Virtually all electronic nautical charts are derived from paper charts that originate in the hydrographic offices of maritime nations. These charts are scanned at high resolution to produce exact electronic digital images of the paper charts and are called raster charts. The scanned raster charts may then be vectorized, i.e., converted from raster to vector data, a considerably more complex process, and these are referred to as vector charts.
Raster charts
Raster charts are exact photographic images of traditional paper charts. They appear the same as the paper chart and contain the same features and information as the original. This is, in fact, their principal advantage. Most users familiar with paper charts feel more comfortable with the raster display. Also, because they are easier to produce they are less expensive than vector charts. Their principal disadvantage is that they require a great deal more storage space, and they are not as “seamless” as vector charts. That is, when you zoom in or out, or when you reach the edge of one chart, the system automatically jumps to the adjacent chart, which may have a different scale or characteristic. This has been largely overcome in recent raster cartography through “quilting” techniques.
Vector charts
Vector information is a digitized database of the items that make up a chart together with the precise location of each feature and the details of its display. The main advantage of vector charts is their inherent flexibility, i.e., the ability to manipulate the data in the database. Vector charts are layered graphic representations of the information contained on the original chart. Outlines of landmasses, elevation and depth contours, spot depths, navigation aids, landmarks, etc., may all appear on separate layers. As you zoom in, increasing layers of detail become available without any loss of image resolution. Also, additional information, such as explanations of obstructions, hazards, etc., can also be added to the chart on their own layer, thus the user determines the amount of detail that appears on the screen. Another significant advantage of vector charts is that the digitized data is highly compressed and takes up little storage space.
PC or chart plotter
Most chart plotters are designed to use proprietary, chip-based cartography from one or two sources (Navionics or C-Map), so the choice is pretty much made for you. If, however, you choose to use a PC-based navigation system, your choices are much broader. Because the PC has more memory (RAM), greater disk storage and a more powerful microprocessor, computers can run more sophisticated software and support charts in a variety of formats. In fact, the lines between vector and raster formats are blurring as more and more navigational software programs are able to read both.
Original chart sources
These are the national hydrographic offices that conduct hydrographic surveys and produce the original charts. In general, these offices will make charts available for download from their Web sites, some (NOAA, NGA) at no cost. First we must understand a few basic terms: ECDIS, ENCs and RNCs.
There are two basic types of electronic charts: those that comply with the International Maritime Organization’s (IMO) requirements for Safety of Life At Sea (SOLAS) class vessels (does not apply to pleasure yachts), known as the Electronic Chart Display and Information System (ECDIS), and all other types of electronic charts, regarded generically as Electronic Chart Systems (ECS).
ECDIS: An ECDIS is considerably more than a chart display system. Chart presentation is only one aspect of an ECDIS. The term “ECDIS” means a navigation information system, which can be accepted as meeting the chart carrying requirements of the SOLAS Convention, thereby obviating the need for a complete inventory of paper charts aboard the vessel. The key element in the system is a computer with video display capability, networked with other pieces of electronic equipment from which it obtains the information necessary to enable the navigator to complete his or her tasks. The ECDIS takes the ship’s position from loran or GPS, the vessel’s heading from the gyrocompass, the turn rate from the turn indicator and the speed from the vessel’s log. It can also take input data from radar, weather or satellite images to produce overlays on official digital chart images. There are two kinds of official digital charts: ENCs and RNCs.
(ENC): ENCs are vectorized Electronic Navigation Charts, standardized as to content, structure and format, and issued for use with ECDIS on the authority of government hydrographic offices. ENCs conform to International Hydrographic Office (IHO) specification IHO S-57, and are therefore acceptable for use in ECDIS.
(RNC): RNCs are Raster Navigation Charts that conform to IHO specifications, and the IMO permits an ECDIS to operate in a raster mode in the absence of ENCs. However, the IMO, in recognition of the limitations of RNCs as compared to ENCs, requires that a ship using an ECDIS must also use an “appropriate folio of up-to-date paper charts” for those areas where the RNC raster mode is employed.
An ECDIS must meet IMO performance standards and pass type approval by the International Electrotechnical Commission (IEC) in order to qualify as a replacement for the paper nautical chart. The IMO has also specifically requested that member governments encourage their national hydrographic offices to produce ENCs.
Which electronic chart type and format is best for you depends on a number of things – boat size, whether you use a dedicated chart plotter or a PC, the navigation software you currently use and your personal preference for chart display characteristics. The primary consideration is: “What are the best possible charts for the areas you plan to sail?” Generally speaking, choosing software that supports cartography from a number of sources increases the chances of finding charts for the areas you’ll visit. All of the vector charts for the entire world can be contained on just one or two CD-ROMs. Should you decide to visit an area for which you have no charts, you can get an “unlock code” from the vendor via e-mail or phone for the charts you need. Not bad insurance! n
Ev Collier is a freelance writer and frequent contributor to Ocean Navigator who lives in Lynnfield, Mass.
