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Don Casey : Chart Reading 101

Electronic Navigation Charts - Part 1 - updated January 2010
by Chuck Husick

Electronic nautical charts are digital computer data files that when processed by compatible software can be displayed for use in marine navigation. Although attractive in appearance, simply displaying electronic chart information on a typically small and sometimes difficult to see viewing screen does not provide sufficient value to justify the acquisition of the charts and plotter hardware. The value of electronic charts stems from the system’s ability to combine the chart information
with navigation position information from a GPS or Loran C, allowing the navigator to see his vessel’s precise, on-chart position.

Many electronic chart systems can display and store a vessel’s track and show waypoints and planned routes. On-chart measurement of bearings and ranges can be determined without resorting to the traditional parallel rules and dividers. Some systems can integrate information from the vessel’s radar, overlaying the radar returns on the chart image. The display system’s software usually allows the chart image to automatically keep pace with the boat’s movements, either showing the boat at the center of the screen or redrawing the chart when the vessel icon reaches a chart boundary. The chart image may be automatically rotated so that the boat’s heading is always at the top of the screen or the chart may be displayed in the more traditional North-Up manner.

Electronic chart systems can add a great deal of capability to any navigation station, provided that the correct type of chart file is selected for use and that the system’s processing and display limitations are taken into account. Screen visibility in bright light and especially in direct sunlight is frequently the most significant limiting factor determining in the real world usefulness of a system.

It is important that users recognize that with few exceptions what the navigator sees is not an official chart. It may contain some or perhaps all of the information contained in the original official Government Hydrographic Office publication. The presentation of the data, and the symbols used, may not resemble what you are used to seeing on an official paper chart.

Electronic charts can be more costly than equivalent paper versions, especially when navigating over large areas. CD-ROM based chart systems allow purchase of a disk containing a great many charts, requiring payment for only those charts needed for a voyage. The unlock codes for additional on disk charts can be purchased when the charts are required. All US raster (paper equivalent) charts and all US ENCs (electronic charts) can be downloaded from the internet at no cost. The chart libary can then be loaded into a chart viewr or chartpotter.

Navigators new to electronic charts may notice a limitation not present when using paper charts. A navigator using a paper chart can assimilate the total navigation picture provided by the chart while at the same time instantly focus on minute detail in any area of the chart. This process can be more difficult with an electronic chartplotter where screen redraw will take more time than we may be used to. Some of the newest chartplotters are able to very rapidly scroll and zoom the chart image, presenting a seamless view close to the experience of selectively viewing a paper chart.

An evaluation of a chartplotter must include an assessment of reliability. The electronic chart data base and the system used to process and display the information must be suitable for the often harsh marine environment. While overall reliability is generally excellent, a power failure can instantly deprive the navigator of critical information. A prudent navigator will provide
both a back-up electrical power source and have an "old fashioned" paper chart and a flashlight close at hand.

The value and utility of electronic charts and their display systems in not in debate. They are rapidly gaining in popularity, capability, and reliability. Systems that comply with stringent international standards, Electronic Chart Display and Information Systems (ECDIS) are used in place of paper charts on large ocean-going, SOLAS vessels. Progress is being made toward allowing the substitution of certain electronic charts for paper charts on small commercial vessels.
There are few if any restrictions on the use of electronic charts by recreational vessels.

Display/Chart Management Choices
Dedicated Chartplotters
The simplest, least costly way to navigate with electronic charts is with the use of a dedicated chartplotter. These devices are available from a number of manufacturers in both hand-held and fixed-mount versions. Some recently introduced products employ trans reflective active matrix LCDs that can be seen clearly in even the brightest sunlight. Some other products use intense, power consuming, heat generating backlighting to allow use in sunlight.

Many of the dedicated chartplotters provide a combination of functions; chart navigation, GPS position data, depth sounder/fish finder and in many units, display of radar data, either as a separate image or overlaid on the chart. It is wise to remember that display screens are typically quite small, too much information on a small screen can become no information.

The Computer Alternative
Electronic cartography is available in forms compatible with desktop and laptop computers. Chartplotter software is available at many levels of capability, extending to systems that manage and display information from the chart database, GPS/Loran C, radar, sonar, wind velocity, and boat speed and engine and vessel systems. Some can also show images from onboard TV cameras and when not needed for navigation, video from VCR's, DVD's, and on
the air TV. Most of the programs are written for use on Windows machines although some programs are available for Mac users.

The suitability of a desktop or laptop computer as a navigation data system will largely depend upon the environment on the boat. Unless specially hardened against the adverse marine environment most computers are unsuitable for navigation station use on a boat. Visibility of the display screen under the most adverse operating conditions can be a critical factor in choosing
a system approach, dedicated chartplotter or general-purpose computer.

The computer alternative can be compelling for larger vessels where the wheelhouse environment allows use of large screen computer monitors, either CRT or LCD types. Unlike dedicated chartplotters the software used with general-purpose computers can accept either raster scan or vector charts. A computer used for navigation can also be used for other purposes.

Chart Formats
Electronic charts may be in raster scan format (a video image of a paper chart) or in vector format where the features that make up a nautical chart have been digitally coded and placed in different "levels" in the memory device. The advantage of the raster scan chart lies largely in its visual identity with the official, paper charts. The vector chart provides two attributes. The data is highly compressed, taking up little space in a digital memory device. The second and today, in a time of low cost digital storage, the most valuable attribute is the ability to show only the data stored on those layers that are needed for navigation. This capability is most valuable when used in conjunction with the small screen displays common on many dedicated chartplotters where the ability to delete extraneous information can make the difference between on-screen clarity and
confusion.

The official U.S. Government-issued charts are now published in digital, vector, format. It is possible to update digital chart files on-the-fly, using e-mail or satellite communication between your boat and a chart supplier. While you may look forward to real-time updates, it’s worth considering that many of the chartered features depicted on a chart were last actually surveyed many, many years ago. The real value of the technological ability to do minute-by-minute updates may lie primarily in updating information about navigation aids, buoys, lights and the like. (It is worth noting that NAVTEX provides an alternate means for automatically receiving this type of information in alpha-numeric form.)

All vector charts are not necessarily identical in content, graphics, format, and accuracy. The production of a vector chart requires manual digitization of each feature on the original official paper chart. While the vector charts used in large ship ECDIS may be precise enough to meet international standards for ship navigation some of the charts commonly used for small boat systems may contain minor errors. A prudent navigator may wish to carefully compare a newly purchased vector chart with an official paper chart for the purpose of detecting any differences.

The non-volatile digital memory modules used in integrated chartplotters are packaged in a number of different packages. As expected, the oldest are usually the largest, reflecting the progress made in miniaturization. Older chartplotters may require adapters into which the new, smaller memory chips are inserted. Common formats include C–MAP Marine CF-85 Cartography in a mini package, Offshore C–MAP NT™ Cartography in a C-Card format and Inland C–MAP NT™ Cartography in C-Card format. Navionics products include Offshore Cartography in both Microchart
and NavChart packaging. Some manufacturers such as Garmin, Lowrance and Furuno use proprietary packaging. CD-ROMs are available in both vector and raster scan formats. Computer-based systems may be able to download charts direct from cartography suppliers, storing the information in the system’s hard disk or on auxiliary storage media. Regardless of the format or
storage media there is no shortage of chart information, with more becoming available almost daily.

Recent additions to the digital chart world include Bathymetrics, providing a pseudo 3-D image of the depths below a vessel and integration of aerial and satellite photography with the chart information. The Furuno NavNet 3D system with TimeZero cartography presents a very dynamic image that can include satellite image data.

The combination of precision position information with electronic management of chart data provides a vessel’s master with a navigation tool of unsurpassed capability. However, it’s important to recognize that regardless of how the data was obtained or how it is presented, it can be no better than the quality of the survey data used in its preparation. Prudence is a required ingredient in all marine navigation and especially for operation in waters not frequented by large ships and commercial vessels. Hydrographic surveying and transfer of information to a chart is a costly and
time consuming process. Don’t always believe that what is depicted on the chart precisely depicts the real world. Take advantage of all available information updates including the Notices to Mariners and especially the almost real-time data transmitted in NAVTEX. Broadcasts.

Some Frequently Asked Questions:

1. Why do some chips cost so much more than others?
The selling price of a memory chip generally varies with the amount of information stored on the chip. The cost of the chip itself is a very minor part of the price; you are paying for information, not hardware. Virtually all chip-stored charts are in vector format; therefore the manufacturer had to pay the cost of converting the original paper chart information into vector format. The price charged for the chip will include recovery of all of the conversion costs. The differing efficiency of the
software programs are used in the conversion process can result in different cost for development of the database to be stored in the finished chip. Each manufacturer may choose a different economic model in deciding
how to recover the initial chart conversion costs, resulting in differing sales prices for roughly the same chart area coverage.

2. What is the difference between a plotter and a chartplotter?
A plotter stores a record of successive vessel positions, most often using data from either a GPS or a Loran C navigation system. The data may be stored as lat/long or TD's and may be shown on the system’s display screen. In some systems a lat/long grid may be shown along with the track of the moving vessel. Some systems will provide an on-screen display of waypoints stored in the system memory. The distance interval between successive stored positions may be adjustable, allowing the navigator to make best use of the available memory. There is a finite limit to how
many vessel positions can be stored in the system’s built-in memory. When all memory locations have been used, most systems will overwrite the oldest data with new information, erasing the track plot represented by the overwritten information. In some systems, especially in those based on general-purpose computer hardware, external memory devices such as floppy disks or CD-RW's may be used to extend plot/track memory to whatever extent may be desired. It is conceivable that a mariner might record and store a continuous record of vessel position extending over months or even years of voyaging. A plotter's most significant value may be in enabling a navigator to precisely retrace a previously sailed and stored route. With its aid, leaving an uncharted cove once entered using careful eyeball navigation and soundings becomes as simple as retracing the track line on the plotter display. Some plotters also allow conversion of track points into set of progressive waypoints that that can be programmed to form a route that can then be automatically followed by coupling plotter to the vessel’s autopilot.

A chartplotter performs in the same way as the plotter, however the vessel position information is superimposed on a chart or in the case of terrestrial navigation, a map. The data storage includes sufficient information to allow subsequent display of the vessel’s position in correct relation to the chart. In addition, a chartplotter usually allows display of range and bearing between points on the display, including both charted features and vessel position and track.

3. Should I buy a dedicated chartplotter or use my laptop with a navigation program and a compatible chart database?
The answer to this question is, it depends. It depends primarily on where on your boat you intend to use the chartplotter. Few, if any laptop screens, can be seen in an open cockpit and may be difficult to see even in some reasonably shaded wheelhouses. A few modified laptops are available with special, high brightness LCD screens however the cost may put off many prospective buyers. Many, but not all chartplotters can be used in a shaded cockpit and a few can even be used in direct sunlight. However, some of the sunlight readable screens consume substantial amounts of electrical power for their backlight and virtually all are significantly more expensive than units intended for use in protected environments. Development work is always underway toward improving the overall visual quality of displays; it pays to keep informed of the latest product
offerings. Using a laptop or a desktop computer does
bring benefits in virtually infinite data storage (using floppy disks or CD-RW's) and in the ease with which new chart programs can be loaded and in the number of different chart databases that can be used. The wheelhouse on a larger vessel may be a suitable environment for a large, CRT computer monitor.

4. Given the recent history of electronic chartplotters and charts, will a system bought today still be useful in three years? Will technological advances make my system obsolete?
A well-designed system put in use today will still be useful in three years and in fact will remain useful as long as the hardware works reliably. After all, paper charts, protractors, and other drafting instruments and a sharp pencil work as well today as they did more than a century ago. Remember, it is very possible that the paper chart and its electronic counterpart are based on surveys and soundings made 100 or more years ago. Technological advances in electronics are a constant part of the progress we enjoy. Obsolescence in this field will have more to do with the user’s desire to have the very latest gadget on his boat than in any breakthrough in technology. The most significant future improvements will result from the use of higher speed microprocessors and new types of display screens.





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