Glossary of Terms
Air temp
Air temperature
AIS
System where vessel information (location, course, speed, etc.) is broadcast by VHF radio, for use by other vessels in navigation and collision avoidance.
Alarm status
Shows the symbol for each alarm that is on. The symbol is normally black and turns red if the alarm triggers.
Apparent wind
The speed and direction from which the wind appears to blow with reference to the bow when the boat is moving (also called relative wind).
Arrival alarm
An alarm signal issued by a GPS/chart plotter that indicates arrival at or at a predetermined distance from a waypoint (see arrival circle).
Arrival circle
An artificial boundary placed around the destination waypoint of the present navigation leg, the entering of which will signal an arrival alarm.
Attention Area
An important area on a chart, such as a restricted anchorage or a shallow area.
Autopilot (pilot)
An autopilot (or pilot) is a mechanical, electrical or hydraulic system which can maintain a vessel on a predetermined course without the need for human intervention. Using a direct or indirect connection with a vessel’s steering mechanism, the autopilot relieves the crew from the task of manually steering the yacht. On sailing boats certain types of autopilot can be programmed to maintain a course defined at a pre-set angle to the wind.
Bathymetric line
A depth contour line on the chart
Chart card
A plug-in card that stores chart data for a region.
Chartplotter
A chartplotter is an electronic navigation system that combines a GPS receiver with the capability to display electronic maritime charts, enabling the boat owner to continuously monitor the position and movement of his craft in relation to the surrounding physical environment, both above and below the water.
C-MAP™ chart card - C-MAP™ user card
user card - A plug-in card that stores waypoints, routes and tracks.
COG - Course Over Ground
The actual direction of progress of a vessel, between two points, with respect to
the surface of the earth. The vessel’s heading may differ from the course over ground due to the effects of wind, tide and currents.
CPA - Closest Point of Approach
The closest distance two vessels will come to each other based on their current course and speed.
dGPS
differential data is received from external DGPS Receiver
DNT
Distance to the next position the NX40/45 is navigating to, either a waypoint or the cursor.
Echo-sounder-Sonar
An echo sounder, or ‘sonar’ in American terminology, is a sound-detecting instrument used to measure the distance between the surface of the water and objects in the water or on the ocean floor. An essential item of navigational and safety equipment, an echo sounder detects objects underwater by emitting a sound pulse via a transducer mounted on the hull or lowered to the required depth and then measuring the reflected echo. By using a set formula – bearing in mind that sound travels more slowly in fresh water than in sea water – the time lapse between emission and echo gives an accurate measurement of sea depth and other marine topography. Modern echo sounders can analyse the pulse return in order to display information including the composition of the sea bed, the presence of obstructions, and the location and size of fish in the water.
Ethernet
Ethernet is a widely-used cable-based technology for transmitting very large amounts of electronic data between units of equipment within a LAN (local area network) and as such can be found in all forms of computing technology across every aspect of modern life.
Capable of transmitting at rates of 10 MB per second and more (versus 0.25 MB p/s for NMEA 2000) it can play a valuable role with marine electronics that process high volumes of data, for example radar, electronic charts and weather overlay information, and it is now common to find such units that now offer both Ethernet and NMEA 2000 connectivity. However there is no marine standard for Ethernet and equipment from different manufacturers may not be able to communicate with each other. In addition, unlike NMEA 2000 it does not have the ability to prioritise the transmission of critical data and so is not recommended for applications such as steering or throttle that require a near-instant response.
EVC – Electronic Vessel Control
enables boat’s engine, transmission, instruments and control systems to communicate and exchange information via a common bus network. Through proper interface to EVC, an autopilot can get sensor data from the EVC, do steering calculations and send rudder commands back to the EVC which brings rudder to commanded angle.
Fishfinder
A fish-finder is an echo sounder (sonar) specifically designed to detect the presence of fish in the underwater environs of the host boat, in addition to measuring the overall depth of the water. A fish-finder uses echo-location to reflect electronic pulses off fish and other underwater features and converts this information into a graphic rendition, nowadays a full-colour high-definition screen or liquid crystal display unit. The image on the screen, which represents individual fish with a small icon or as a series of arcs, enables fishermen to identify suitable targets and lower their baits or lures to the correct depth.
Goto
A simple way of navigating straight to a waypoint or to the cursor position.
GPS
Global Positioning System. A satellite-based navigation tool. This system is based on satellites in fixed orbits, circling the earth at an altitude of approximately 20,200 km. The system will provide the user with 24 hour a day all weather position coverage, with an accuracy of 5 to 30 meters.
Instruments
Marine instruments enable the sailor to measure and plot various aspects of his on-the-water experience in real time. Typically these include performance indicators such as wind speed and direction, boat speed and heading, and depth. More sophisticated systems can then take these basic measurements and calculate and display additional information such as true and apparent wind speed, course over ground, speed over ground and much more. Marine instruments are generally a combination of sensor, processor and a relatively small display unit or dial which can be mounted both above and below deck.
Leg
The straight segments of a route between waypoints. A route with four waypoints has three legs.
Magnetic deviation
A local magnetic field on board a vessel. Can interfere with the earth’s magnetic field and create compass readings that may deviate from the actual magnetic heading. The deviation will vary with the actual heading.
Magnetic heading
heading relative to magnetic north
Magnetic variation
A magnetic compass points to the magnetic north pole. The difference between this direction and true north is the magnetic variation. The amount and direction of this variation is dependent upon where on the earth you are located.
Measurement units
m meters
ft feet, 1 foot is 0.3048 meter
fm fathoms, 1 fathom is 1.83 meters
nm nautical mile, 1 nm is 1852 meters
kn knots, nautical mile per hour
km kilometer, 1 km is 1000 meters
kh kilometer per hour
mi statute mile, 1 mile is 1609 meters
mh mile per hour
MMSI
Maritime Mobile Service Identity. Unique identification number for a vessel, for use in Digital Selective Calling.
MOB
Man overboard.
MOB function
Starts navigating back to the place where someone fell overboard.
Multifunction Display
Multifunction displays (MFDs) are displays that have the capability to act as more than one type of electronic equipment. In effect they can, for example, be a chartplotter one moment and a radar display or echosounder the next. In recent years MFDs have become much more affordable, with screen sizes in the leisure market ranging from five to nineteen inches.
NMEA
National Marine Electronics Association.
NMEA0183
(NMEA0183) NMEA stands for National Marine Electronics Association (of the USA). NMEA 0183 was first introduced in 1983 as a voluntary industry standard for data communications among shipboard electronic devices. It uses a simple ASCII, serial communications protocol that defines how data is transmitted in a "sentence" from one ‘talker’ to one or more ‘listeners’ at a time, and therefore cannot be used to create networks. Data transmission is slow by today’s standard at 4800 bits / second and the standard does not allow for multiple ‘talkers’. However it is still in widespread use and is perfectly adequate for situations where one piece of equipment, for example a hand-held GPS, is to be connected to another such as an on-board chartplotter where the user wishes to integrate the two sets of data.
Generally, however, NMEA 0183 has been superseded by the NMEA 2000 networking standard, although many devices are designed to communicate using either standard.
NMEA2000
(NMEA2000) Currently the accepted standard across the international marine industry, NMEA 2000 is much more sophisticated than NMEA 0183 in that it allows multiple units to simultaneously both transmit and receive data. With the inclusion of multifunction displays into a networked system the user can then choose any combination of data outputs to be displayed at any position or for any situation. It is NMEA 2000 that has made possible the development of the integrated navigation and control systems that are now being fitted on craft of almost every size and application.
NMEA 2000 has a range of advantages over its precursor, NMEA 0183. The cables carry the current as well as the data, reducing both the cabling requirement and the risk of electromagnetic interference. Devices are connected using CAN (controller area network) technology and NMEA 2000 not only allows the transmission of data at vastly great speeds than NMEA 0183 (250,000 bits / second versus 4,800 b/s), but also in a more compact form, making it far more suitable for complex, multi-unit systems.
Finally, as a common standard NMEA 2000 allows the boat owner to interconnect equipment from different manufacturers.
Port side
– left (red)
Range (radar)
The distance between the center of the radar image and the outer range ring. The scale is shown in the top left corner of the radar window.
More about radar here.
Range rings
Concentric circles extending from your boat (usually at the center of the radar window) and used to estimate distances to objects. The scale is shown in the top left corner of the radar window.
Raster charts
– the original type of electronic chart, raster charts are produced by converting paper charts to digital images by scanner, and therefore look identical to a traditional chart in every way. While having the virtue of simplicity, and are instantly recognisable by anyone accustomed to using paper charts, they are not interactive and over time are being superseded by the more versatile vector charts.
Route
Two or more waypoints linked in sequence to form a course for the boat.
SDGPS
Satellite Differential Global Positioning System - will provide position corrections from received satellite signals (WAAS, EGNOS and MSAS).
Echosounder
An echo sounder, or ‘sonar’ in American terminology, is a sound-detecting instrument used to measure the distance between the surface of the water and objects in the water or on the ocean floor. An essential item of navigational and safety equipment, an echo sounder detects objects underwater by emitting a sound pulse via a transducer mounted on the hull or lowered to the required depth and then measuring the reflected echo. By using a set formula – bearing in mind that sound travels more slowly in fresh water than in sea water – the time lapse between emission and echo gives an accurate measurement of sea depth and other marine topography. Modern echo sounders can analyse the pulse return in order to display information including the composition of the sea bed, the presence of obstructions, and the location and size of fish in the water.Sounder status
A summary of echosounder settings.
Starboard side
right (green).
TCPA
Time to Closest Point of Approach. Time until the closest point of approach for two vessels.
Transducer
the transducer serves as the acoustic “loudspeaker” and “microphone” to send and receive the signals through the water. They are most often made from ceramic elements carefully built into a robust housing. The ceramic elements change shape when a voltage is applied across them and they also generate a voltage when they encounter sound waves.
UTC
Universal Time Coordinates, which is equal to standard time in London (GMT). UTC is not affected by the local summertime adjustments.
Vector charts
– the most common type of electronic chart, vector charts are used by all professional electronic charting systems authorised under IMO regulations, as well as most, more advanced leisure systems. As computer-generated charts they contain all the information required for safe navigation and are often interactive, allowing the user to use a cursor to click on elements to obtain more information, or zoom in to obtain greater detail. Increasingly vector charting packages contain additional information such as satellite imagery, port information, tidal data and the ability to manage views and 3D imagery.
VMG
Velocity Made Good. A vessel’s true progress towards a waypoint taken into consideration all factors like leeway and current.
Waypoint
A discrete point, stored in a navigator, located on the surface of the earth. Normally this point will be identified by Lat/Lon coordinates although in some systems it may be shown by T.D.'s.
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