Raster and vector charts: definitions
For reference:
A raster chart is a direct electronic reproduction of an official paper chart (UKHO, Imray, etc.), drawn by a professional cartographer and presented digitally exactly as published.
A vector chart is a digitally re‑encoded version of chart information derived from raster sources. The data is stored as objects and rendered by a computer in real time, rather than being a fixed cartographic image.
Why this matters
This article addresses safety‑critical differences between the level of detail available on vector charts—commonly embedded in modern chart plotter, and the richer, more complete information available from raster charts and other sources.
These differences become particularly important when navigating close to the shore, over shoals, or in drying waters, where precise chart interpretation is essential.
The skipper's responsibility
Being the skipper of a small craft carries clear responsibilities. The skipper is accountable for:
- The safety of the vessel and crew
- Carrying appropriate navigation equipment
- Ensuring charts are current and suitable
- Preparing a proper passage plan, taking account of weather, tides, and local conditions
This article assumes the reader is competent in traditional navigation techniques, including:
- Using paper charts
- Plotting positions
- Taking bearings from shore marks
- Allowing for tidal streams
These are fundamental skills that should be mastered before going to sea. A competent skipper should be capable of making a coastal passage without reliance on GPS.
GPS and chart plotters: useful, but not infallible
Today, GPS receivers are ubiquitous, and compact chart plotters—both dedicated units and software‑based systems—are widely used. For what they are, these devices are highly capable. However, they also have inherent limitations, particularly related to:
- Screen size
- Scale‑dependent rendering
- The way vector charts are displayed
There is a reason paper charts are large: they provide spatial awareness of surrounding hazards that is difficult to replicate on a small display.
Two certainties about chart plotters
If you purchase a dedicated chart plotter, whether a small portable unit or a larger, permanently installed syste, you can be certain of two things:
- The manufacturer will disclaim its use as the sole means of navigation.
- The embedded charts will be vector charts (ENCs).
At this point, many users reasonably ask: What exactly have I paid for?
Appropriate user of chart plotters
A well‑installed chart plotter, ideally with NMEA data feeds for position, depth, wind, AIS, and possibly radar, can be an excellent situational awareness tool, particularly when positioned at the helm.
Used sensibly, such systems are a valuable aid to safety.
The risk arises when a skipper treats what is displayed on a small chart plotter screen as complete and authoritative, and relies on it beyond its practical limits. This is poor seamanship and can lead to dangerous situations, precisely where vessels are most vulnerable: near the shore.
Maximum attention to navigation detail is required when:
- Closing the land
- Crossing shoals
- Navigating drying or poorly buoyed areas
Small‑craft skippers regularly encounter these conditions and should equip themselves with multiple, high‑quality sources of information.
Raster vs vector: what the charts actually show
If you regularly navigate across shallow or drying waters, the differences between raster and vector charts become striking when viewed side‑by‑side.
In our examples:
- Column 1: Official Admiralty raster chart
- Column 2: The same area displayed using official S‑57 vector data
- Column 3: Another widely used commercial vector chart system
The locations show, such as Wootton Rocks and Bembridge Ledges, are areas we know well. Similar results can be expected elsewhere.
Why vector charts lose detail
As stated in a white paper by CherSoft, developers of UKHO‑based navigation products:
“At the UK Hydrographic Office, and many other national Hydrographic Offices, vector data (ENC) is created by tracing over the raster data. The significance of this cannot be overstated.
"The best-quality image possible from this data can only ever approach the original raster image.
"When the electronic navigation chart (ENC) is displayed, even with all display options enabled, the total amount of information will still only approximate that available from the raster image.”
This observation remains valid today, including under the newer S‑101 ENC standard, which improves data structuring but does not eliminate the fundamental limitations of late‑rendered cartography.
Why vectar charts are still used
Vector charts are used in chart plotters for practical reasons:
- Smaller file sizes
- Seamless scrolling across regions
- Efficient handling on low‑power devices
However, instead of a cartographer deciding how much detail to show at a given scale, a computer algorithm makes that decision, sometimes omitting information that may be critical in specific circumstances.
Raster charts, by contrast, are not seamless and can seem less elegant, but they present a carefully composed, human‑designed chart image.
Practical advice
Our advice is straightforward:
- Do not rely entirely on vector chart plotters.
- If planning a route on a vector system, inspect it at the largest available scales.
- You may, and often will, find that a route crosses hazards that are not visible at smaller scales.
- Be particularly cautious with drying heights and shallow contours on vector charts.
- Cross‑check with raster charts wherever possible.
Given the relatively low cost of electronic charts today, the best solution is to carry both raster and vector systems on board.
Many skippers successfully use:
- A dedicated plotter with vector charts at the helm, and
- A tablet or laptop running raster charts as a planning and verification tool
Choosing electronic chart systems
If using an app with raster charts, ensure that it provides:
- Clear latitude and longitude read‑outs (for paper chart plotting if required)
- Visibility of GNSS accuracy or fix quality
- Easy route planning and waypoint management
- Distance and bearing tools
For skippers with a dedicated plotter, adding a large Android or iOS tablet running marine navigation software with raster charts makes an excellent and cost‑effective backup.
Real‑world experience
One customer summarised their experience as follows:
“The stars of the show were an older Aurohelm 200 tiller pilot and a Nexus 7 tablet with a low‑cost app and Admiralty charts from VisitMyHarbour. It never missed a bea, unlike the Garmin, which developed backlight and contrast issues.”
Another noted the difficulty and cost of updating proprietary plotters:
“I purchased a £50 chart update which arrived as a blank SD card. After repeated failed downloads and support calls, I ended up sailing with out‑of‑date charts. The very next day, a new navigation mark was visible on SeaClear but not on the Garmin.”
The conclusion was clear: Raster chart updates were simpler, more reliable, and more economical.
Final thought
“You may often find that your route is taking you across a danger you have not seen at the scale you are using.”
That statement is not hypothetica, it is based on repeated real‑world experience.
Modern electronic navigation tools are powerful aids, but good seamanship still depends on understanding their limitations, cross‑checking information, and maintaining proper navigational discipline, especially where it matters most: close to the shore.