This text is intended as an introduction to electronic charts. It is not an introduction to the use of chartplotters or to the use of applications showing electronic charts. It is an introduction to charts. It covers how they are designed, their types and formats. This knowledge helps to understand how to use them, but also to understand their limitations… whatever the software that interprets them.
Although paper charts are becoming a thing of the past, and electronic substitutes for paper charts are deemed acceptable in many countries, Canadian regulations still require appropriately sized paper charts to be carried on board. There are exceptions, but for boaters exploring new places, it’s an obligation. It’s not forbidden to hope for change, or to use your favorite electronic navigation application, but in the eyes of the law, you must have paper charts on board!
Summary of differences
If you don’t feel like reading the whole text, this little table summarizes the differences between the different types of charts.
| Paper charts | Raster charts | Vector charts | |
| Electronic? | No! | Yes! | Yes! |
| Main characteristic | It’s an image printed on paper. | It is an image displayed on a screen. | It is a computer object that changes according to the context. |
| Format | Paper. | Any image format (jpg, png, etc.) | S57, S100 and others. |
| What happens if we zoom in? | N/A | Pixels will appear: the map has a limit. No new objects will appear. | The map will appear as sharp as ever. Some objects will appear. |
Raster charts
Raster charts are are high-resolution images of charts. Initially, electronic charts were nothing more than scans of paper charts. Raster charts later refined the idea by printing maps directly onto a digital image format.
Like paper charts, objects such as buoys and navigation symbols were carefully chosen to maximize clarity and the information conveyed to readers. This is a choice made by the chart’s designers. Like photos or digitized drawings, these are still images, meaning that the navigation symbols remain, no matter how far you zoom in on the screen.
For chart designers, a raster chart is a convenient transition to electronics: instead of printing on paper, all that is needed is to print the image on file… and the chart design process remains essentially the same.
You can easily identify raster charts by zooming in on your navigation software: if you zoom in excessively, the screen resolution will exceed the image resolution, and the chart will look pixelated (images below).
Some American charts, published by the National Oceanographic and Atmospheric Administration (NOOA), are in raster format, but will soon be discontinued. Some Caribbean countries also have raster charts.
Vector charts
Vector charts are radically different in concept. Basically, they are databases. These data represent either the geographical aspects of the chart (tidal flats, land, water, etc.), or navigational symbols (buoy symbols, etc.). These data are coded in the form of mathematical formulae, and are drawn whenever the computer or software displaying them needs them.
Vector charts are thus dynamic objects: when the chart is enlarged on the screen, a navigation symbol (e.g. a buoy) or a depth plot will retain of similar size. Drawings are redrawn according to size. In this way, there is no pixelation or loss of resolution. The chart is always sharp!
At first glance, this is an advantage in terms of legibility: the screen will always display a legible chart, whatever the scale required. However, we must beware of this risk: the chart doesn’t necessarily display greater precision: 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 water depth through a contour line, but this depth is the result of a calculation drawn from the stored data… and not the actual depth. So beware of excessive zooming: although the chart is always sharply drawn, it doesn’t necessarily reflect accurate information.
Vector charts also have an important difference from other chart formats. Unlike fixed formats, where people have carefully chosen the positioning of symbols, the symbols displayed on a vector chart depend on a computer program. It is this program that decides whether the symbol is displayed or not.
The most important case is illustrated below, where the information displayed varies with the zoom level. The chart shows Port meunier, southwest of Anticosti Island. The scale is 3 nautical miles, 1 nautical mile and half a nautical mile respectively. Note the appearance of range light symbols, buoys, rocks and depths. To use these charts correctly, it’s good practice to zoom in further to see whether or not new symbols appear. You should also remember to avoid excessive zooming for navigation purposes.
S57 and S100 formats
Vector charts are organized in two formats: the current format (S57) and the future format (S100). Conceptually, these formats organize data into “layers” (geographic, bathymetric, currents, navigation symbols, text, etc.) in a similar way to a photo editing software. These layers can be displayed or removed.
The two main additions to S100 maps are the ability to handle encrypted data and to process dynamic data. The first is useful for protecting proprietary data… or state secrets. The second addition enables you to generate cards with near-real-time data. For example, currents can be displayed at the current time. In Canada, the St. Lawrence Global Observatory is the pilot organization experimenting with the new S100 format. Eventually, it will be integrated into commercial boating software.
