Introduction to electronic charts

This text is intended as an introduction to electronic navigational charts (ENCs). It covers how they are designed. This knowledge helps understanding how to use them and their limitations. It is an essential reading before using our introduction to electronic navigation applications. You can also read a similar text on the Canadian Hydrographic Service website.

Paper charts are disappearing. Electronic charts and applications are considered acceptable in several countries, but Canadian regulations still require paper charts to be carried onboard. There are exceptions, but for boaters exploring new places, it is an obligation. There are no reason not to hope for change, or not to use your favourite electronic navigation application, but to comply with the regulations, you have to have paper charts on board!

Summary of differences

If you don’t feel like reading the whole text, this little table summarises the differences between the different types of chart.

Raster charts

Raster charts are high-definition images of charts. They are images. Originally, electronic charts were scans of paper charts. Raster charts were later refined by printing the computer file directly onto a digital image format.

Apart from the fact that they can be displayed on screen, they are in principle identical to paper charts. Like digitised photos or drawings, they are still images, meaning that the navigation symbols remain no matter how far you zoom in on the screen. For example, buoys and bathymetry information have been carefully displayed to maximise clarity, safety and legibility. The display is fixed and is a choice that is made when the chart is designed.

From your electronic navigation application, you can test whether you are working with a raster chart by zooming in excessively. With a raster chart, the screen resolution will eventually exceed the image resolution and the chart will look pixelated (see images below).

Some American charts are in raster format. Some Caribbean countries also have charts in raster format. However, these formats are primarily a model for the transition from paper to electronics. Eventually, they will be all replaced by vector charts.

Vector charts

Vector charts are radically different objets. They are not images, but databases. These data represent either the geographical aspects of the chart (tidal flats, land, water, etc.), or navigation symbols (buoy symbols, etc.). These data symbols are coded in the form of mathematical formulae. Each time a navigation application needs to display a symbol, the computer calculates the drawing to be displayed at the appropriate size, and then rendered on screen.

Vector charts are therefore dynamic objects that can be used to modify what is displayed. For example, when the chart is enlarged on the screen, a navigation symbol (e.g. a buoy) or a depth contour will retain a similar size. Drawings are redrawn according to size. In this way, there is no pixelation and no loss of resolution.

Advantages and disadvantages

False precision

Redrawing the symbols according to the scale of the map is an advantage in terms of legibility: the screen will always display a legible chart. However, there is a risk: the chart will not necessarily be more accurate: it simply redraws the data it holds.

This characteristic is therefore an inherent danger in navigation: zooming in on a region of the chart may show a given isobath, but the line is the result of a calculation. So beware of excessive zooming: although the chart is always sharply drawn, it does not necessarily reflect such precise information. In reality, an isobath may be slightly offset from the calculated drawing.

Display varies with scale

Vector charts also have another important difference from paper or raster charts: the symbols displayed on a vector chart are drawn according to the parameters of your electronic navigation application. It is your application that decides whether a symbol is displayed or not.

I illustrate two examples below. The first example shows the equivalent of chart #1233 at low resolution and at a slightly higher resolution. In the low resolution screenshot, the electronic navigation application (here SEAiq) does not display the water depth at chart datum. The following image shows, however, that by zooming in a little, the depths do appear.

I’ll show a second example, this time using the Navionics (Boating) electronic application. The map shows Port meunier, south-west of Anticosti Island. The scale in each image is respectively 3 nautical miles, 1 nautical mile and half a nautical mile respectively. As the resolution increases (by zooming in), the symbols for range lights, buoys, rocks and depths appear in succession.

To extract all the information from the chart, it is a good practice to zoom in to see whether or not new symbols appear. Once you have scanned the charts to identify the symbols, it is good pratice to return to an appropriate zoom, so as to avoid the problem of false precision.

The S57 and S100 formats

Vector charts come in two formats. The current format complies with the International Hydrographic Service‘s S57 standard. The new chart format is called S100 and is gradually becoming available to the general public. In the United States, some S100 charts are already available, while in Canada, deployment is scheduled for 2026, as of December 2025.

The S57 and S100 formats organise data in logical groups, often called layers: the geographic layer, the bathymetric layer, the currents layer, the navigation symbols layer, and so on. The idea of layers is similar to that of photo editing software. These layers can be displayed or removed, depending on the options in your electronic navigation application.

The two main additions to the S100 maps are the ability to process encrypted data and to process dynamic data. The first is useful for protecting proprietary data or state secrets (e.g. military charts). The second addition enables charts to be generated with real-time data. For example, you can display currents in real time. In Canada, the St. Lawrence Global Observatory is the pilot organisation experimenting with the new S100 format.

How to decode symbols on a chart?

Electronic navigation applications offer ways of interpreting the symbols displayed on a chart. These methods vary from one application to another. Sometimes a crosshair is moved over the object, sometimes a simple click is made on the object, but in general, some sort of interaction tells the app to reveal the signification of a symbol.

That said, all symbols displayed on a navigation chart are recorded in a master document of the Canadian Hydrographic Service: chart #1. By selecting a section of the document, you can understand the meaning of any symbol on Canadian charts. Below, an extract from chart #1 shows how to interpret the symbols for certain special buoys.