ARCS Hydrographic Chart Raster Format Explained
Before electronic chart displays became standard equipment on bridges worldwide, mariners faced a stark choice: keep paying for paper charts and a chart table full of corrections, or find a digital equivalent that regulators would actually accept. The ARCS Hydrographic Chart Raster Format gave them that bridge. Developed by the United Kingdom Hydrographic Office, ARCS turned official Admiralty paper charts into scanned digital images that could be displayed on a screen, correct down to the pixel, and legally carried in place of paper on many vessels.
ARCS stands for Admiralty Raster Chart Service, and understanding it requires separating raster from vector chart technology, a distinction that still trips up newer officers. A raster chart is essentially a high-resolution photograph of the original paper chart. Every sounding, every buoy symbol, every notation appears exactly as the cartographer drew it, because the digital file is literally a scanned image rather than a database of coordinates and attributes. Vector charts, by contrast, store chart information as discrete objects that software can query, reclassify, and layer selectively. ARCS has no such intelligence built in. What you see is what the chart compiler intended you to see, nothing more.
How the ARCS Format Actually Works
Each ARCS chart cell corresponds to a specific Admiralty paper chart, scanned at a resolution sufficient to preserve the fine detail hydrographers rely on, things like depth contours, rock symbols, and traffic separation scheme boundaries. The files are distributed on CD-ROM or through digital download, encrypted so that only licensed users with the correct permit can unlock and display them. Shipowners typically subscribe to a chart folio covering their trading routes, and updates arrive weekly through Notices to Mariners, either as full replacement cells or as patch corrections applied directly within compatible ECDIS or chart-viewing software.
Because the format is image-based, the software displaying ARCS cannot reinterpret the data the way it can with vector charts. There is no automatic route-checking against depth contours, no ability to turn off clutter like text labels while keeping depth soundings visible, and no alarm triggered when a planned track crosses a charted hazard. The officer of the watch still has to read the chart the old-fashioned way, visually, even though it appears on a monitor rather than unrolled across a chart table. This limitation shaped how regulators and shipping companies have treated ARCS over the years, generally as a transitional or supplementary tool rather than a full replacement for vector-based Electronic Navigational Charts.
Where ARCS Fits in Real-World Navigation
For much of the 2000s and into the 2010s, ARCS filled a genuine operational gap. Vessels trading in waters where official ENC coverage was incomplete, patchy outside major shipping lanes and ports, could still run a fully digital chart table using ARCS cells alongside paper backups. Many operators of RCDS, or Raster Chart Display Systems, used ARCS as the primary chart source under SOLAS carriage requirements, satisfying regulations while avoiding the cost and complexity of maintaining full paper chart folios across a global fleet. Fishing fleets, workboats, and smaller commercial vessels that did not need full ECDIS capability found ARCS a practical, lower-cost way to modernize their bridge without sacrificing chart accuracy or Admiralty authority.
Why ARCS Still Matters as Vector Charts Dominate
Global ENC coverage has expanded dramatically, and IMO carriage requirements now push larger vessels firmly toward vector-based ECDIS as the primary means of navigation. That shift has steadily reduced reliance on ARCS, and the UK Hydrographic Office has signaled its long-term transition away from the format in favor of Admiralty Vector Chart Service products. Yet ARCS has not vanished. Remote regions, older vessels not fitted with full ECDIS, and operators wanting a familiar backup system still find value in raster charts precisely because they look and behave like the paper original, reducing the training curve and interpretation errors that can accompany more complex vector symbology. Understanding ARCS also matters historically, since much of today’s digital navigation culture grew directly out of the compromises raster charting forced the industry to confront.
As hydrographic offices complete their push toward fully vector-based chart ecosystems, ARCS will likely fade into a legacy role, remembered as the format that proved digital charting could work commercially and operationally. For mariners and surveyors alike, it remains a useful reminder that the transition from paper to pixels was never instantaneous, and that practical, imperfect solutions often pave the way for the more sophisticated systems that follow.
