Charts for the Chartplotter

Charts are the lifeblood of chartplotters. Almost anything you will do on a chartplotter requires that a chart be loaded and on the screen. Individual things, such as your boat position, radar targets or other specific points, etc., will be overlaid on the chart.  

There are both surface and bottom (bathymetric) charts. There are various names for  charts that show the surface, including coastal, "grow-out", and navigational. Many chartplotters will take different kinds of charts.

Charts are of two basic types:  raster, which is a picture of a physical chart, and vector, which is a set of instructions to the computer that tell it how to construct the chart. Both types have advantages and disadvantages, discussed below.  

It is also worth distinguishing between surface charts (i.e., the basic navigational chart) and bathymetric charts of the bottom.

We recommend that you use the standards for electronic charts: BSB for raster and S-57 for vector, although there are some supplemental standards for specific purposes. C-Map does have proprietary formats that are de facto standards and reasonable to use. Avoid proprietary formats used only by one chartplotter manufacturer.  Many chartplotters, especially for raster charts, require you to run the electronic chart through a converter to put it into a form the chartplotter can use.

Smaller vessels, not legally required to keep a paper chart, may use electronic chartplotters freely. Larger vessels, or passenger vessels subject to the Safety of Life at Sea (SOLAS) regulations, must either use a paper chart with the chartplotter, or use a government-accredited electronic chart. S-57 is the International Hydrographic Organization transfer standard for chart information, although not all chartplotters understand it.

Surface charts

The table below reviews current support, by product, for raster versus vector charts. See specific product descriptions for the standards supported. Maptech Chart Navigator and Rose Point Coastal Explorer are the same program, but sold by different companies..


Raster charts

Chartplotters that take raster charts have the advantage that if no electronic chart is available for an area of interest, users scan an existing chart, or have a company with the proper equipment for scanning create them. The raster chart is a precisely dimensioned picture of a chart, but it is a picture; the chartplotter has no knowledge of the meaning of any component of the chart.

In the United States, all official nautical paper charts produced by NOAA's Office of Coast Survey (OCS) are available in raster format. A fundamental tool of marine navigation, NOAA's Raster Navigational Charts (NOAA RNCs™) are produced by scanning at high resolution the original color separates, which are used to print the paper charts. NOAA adds to the digital raster file such features as data describing the chart, its datum, projection, and its geo-reference. Geo-referencing enables a computer-based navigation system that is connected to a GPS to locate and display on the chart image on screen the vessel's exact position.,

Computer-based navigation systems that use official raster data, such as NOAA RNCs, may also be referred to as Raster Chart Display Systems (RCDS).  RCDS use raster nautical charts and electronic positioning to provide an integrated navigational tool. Today, national hydrographic offices such as OCS endorse the use of RCDS to enhance safe navigation. 

Raster charts for the United States can be downloaded free from NOAA, or bought on CD-ROM if the user does not have a high-speed Internet connection.
Chart Loaders
It may be necessary to run certain charts, such as the BSB raster charts from NOAA, through a converter program, specific to the chartplotter, before the chartplotter can use them. For example, SeeClear II requires the charts to be loaded via MapCal II.

Some raster chartplotters allow the user, with a high-resolution scanner, to produce an overlay that the software will superimpose onto the basic chart. To make the overlay work, there typically must be at least three known and precise geographic points on the overlay.

This would be useful, for example, for a harbormaster to place a set of mooring points onto a surface chart.
Related to overlays is the concept of boundaries, which is easy in vector graphics but can be implemented by some raster chartplotters. A boundary is more complex than an overlay, in that the chartplotter should raise an alarm condition if programmed to do so when nearing a boundary, crossing a boundary, etc.

One use of boundaries is the shoreline, which clearly cannot be crossed. Another application, however, iis with such things as restricted fishing areas, so a fishing vessel can tell if it is inside or outside such areas. Since boundaries are usually on charts drawn for the purpose, which are not full navigational chart, an overlay feature may be useful for introducing the boundaries to the navigational chart.

Vector charts

A vector chart breaks down the data in a chart into series of digital lines, polygons, etc., that maintain three-dimensional relationship. Using a vector chart, the chartplotter can understand the difference between water and land, or in water depth, so it can sense that a course will run the vessel aground. Vector charts require much more computer power to create, and more to view than do raster charts. They do take less storage than raster charts, and, of course, offer much more knowledge to the chartplotter program.  Vector charts tend not to blur at extreme enlargement.

Since these better meet the legal needs of replacing paper charts, they are also called electronic navigational charst (ENC), Such a chart is a vector-based digital file containing marine features suitable for marine navigation. It is based on the International Hydrographic Organization (IHO) S-57 standard. The ENC is intended for use in electronic charting systems (ECS) as well as Electronic Chart Display and Information Systems (ECDIS). ENCs can also be used in geographic information systems (GIS) as base map data.  7Cs, a primarily European standard, may become more common in the US.

The NOAA ENC program is building the ENC production database from a combination of charted information as well as original “source” information. NOAA has compiled critical features such as channel limits, aids to navigation and obstructions from the original documents that were used to put the feature on the paper chart. This means that a feature such as a federally maintained channel was digitized from a 1:2,400 scale drawing as opposed to a 1:20,000 scale chart. The objective is to use more accurate information for features that are critical to the safety of navigation.

Bathymetric charts

Bathymetric charts are three-dimensional charts from the surface to the bottom. In practice, they have to be vector charts. Think of them as underwater topographic maps, showing bottom contours, peaks, etc.  

While fishermen may not need to know every feature, if they are in a bottom-dragging fishery, they do need to know the characteristics of the bottom (e.g., mud, rock, coral, sand).