Chart Plotters
by Chuck Husick

During the past 10 years electronic chart plotters have become an increasingly important part of the helm station on many recreational and commercial vessels. Plotters very widely in capability, ranging from the very small screens in hand-held GPS units to full color, large screen computer monitor displays and the International Maritime Organization approved Electronic Chart and Information Display Systems (ECDIS) used on large ocean going vessels. Deciding if a chart plotter belongs on your boat and if so what capabilities are appropriate for your navigation requirements can be a confusing task.

During your evaluation of chart plotters you should keep one underlying fact in mind. Unless you are boating on a small pond or lake, a marine chart, issued by a Government Hydrographic Office (the National Oceanographic and Atmospheric Administration - NOAA in the US), is a necessary part of your boating safety equipment. These charts show boundaries of land and water, water depths and contour lines, type, identification and location of aids to navigation, position of channels, danger and prohibited areas and locations of shore-side facilities. Elevations and contour lines on adjacent land are invaluable aids when using radar for navigation. While electronic charts may contain useful information, such as marina data, not on official charts, the official chart may contain vital information that may be absent on some electronic charts.

A chart plotter's greatest value is in its ability to convert the precise but abstract position information supplied by the GPS or Loran into an easily understood picture of the vessel's position in relation to its surroundings. This improves the navigator's situational awareness, his ability to correlate his vessel's position in relation to surrounding land, channel boundaries and various navigation aids and other vessels. Even low cost chart plotters that depict vessel position on a minimal content chart can greatly aid the user in "finding" his vessel's position on the chart being used for navigation. More complex plotters, full detail charts can do much more, including voyage planning, rapid input of waypoints, calculation of distances, courses and preparation of voyage time estimates.

Chart plotter displays vary widely in size, type, resolution and usefulness in different lighting conditions. In general, monochrome LCDs provide good to excellent visibility in all situations, including the most difficult, full sunlight conditions encountered on small boats and on the flybridge of larger vessels. Except for display screens specifically rated for use in full, direct sunlight (and usually having a specified screen brightness of 1400 nits or greater) color LCD and CRT screens are best used in subdued lighting conditions.

Screen size can be critical in determining the real-world usefulness of a chart plotter. You can evaluate the usefulness of a given size screen before buying one by making a copy of a familiar chart, and cutting out a section of the same dimensions as the screen size being considered. Remember that with the original paper chart, looking at adjacent areas required only a glance while with the chart plotter time will be required for a redraw of the screen.

Users of chart plotters other than fully approved ECDIS are invariably greeted with a warning message stating that the system is not to be used as the sole means for navigation. This warning is usually present for two reasons. First, the electronic chart may not be an "official" chart (only charts issued by the appropriate Government Hydrographic Office are legal for navigation). Second, the manufacturer's legal department wants to protect the company in the event the user has an accident that in any way results from or can be connected to use of the system for navigation. The on screen warning and the fact that electronic systems are not infallible should provide sufficient motive to always have a paper chart on board and to maintain a position plot or position log.

The decision to use a chart plotter entails the need to choose the type of chart information to be used. Basic, limited capability systems are often restricted to using only one type of chart format, usually in the form of a plug-in memory chip. The more capable, general-purpose computer based systems can use any one of a number of chart data sources, allowing the user to choose between raster scanned charts and the various vector scanned chart products.

Raster charts are photographically precise copies of the standard Hydrographic Office charts. They contain everything that is shown on the paper chart, with virtually no chance for error. All symbols and markings will be familiar to anyone used to using conventional paper charts. Vector charts are produced from the original paper charts, however the data is recorded in separate digital file layers, enabling the user to select the types of information that will appear on the screen. Used with due care, this capability can improve readability on small and limited resolution screens.

The initial popularity of vector format charts was largely the result of the limited memory capacity and processor capability of early to market chart plotters. Vector chart files are much smaller than those required for raster charts. In addition, vector files could be updated more easily than raster files. However the reduction in memory cost and increase in processor speed and capability has made the use of raster charts practical and in some instances, preferred. Both vector and raster format charts can now be updated without difficulty. When evaluating a chart plotter carefully evaluate the speed with which the screen can be redrawn to show a new chart or a different magnification level of the chart in use.

Chart plotter systems can provide large amounts of useful information in addition to the basic marine chart data. Aerial photographs keyed to the chart area being used may be included for popular harbor areas. Lists of
marina and other shore side facilities similar to those contained in cruising guides are available with a key or mouse click. Light list, tide and current information is increasingly available for the more sophisticated chart data base systems. Some systems can replace an entire navigation library, including chart and text references. However always bear in mind that greater capability is accompanied by greater complexity in both the hardware and the software. Regardless of the system you choose for your vessel, the basics, charts, coast pilot, tide and current tables, light list etc should be on board, in paper form, accompanied with a couple of flashlights and lots of spare batteries and bulbs.

Some chart plotter systems integrate information from other on board systems including relative wind speed and direction, hull speed, water temperature and radar data. In fully integrated systems it is possible to transfer waypoints between the chart plotter, GPS and radar and to combine the displays, including overlaying the radar data on the chart. The success of radar overlay on a chart will depend in part on the quality of the vessel's magnetic heading sensor system. The overlay may work well when proceeding on a straight course, however performance can be seriously degraded if the accuracy of the heading information degrades during turning maneuvers.