Navigation Charts - Part 1 - updated January 2010
by Chuck Husick
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 systems ability to combine the chart information
with navigation position information from a GPS or Loran C, allowing
the navigator to see his vessels precise, on-chart position.
Many electronic chart systems can display and store a vessels
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 vessels radar, overlaying the radar returns
on the chart image. The display systems software usually allows
the chart image to automatically keep pace with the boats
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 boats 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 systems 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
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.
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
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, its 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
CMAP Marine CF-85 Cartography in a mini package, Offshore
CMAP NT Cartography in a C-Card format and Inland CMAP
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 systems 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.
of precision position information with electronic management of
chart data provides a vessels master with a navigation tool
of unsurpassed capability. However, its 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. Dont 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.
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 systems
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 systems 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 vessels autopilot.
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 vessels 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 users
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.