuous verification that the compass being steered is reading true against a known reference<\/p>\n

Interfaces:<\/strong> Magnetic compass, gyrocompass, GPS heading sensor, chart plotter, ECDIS, deviation card, celestial almanac<\/p>\n

Operational Criticality:<\/strong> High \u2014 an undetected compass error silently corrupts every course steered and every bearing logged<\/p>\n

Failure Consequence:<\/strong> Accumulated navigational error not apparent until a discrepancy with a fixed reference or, in the worst case, a grounding or collision whose post-incident track reconstruction reveals a steady unexplained offset throughout the watch<\/p>\n<\/div>\n

A compass does not tell you where you are going.<\/em>
It tells you which way the ship is pointing \u2014 and only if it has been checked.<\/em><\/p>\n

Introduction<\/h2>\n

Compass error checking is one of the oldest disciplines in watchkeeping, and one of the most neglected. The gyro is running, the autopilot is engaged, the ECDIS track looks clean. Nothing is alarming. The watch feels quiet. In that quiet, the practice of actually verifying the compass \u2014 magnetic and gyro both \u2014 slides from discipline into ritual, and from ritual into fiction.<\/p>\n

The log entry gets made. The number might come from a genuine observation, from the deviation card without any comparison, or from the entry on the previous page. On a passage with no coastal features and a cloudy sky, weeks can pass before anyone takes a bearing that is worth anything. By then the vessel has been steered on a heading whose actual truth is unknown to the officers who logged it as verified.<\/p>\n

This article is about the practice: how compass error is actually obtained at sea, which methods are reliable in which conditions, what the relationship is between the observed error and the deviation card, and why a significant error discovered during a watch is not simply an administrative inconvenience but a navigation event that requires immediate action on the chart.<\/p>\n

Contents<\/h2>\n
    \n
  • 1. What compass error is and what it is not<\/li>\n
  • 2. Azimuth of a heavenly body<\/li>\n
  • 3. Transit bearings ashore<\/li>\n
  • 4. Using the gyro\/magnetic comparison<\/li>\n
  • 5. The deviation card and its limits<\/li>\n
  • 6. When methods become unreliable<\/li>\n
  • 7. Discovering a significant error mid-watch<\/li>\n
  • 8. The log entry problem<\/li>\n
  • 9. Closing Reality<\/li>\n<\/ul>\n

    1. What compass error is and what it is not<\/h2>\n

    Compass error is the total angular difference between the direction the compass indicates and true north. It is the sum of variation and deviation. Variation is a property of the place; deviation is a property of the ship on that heading. Neither is fixed. Variation changes with geographic position \u2014 slowly over years, more noticeably over long passages. Deviation changes with every alteration of course.<\/p>\n

    The gyrocompass has neither variation nor deviation in the magnetic sense, but it has its own errors: latitude error, speed error, and the slow accumulative drift that characterises any gyro that has been running for days without reset. Comparing gyro to magnetic compass gives a useful cross-check, but only if at least one of them has been independently verified against a true reference.<\/p>\n

    That is the point most officers grasp intellectually and underweight in practice. A gyro\/magnetic comparison tells you the difference between the two. It does not tell you whether either one is correct.<\/p>\n

    Two instruments agreeing is not the same as two instruments being right.<\/em><\/p>\n

    2. Azimuth of a heavenly body<\/h2>\n

    The azimuth method remains the most reliable way of obtaining compass error at sea when no fixed terrestrial references are available. The principle is straightforward: the computed true bearing of the sun, moon, star, or planet at the moment of observation is compared with the bearing observed by compass. The difference is compass error.<\/p>\n

    In practice the method requires the observer to be at the compass, taking the bearing at the instant of observation, not estimating it from memory while writing down the time. A bearing taken two or three minutes before the time logged introduces an error that defeats the purpose. With the sun near the horizon and moving quickly in azimuth \u2014 particularly in low latitudes \u2014 a timing error of sixty seconds can shift the computed azimuth by more than a degree.<\/p>\n

    Amplitude \u2014 the bearing of the sun at the moment of rising or setting \u2014 is a simplified form of the azimuth method. It is quick and does not require an accurate time, since the sun’s rate of azimuth change is near zero at the horizon. The observed compass bearing at the moment of rising or setting is compared with the computed amplitude. The drawback is atmospheric refraction, which lifts the sun’s visible disc above the horizon before it has geometrically risen. Observing amplitude requires the sun’s lower limb to be approximately half a diameter above the visible horizon, not at it.<\/p>\n

    The azimuth of a star observed well clear of the horizon \u2014 between about 15\u00b0 and 45\u00b0 altitude \u2014 is generally the most accurate single observation available to a watchkeeper with a compass bearing ring and a corrected almanac. The star is a point source, the bearing is unambiguous, and refraction at those altitudes is small and well-tabulated.<\/p>\n

    3. Transit bearings ashore<\/h2>\n

    In coastal waters, the transit \u2014 two charted objects in line \u2014 provides compass error without any computation beyond looking up the chart. When two objects are in transit, their true bearing is read directly from the chart by drawing a line through them and measuring it. The observed compass bearing of that same line gives compass error immediately.<\/p>\n

    The method is simple but its conditions are exacting. Both objects must be positively identified. Both must appear on a chart of adequate scale. The line between them must be precisely the line of observation \u2014 which means the observer must be genuinely in line, not approximately in line. A bearing taken when the objects are near the beam is far more sensitive to small positional errors than one taken when the line is closer to ahead or astern, and a small uncertainty in the vessel’s athwartships position produces a disproportionate bearing error.<\/p>\n

    A transit taken too close to the beam is not an error check. It is a source of error.<\/em><\/p>\n

    For this reason, transits used for compass error work should, wherever possible, subtend a bearing within roughly 45\u00b0 of the ship’s heading. Lines near the beam \u2014 particularly on a vessel making leeway or crabbing on a tidal set \u2014 are unreliable even when the objects appear to be in line from the bridge wing. The parallax between the compass position and the bridge wing observation position compounds the geometry.<\/p>\n

    The other requirement is that the objects be far enough apart, and far enough from the ship, that identification is certain. Two church spires that look similar, or a lighthouse and a building that could be confused in poor visibility, introduce an error that no amount of technique corrects.<\/p>\n

    4. Using the gyro\/magnetic comparison<\/h2>\n

    Every ship with both a gyrocompass and a magnetic compass should be logging the gyro\/magnetic difference at regular intervals. On a passage with multiple course alterations, a comparison on each heading provides a running picture of whether the deviation is behaving as the card predicts.<\/p>\n

    The value of this comparison is real but bounded. It detects relative change. If both instruments are offset by the same amount \u2014 an unlikely but not impossible scenario \u2014 the comparison shows nothing amiss. If the gyro is drifting and the magnetic compass is steady, the comparison will change, but the change looks identical to what would be produced by a deviation shift on the magnetic. Distinguishing between the two requires at least one independent check against a true reference.<\/p>\n

    The practical use of the gyro\/magnetic comparison is as a sentinel. A discrepancy that falls outside what the deviation card predicts for the current heading is a prompt to investigate immediately, not to log and continue. A discrepancy that has been widening slowly over successive watches \u2014 even if each individual entry looks plausible \u2014 is the signature of gyro drift that has not been caught because each watch officer compared against the previous entry rather than against a computed truth.<\/p>\n

    Gyro drift is insidious precisely because it is slow.<\/em><\/p>\n

    5. The deviation card and its limits<\/h2>\n

    The deviation card is a snapshot of the magnetic compass’s behaviour at the time the last compass adjustment or swing was carried out. It is not a guarantee of current behaviour.<\/p>\n

    Deviation changes when the ship’s magnetic signature changes. A long passage through high magnetic latitudes, a cargo change that significantly redistributes ferrous mass, major repair or modification involving steelwork, a lightning strike \u2014 all of these can alter the deviation pattern. A card that was accurate when swung may be substantially wrong now, and there is no alarm that fires to announce the change.<\/p>\n

    The deviation card is a starting point for estimating what the deviation should be on a given heading. When an azimuth or transit check shows a compass error that does not match the combination of charted variation and card deviation, the discrepancy demands explanation. Either the observation was made poorly, the card is out of date, or there has been a change in the compass’s magnetic environment. None of those explanations is comfortable. All of them require action.<\/p>\n

    A deviation card that has not been updated in two or more years should be treated with suspicion on any vessel that has had significant cargo or structural changes. The STCW requirement to check compass error regularly exists precisely because the card alone is not sufficient.<\/p>\n

    6. When methods become unreliable<\/h2>\n

    The azimuth method fails in overcast conditions with no visible celestial bodies. It also fails when the observer lacks a reliable time source or an accurate almanac reduction \u2014 less common now than it once was, but GPS time and electronic almanacs introduce their own dependency on systems being operational and correctly set.<\/p>\n

    Transit bearings fail when no suitable pairs of charted objects exist, when the chart scale is too small to measure the transit bearing accurately, when identification is uncertain, and when the geometry is unfavourable. Offshore, they are usually unavailable. In congested coastal waters with poor visibility, they may be geometrically available but practically dangerous to execute \u2014 the time spent lining up objects on the bridge wing is time not spent managing traffic.<\/p>\n

    The gyro\/magnetic comparison, as noted, fails to detect correlated errors in both instruments. It also fails when the magnetic compass is disturbed by a local anomaly \u2014 a vessel overtaking close aboard with a different magnetic signature, an anchor chain that has shifted, or electrical equipment that has been switched on near the compass binnacle.<\/p>\n

    There is no single method that is reliable in all conditions. The watchkeeper who has only one technique for obtaining compass error will eventually be in conditions where that technique is unavailable or degraded, and will either make something up or copy forward from a better day.<\/p>\n

    7. Discovering a significant error mid-watch<\/h2>\n

    A compass error discovered during a watch \u2014 one that differs significantly from what has been logged and assumed \u2014 is not a logging problem. It is a navigational event.<\/p>\n

    The immediate question is not how to record it. The immediate question is: what is the ship’s actual position? Every course steered since the last reliable fix has been steered on a heading that was offset by a potentially unknown amount. Depending on the time elapsed, the vessel’s speed, and the waters being navigated, that offset may have placed the ship materially away from its assumed track.<\/p>\n

    An uncorrected compass error does not stay in the log. It goes into the water under the keel.<\/em><\/p>\n

    The correct response is to fix the position independently \u2014 by GPS, radar, visual bearing, or depth \u2014 and compare it with the DR position that the erroneous course would have produced. The difference quantifies the accumulated error. If the vessel is in confined waters or approaching shoaling ground, the OOW must slow down or stop until the picture is clear. The master must be informed.<\/p>\n

    The passage plan must then be reviewed. A consistent compass error that has been present since the last coastal fix means every waypoint arrival time, every planned track, and every safety margin calculated from that track needs to be reassessed against the actual rather than assumed position. This is not paperwork. This is the difference between navigating and assuming.<\/p>\n

    8. The log entry problem<\/h2>\n

    The practice of copying compass error log entries from the previous watch is not confined to careless or junior officers. It occurs on well-run ships, in good weather, because conditions are stable and the previous entry looks reasonable. Over a long passage in clear weather with good celestial opportunities, a watch officer who has actually done the work sets an example that the next watch copies \u2014 not the observation, just the number.<\/p>\n

    The failure mode is not dramatic. The log looks consistent. The entries show sensible values on various headings, cross-referenced against the deviation card, with small gyro\/magnetic differences. Everything appears to be in order. The first indication that the numbers have been fabricated may be the grounding investigation that reconstructs the track.<\/p>\n

    Certain patterns in compass logs should prompt scrutiny. Identical values across multiple watches despite course changes that would alter deviation. Errors that match the deviation card precisely and consistently \u2014 actual observations rarely match the card exactly. Absence of any observed error during a period of documented celestial visibility.<\/p>\n

    The purpose of the compass error log is not to demonstrate that the work was done. The purpose is to track the actual behaviour of the compass over time so that trends \u2014 developing deviation anomalies, progressive gyro drift \u2014 can be detected before they become navigational hazards. A log that is fabricated destroys that function entirely, regardless of how professionally it is formatted.<\/p>\n

    A log filled with numbers that were never observed is worse than a blank page. A blank page is honest about what it does not know.<\/em><\/p>\n

    9. Closing Reality<\/h2>\n

    Compass error checking is one of the few watchkeeping disciplines where the navigator holds a direct line of responsibility between observation and safety. No alarm will sound when the deviation drifts. No system will flag that the gyro has been accumulating error for three days. No audit will catch a log filled with copied entries until something goes wrong downstream.<\/p>\n

    The azimuth method, the transit, the gyro\/magnetic comparison \u2014 none of them is difficult. All of them require being done, with attention, at intervals that actually catch changes before those changes matter. The discipline is simple. What erodes it is the apparent stability of a vessel that is steering a steady course in calm weather with the ECDIS looking clean and no alarms active.<\/p>\n

    That apparent stability is exactly when the check is most important, because it is exactly when the check is most likely to be skipped.<\/p>\n

    The compass does not know whether it is being watched.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

    The watchkeeping discipline of checking compass error: azimuths, transits, deviation cards, and why a discovered error mid-watch is a passage planning problem.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"fifu_image_url":"","fifu_image_alt":"","c2c-post-author-ip":"2a02:c7c:2ef8:2400:931:afb1:9971:4a62","footnotes":""},"categories":[10,1],"tags":[9172,9102,9171,9170,9173,9174,2726,9123],"class_list":["post-51627","post","type-post","status-publish","format-standard","hentry","category-bridge","category-latest","tag-azimuth","tag-bridge-procedures","tag-compass-error","tag-compasses","tag-deviation","tag-gyrocompass","tag-navigation","tag-watchkeeping"],"acf":[],"_links":{"self":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts\/51627","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Fcomments&post=51627"}],"version-history":[{"count":1,"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts\/51627\/revisions"}],"predecessor-version":[{"id":51634,"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts\/51627\/revisions\/51634"}],"wp:attachment":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Fmedia&parent=51627"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Fcategories&post=51627"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Ftags&post=51627"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}