{"id":51604,"date":"2026-04-17T21:26:20","date_gmt":"2026-04-17T20:26:20","guid":{"rendered":"https:\/\/maritimehub.co.uk\/?p=51604"},"modified":"2026-04-17T21:26:20","modified_gmt":"2026-04-17T20:26:20","slug":"enc-vs-rnc-understanding-chart-types","status":"publish","type":"post","link":"https:\/\/maritimehub.co.uk\/enc-vs-rnc-understanding-chart-types\/","title":{"rendered":"ENC vs RNC \u2013 understanding chart types"},"content":{"rendered":"
\n

BRIDGE \u2192 ECDIS & Charts<\/strong><\/p>\n

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

System Group:<\/strong> Navigation \/ Chart Display and Information<\/p>\n

Primary Role:<\/strong> Provision of georeferenced nautical chart data for passage planning and navigation<\/p>\n

Interfaces:<\/strong> ECDIS, GPS\/GNSS, AIS, ARPA, echo sounder, voyage management systems, port state control documentation<\/p>\n

Operational Criticality:<\/strong> Absolute \u2013 chart data underpins every navigational decision from berth to berth<\/p>\n

Failure Consequence:<\/strong> Use of incorrect chart type or misunderstood display scale leads to position errors, undetected hazards, grounding; RNC-based ECDIS operation invalidates trading certificates under SOLAS V<\/p>\n<\/div>\n

A chart is not an image of the sea.
It is a structured record of what was measured, when, and to what standard.<\/em><\/p>\n

Introduction<\/h2>\n

The transition to mandatory ECDIS carriage brought with it a confusion that persists on the bridge today. Two chart types load into the same display. Both produce a recognisable picture of coastline and soundings. Both scroll, zoom, and overlay GPS positions. From the front of a darkened wheelhouse at 0300, they look identical.<\/p>\n

They are not identical. The difference is not cosmetic or administrative. It reaches into how hazards are detected, how alarms are generated, how far a displayed position can be trusted, and whether the ship is legally navigating to the SOLAS standard at all. Conflating the two is one of the most consequential misunderstandings in modern bridge practice.<\/p>\n

Port state control examiners know this. Vetting inspectors know this. The officers who learned ECDIS through a four-hour familiarisation course and have never been corrected often do not.<\/p>\n

Contents<\/h2>\n
    \n
  • 1. What the terms actually mean<\/li>\n
  • 2. Data structure and why it determines everything<\/li>\n
  • 3. Official and unofficial charts \u2013 a distinction with legal teeth<\/li>\n
  • 4. Datum: chart datum and horizontal datum<\/li>\n
  • 5. Scale, compilation scale, and the zoom trap<\/li>\n
  • 6. Why an RNC passage plan is not an ECDIS passage plan<\/li>\n
  • 7. Safety contours, safety depth, and the ENC advantage<\/li>\n
  • 8. Practical implications for the watchkeeper<\/li>\n
  • Closing Reality<\/li>\n<\/ul>\n

    1. What the terms actually mean<\/h2>\n

    ENC stands for Electronic Navigational Chart. It is a vector dataset compiled to IHO S-57 (or the newer S-101) specification, produced by or under the authority of a national hydrographic office, distributed in a format that ECDIS systems are type-approved to read. It is the only chart type that satisfies the SOLAS requirement for ECDIS carriage.<\/p>\n

    RNC stands for Raster Navigational Chart. It is a digital scan of a paper chart \u2013 a georeferenced image, nothing more. The BSH produces them. The UKHO produces ARCS. The NGA produces RNC products for US waters. They are legitimate products from authoritative sources. They are not ENCs and cannot substitute for them.<\/p>\n

    The terminology matters because the IMO performance standards for ECDIS, and SOLAS regulation V\/19, are explicit: the chart data used must be the equivalent of the up-to-date official chart. For ECDIS, that means ENC. An ECDIS running on RNCs does not meet the carriage requirement. The ship is, in the eyes of the convention, navigating without charts.<\/p>\n

    2. Data structure and why it determines everything<\/h2>\n

    A raster chart is a grid of pixels. Each pixel holds colour information. The ECDIS knows where the image sits geographically, but it has no knowledge of what the colours represent. A sounding, a separation scheme boundary, a wreck symbol, a depth contour \u2013 to the raster system, these are all equally meaningless arrangements of coloured dots.<\/p>\n

    A vector ENC is a database. Every object in it has attributes. A sounding is not a number printed on a chart \u2013 it is a depth value, with a positional accuracy indicator, a source survey date, and a quality code. A wreck is not a symbol; it is an object with a classification (dangerous, non-dangerous, depth unknown), coordinates, and metadata linking back to the source data. A depth contour is a geometric line with a depth value attached to it that the system can interrogate.<\/p>\n

    This structure is what makes ECDIS anti-grounding functions possible. The safety contour alarm, the look-ahead guard zone, the shallow water highlighting \u2013 all of these depend on the system being able to ask a question of the chart data. With an ENC, it can ask: is there an object within this corridor with a depth less than my safety contour value? With an RNC, it cannot ask anything. It can only display.<\/p>\n

    A raster chart is a picture. An ENC is a database. Navigating on a picture and navigating on a database are not the same act.<\/em><\/p>\n

    3. Official and unofficial charts \u2013 a distinction with legal teeth<\/h2>\n

    Not all ENCs are official ENCs. This is where the market has introduced a dangerous ambiguity. Several commercial chart producers supply vector chart data that loads into ECDIS-like displays, overlays GPS positions convincingly, and provides a navigational picture. Some of these products are explicitly not ENCs in the SOLAS sense. They are unofficial vector charts \u2013 sometimes called navigational charts or enhanced charts \u2013 compiled from various sources and not issued under hydrographic office authority.<\/p>\n

    The ECDIS performance standards require the use of official charts from authorised distributors. AVCS (the UKHO’s service) and PRIMAR, IC-ENC, and similar regional services distribute ENCs that carry the issuing hydrographic office’s authority. A chart that did not originate from that pipeline, regardless of how it displays, is not an official ENC.<\/p>\n

    The practical test is straightforward: does the ENC cell carry an issuing authority code from a recognised IHO member state hydrographic office? Is it distributed through an authorised RENC or authorised distributor? Is it updated through the SENC update mechanism? If any of these answers is uncertain, the chart’s status is uncertain.<\/p>\n

    Some vessels run unofficial vector data specifically in areas where ENC coverage is sparse or where ENC compilation quality is known to be poor. This is an understandable operational response to a real problem. It does not make the data official, and it does not satisfy the carriage requirement. The master who understands this and makes a documented risk-based decision is in a different position from the officer who simply does not know the difference.<\/p>\n

    4. Datum: chart datum and horizontal datum<\/h2>\n

    Two separate datum concepts apply to any chart, and both carry navigational consequence.<\/p>\n

    Chart datum governs the vertical reference for soundings and drying heights. By convention, chart datum is set near the level of lowest astronomical tide, ensuring that charted depths represent a worst-case available water depth. This is not a navigational technicality \u2013 it is the mechanism that gives a sounding its safety meaning. A charted depth of 4.5 metres means approximately 4.5 metres at the worst tidal state, not at mean sea level.<\/p>\n

    Horizontal datum governs the geographic coordinate system to which positions on the chart are referenced. GNSS receivers output positions in WGS84. Most ENCs are also referenced to WGS84, which is why GPS positions plot directly onto the chart without correction. Many older paper charts, and the RNCs scanned from them, use local geodetic datums \u2013 ED50 in Europe, Tokyo Datum in Japan, Indian Datum in parts of South Asia. The shift between WGS84 and a local datum can be 50 to 300 metres depending on location.<\/p>\n

    A GPS position plotted on a chart referenced to a different horizontal datum will be offset. In open water this is an acceptable navigation discrepancy. In confined waters near a shoal or a breakwater, a 200-metre offset is a grounding risk that the display will not warn about because it does not know it exists.<\/p>\n

    ENCs carry the horizontal datum in the metadata. A properly functioning ECDIS applies any necessary WGS84 transformation automatically. An RNC displayed in ECDIS may or may not carry adequate datum shift information, and even where it does, the system’s handling of that shift requires verification. The datum note on a raster chart image is text on a scan. The system cannot read it.<\/p>\n

    The GPS position is always in WGS84. The question is whether the chart agrees.<\/em><\/p>\n

    5. Scale, compilation scale, and the zoom trap<\/h2>\n

    Three scale concepts must be kept separate, because ECDIS conflates them visually while treating them very differently in terms of safety function.<\/p>\n

    Compilation scale is the scale at which the source survey data was collected and at which the chart was produced. An ENC cell compiled at 1:50,000 contains positional accuracy and sounding density appropriate to that scale. The data cannot support navigational decisions that require the accuracy of a 1:5,000 harbour chart, regardless of how the display is set.<\/p>\n

    Display scale is the scale at which the ECDIS is currently rendering the chart. The officer can zoom in until a harbour fills the entire screen. The display scale may read 1:2,000. The compilation scale of the underlying data remains 1:50,000. Zooming in has not improved the data. It has stretched it.<\/p>\n

    On an RNC, zooming beyond compilation scale degrades a pixel image and provides no additional information. A sounding that was a small printed number at the paper chart’s intended scale becomes a blurred artefact. Contour lines lose their precision. The officer sees what appears to be a detailed picture of a berth approach while navigating on data never intended for that purpose.<\/p>\n

    On an ENC, the consequence of over-scaling is different but equally dangerous. The database may not contain objects below a certain significance threshold at the compilation scale. A 4-metre shoal that would appear on a 1:10,000 harbour chart may not exist as an object in a 1:50,000 coastal cell. The display looks clean. The safety contour does not alarm. The shoal is simply absent from the data.<\/p>\n

    ECDIS systems are required to warn the officer when the display scale exceeds the compilation scale of the ENC in use. This warning \u2013 the over-scale indication \u2013 is a critical safety signal. On many bridges it has been normalised to the point of invisibility. The display carries the indicator permanently during port approaches and nobody responds to it.<\/p>\n

    Over-scale is not a display preference. It is a warning that the chart cannot support the navigation being attempted.<\/em><\/p>\n

    6. Why an RNC passage plan is not an ECDIS passage plan<\/h2>\n

    The IMO and flag state guidance on ECDIS passage planning is built on the assumption that the underlying chart is an ENC. The entire safety checking framework \u2013 automatic cross-track alarm generation, look-ahead hazard detection, safety contour verification along the route \u2013 depends on the system being able to interrogate the chart data against ship parameters.<\/p>\n

    When a route is planned over an RNC, the ECDIS can draw the line on the picture. It cannot check the route. It cannot detect that the planned track passes over a wreck because it cannot see the wreck as an object. It cannot verify that the safety contour is not crossed because there is no machine-readable contour to interrogate. The graphical display of a planned route over an RNC has precisely the same safety value as drawing a pencil line on a paper chart and photographing it.<\/p>\n

    This matters acutely during port state control examinations and vetting inspections. The inspector who asks to see the ECDIS route check and finds an RNC underneath the planned track has found a compliance failure that extends beyond chart type selection. It calls into question the passage planning methodology, the officer’s understanding of the system, and potentially the company SMS.<\/p>\n

    The argument sometimes heard \u2013 that RNCs are used as a backup where ENC coverage is poor \u2013 does not address the passage planning requirement. Using an RNC as a supplementary display alongside paper charts in a non-ECDIS system is a legitimate operational choice. Using an RNC as the primary chart source in an ECDIS and then certifying that an ECDIS passage plan has been completed is not.<\/p>\n

    7. Safety contours, safety depth, and the ENC advantage<\/h2>\n

    The safety contour is the line in the ENC data at which the ECDIS changes its depth area shading and triggers an alarm on crossing. It is set by the navigator based on the ship’s draught plus an underkeel clearance margin. The system finds the nearest available depth contour in the ENC that equals or exceeds this value and uses that as the safety contour. In areas of sparse depth data, the safety contour may be set at a contour significantly deeper than intended \u2013 a common source of confusion and a reason why understanding ENC data quality is inseparable from setting safety parameters correctly.<\/p>\n

    Safety depth governs the threshold at which individual spot soundings are highlighted as shallow. Soundings shallower than the safety depth value appear in a different colour. Again, this depends entirely on the sounding being a discrete database object with a known depth value. An RNC provides none of this.<\/p>\n

    The ENC dataset also carries information on areas of restricted data accuracy. Zone of Confidence (ZOC) coding tells the navigator how reliable the sounding data is, based on the survey methodology, equipment, and date. A ZOC A1 area has centimetric position accuracy and full seafloor coverage. A ZOC D area has data of uncertain quality, possibly surveyed by lead line in the nineteenth century. Both can appear on the same ENC cell, and the contours will look equally solid on the display.<\/p>\n

    Knowing where the data comes from and when it was collected is a navigational skill the ENC system supports but cannot replace. ZOC information is available in the ENC pick-report for any area. It is rarely checked. The absence of a shoal from the display should never be read as confirmation that no shoal exists.<\/p>\n

    8. Practical implications for the watchkeeper<\/h2>\n

    Before every passage plan is completed, the chart source for every section of the route should be confirmed as official ENC. The cell status panel in the ECDIS catalogue shows cell origin, issue date, and update status. This is not a task to delegate and forget.<\/p>\n

    ENC coverage is not uniform. Coastal cells from major trading nations are generally current, well-surveyed, and updated frequently. Cells for some secondary ports, river approaches, and developing-nation coastlines may be sparse in coverage, compiled from old surveys, or simply absent. In these areas, the responsible approach is to supplement ENC navigation with paper charts, pilot input, and an honest assessment of what the ECDIS does and does not know. The default assumption that the ECDIS holds complete and accurate information is unjustified and occasionally fatal.<\/p>\n

    Where RNCs are in use \u2013 on vessels not subject to ECDIS mandatory carriage, or on systems that are not type-approved ECDIS \u2013 the navigator must understand explicitly that none of the automated safety functions apply. The system is a chart plotter. It shows position on a picture. The navigator provides all the judgement that an ECDIS would provide automatically from interrogating the database.<\/p>\n

    Update discipline is not the same for ENCs and RNCs. ENCs are updated through a continuous weekly update cycle distributed by the chart service. These updates are encoded as incremental changes to the database. An ENC that missed last week’s Notice to Mariners is visually identical to one that has been updated \u2013 the system must be checked to confirm current status. An RNC has no equivalent update mechanism; corrections must be applied manually to the paper chart from which the scan was made, which in practice means they are not applied at all.<\/p>\n

    An uncorrected chart does not announce itself. The display looks the same whether the data is current or twenty years old.<\/em><\/p>\n

    Closing Reality<\/h2>\n

    ENC and RNC are not interchangeable options within the same system. One is a structured, interrogable, safety-function-enabling dataset that satisfies the SOLAS carriage requirement. The other is a georeferenced image that supports visual navigation and nothing more. Displaying either on a certified ECDIS screen does not elevate the lesser to the status of the greater.<\/p>\n

    The danger is not that officers choose RNCs knowingly. The danger is that a decade of ECDIS familiarisation training has produced navigators who can operate the system without understanding what the system is operating on. The chart type changes what the machine can do. It also changes what the law considers the ship to be doing.<\/p>\n

    Over-scaling beyond compilation data, navigating on unofficial vector products, treating an RNC route check as equivalent to an ENC hazard scan \u2013 these are not minor procedural shortfalls. They are the conditions under which ships run aground in clear weather with functioning GPS and a lit-up ECDIS display.<\/p>\n

    The chart is not just the background. It is the safety system.<\/p>\n","protected":false},"excerpt":{"rendered":"

    Vector ENCs and raster RNCs look similar on screen but are fundamentally different in structure, safety function, and legal standing under SOLAS.<\/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":[9133,9132,9134,8949,2726,8959,9131],"class_list":["post-51604","post","type-post","status-publish","format-standard","hentry","category-bridge","category-latest","tag-chart-datum","tag-chart-types","tag-compilation-scale","tag-ecdis","tag-navigation","tag-passage-planning","tag-rnc"],"acf":[],"_links":{"self":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts\/51604","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=51604"}],"version-history":[{"count":1,"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts\/51604\/revisions"}],"predecessor-version":[{"id":51620,"href":"https:\/\/maritimehub.co.uk\/?rest_route=\/wp\/v2\/posts\/51604\/revisions\/51620"}],"wp:attachment":[{"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Fmedia&parent=51604"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Fcategories&post=51604"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/maritimehub.co.uk\/?rest_route=%2Fwp%2Fv2%2Ftags&post=51604"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}