Paper charts. Raster charts. Vector charts. GPS handhelds. U.S. Coast Guard approved charts. Which one do you use? Where can you get them? What's the difference between them? With so many navigational charts and terms to learn, embarking on the boating world can seem like moving to another culture with a different language. In a sense, it is, but there is help to sort through it all--numerous books, websites and online courses, and courses sponsored by the U.S. Coast Guard. This article is simply an overview and a resource to locate more information for our Bayliner friends.
What exactly is a nautical chart? The navigational chart is one of the most fundamental tools available to the boater. When a map goes to sea it becomes a chart. (True boatmen never use the term "map," lest they be mocked.) A nautical chart is a graphic portrayal of the marine environment. In addition to providing the most basic elements, a chart is a working document used by the mariner for safe navigation. In conjunction with supplemental navigational aids, it is used to lay out courses and navigate ships by the shortest and most economically safe route.
A chart shows the nature and form of the coast, the depths of the water and general character and configuration of the sea bottom, locations of dangers to navigation, the rise and fall of the tides, locations of man-made aids to navigation and the characteristics of the earth's magnetism. In order to decipher all these elements of a chart, you must first have a basic working knowledge of chart terms.
Navigational Terminology:
One of the first things to navigate is the terminology chartmakers use. Familiarize yourself with these frequently used terms and you're well on your way to graduating from novice status.
- Aids to Navigation: Artificial objects to supplement natural landmarks to
indicate safe and unsafe waters, i.e. buoys, daybeacons and daymarks.
- Bearing: The direction of an object expressed either as a true bearing as shown on the chart, or as a bearing relative to the heading of the boat. It is measured clockwise from 0º to 360º.
- Compass: Navigation instrument, either magnetic (showing magnetic north) or gyro (showing true north).
- Compass Rose: The resulting figure when the complete 360° directional system is developed as a circle with each degree graduated upon it, and with the 000° indicated as true north. True north is also known as true rose. This is printed on nautical charts for determining direction.
- Contours: Lines that connect points on a chart to show surfaces at the same elevation.
- Dead Reckoning: The navigation procedure to plot a course based solely on mathematical calculations.
- Knot: A measure of speed equal to one nautical mile (6,076 feet) per hour.
- Latitude: The distance measured as an angle north or south of the equator.
- Longitude: The distance measured as an angle east or west of Greenwich, England.
- Magnetic North: An average position of the North Pole that fluctuates according to the Earth's magnetic field. The pole wanders daily in a roughly elliptical path around this average position and may frequently be as much as 80 kilometers away from this position. The earth's north and south magnetic poles do not coincide with its north and south geographic poles (true poles). Only a few places on the globe will a compass point to the true north.
- Mercator Projections: The most widely used charting method developed by Gerard Mercator in the 16th century. It is the method of portraying the land areas of the earth on a flat chart having a rectangular grid of latitudes and longitudes. Rhumb lines are laid down as straight lines and specify locations by reference to rectangular coordinates. Nearly all the charts developed for small craft are made with Mercator projections.
- Meridian: A great circle on the surface of the earth passing through the poles and any give place; the half of such a circle included between the poles; a representation of such a circle or half circle numbered for longitude on a chart.
- Nautical Chart Symbols: Symbols used on Coast Survey charts and those of many other nations.
- Nautical Mile: The distance of one degree of latitude; approximately 6,076 feet; about 796 longer than the statute mile of 5,280 feet.
- Navigable Water: Legally, water that connects with the sea or permits navigation between two states or to a foreign country.
- Navigation: The art and science of conducting a boat safely from one point to another. True navigation applies when land is far over the horizon and invisible. Traditionally, celestial bodies were used as guides, which required mathematical computations in order to plot positioning. In modern times we also use satellites (GPS) to plot the positioning.
- Pilotage: Navigation by use of visible references, guides fixed to the land that can be seen and used directly or with an electronic aid.
- Rhumb line: A course line on a chart crossing all meridians at the same angle.
- True North: The north end of the earth's axis. Also called North Geographic Pole. The direction indicated by 000° (or 360°) on the true compass rose.
- Variation: The angular difference between the magnetic meridian and the geographic meridian at a particular location.
Electronic and Traditional Positioning Systems
Okay, once you've got the terms down, or at least stored in a place where you can reference them, you'll want to learn how to plot your position both electronically with Global Positioning System (GPS) and with the traditional methods.
GPSs are great for navigation. However, they are not stand alones, so there are a few things to know when using the GPS with a nautical chart:
- Always change the units on the GPS from statute miles to nautical miles.
- Remember GPS devices read directions in either true north or magnetic north. They are often preset to the true north option.
- Most GPS devices are preset to give locations in degrees and minutes with two decimal places (with an option for degrees, minutes and seconds), most newer charts measure distances in degrees and fractions of minutes. Some older charts may measure distances in degrees, minutes and seconds. Thus, a GPS reading of 79 degrees, 50.30 minutes corresponds to a chart location of 79 degrees 50 minutes and 18 seconds. It is not located halfway between 50 and 51 minutes.
Even with all of today's electronic navigational aids, it is important to be familiar with nautical charts and to know how to use them. Here are a few simple skills that everyone on board should be able to perform if needed.
1) Navigating with a Chart
a) In navigating with a chart, you'll need to gather these three items first: a chart, a set of dividers and a set of parallel slides.
b) Use a chart approved by the U.S. Coast Guard for navigation. The "Not to be used for navigation" charts can provide round-a-bout info, but they are not detailed enough and are not necessarily to scale.
c) Locate the chart's scale to find the distance between two points: Simply spread your dividers to the two points. Move them over to the scale and they will tell you the distance. Most scales are in nautical miles.
2) Using the Compass Rose
a) A compass rose helps you find headings to and from specific points on the chart. The inner ring reflects magnetic bearing while the outer ring is usually true. (Most, if not all, charts reflect associations to true north rather than magnetic.)
b) Finding bearings: Place the edge of your parallel slide so it lines up with both points. Walk the slide to the nearest compass rose, keeping it from sliding out of line until an edge is right on the center point of the compass rose. Look on the outer ring for the bearings to or from either point. NOTE: This does not allow for the direction and speed of the current that affects set and drift.
3) Plot a Position Using a GPS Unit
a) Some people prefer to use the parallel slide to do this, while others prefer to use the dividers, especially on heaving vessels at sea. (Note: The lines of latitude and longitude printed across the chart and the latitude scales printed along the outer edges.)
b) With the GPS readings, find the latitude on the right or left edge of the chart. Spread your dividers to the nearest line of latitude.
c) Find your longitude on the upper or lower edges and follow it to the approximate area of your latitude. Move your dividers to the area and make a small line where the latitude is.
d) Line up your dividers on the nearest line of longitude and spread your dividers to your longitude point.
e) Follow the line of longitude down to where your latitude mark is and make a small line through your latitude line. Where the two lines intersect is your position.
f) To find the coordinates for a point on the map, simply reverse the process. Practice will make this easier.
4) Soundings on the Chart
a) Find the key on the chart and note in what measurements soundings are recorded. (Water depth is measured in feet, meters or fathoms. Six feet equals one fathom.) This is critical. For instance, is the depth where you are going, which has a recorded sounding of three, 3 feet or 3 fathoms (18 feet)? Inquiring minds need to know.
b) Charts use degrees, minutes and seconds of longitude and latitude in pinpointing locations. The top and bottom of the chart display the degrees of longitude (running east to west), while the right and left sides of the chart display the degrees in latitude (running north to south).
c) Before entering a harbor, dock or anchorage go online and obtain free tide tables and graphs, detailed nautical maps and aerial photographs for greater understanding of the marine environment you are about to enter.
Helpful Courses:
Basic Coastal Navigation Course (online availability varies by state):
The U. S. Coast Guard Auxiliary's Basic Coastal Navigation (BCN) and Navigating with GPS (GPS) courses are comprehensive courses that introduce the boater to the methods needed to pilot a boat safely. Go to http://www.cgaux.org/cgauxweb/classes/master.shtml and find your state. When you receive your state's page, look for "BCN" or "GPS" under course title.
Beginner Navigational Courses Available online:
Boat-Ed (State-by-state websites; some states do not offer navigational courses, but courses can be found at other state sites.) This website is found at Boat Minnesota: http://www.boat-ed.com/mn/course/p3-9_riverdamsnav.htm
Navigate!: http://home.att.net/~agligani/navigation/navigate.html
Boat Safe: http://www.boatsafe.com/navigation/index.htm
Starpath School of Navigation: http://www.starpath.com/catalog/courses/courses_index.htm
Helpful Publications
Chart No. 1-Nautical Chart Symbols Abbreviations and Terms: A comprehensive reference publication containing the basic chart elements, which incorporates chart symbols used on Coast Survey charts and those of many other nations. You may order this publication from the National Oceanic and Atmospheric Administration (NOAA) or view it online in PDF format: http://nauticalcharts.noaa.gov
Nautical Chart User's Manual: Equally useful is another publication that provides a wealth of technical information concerning the structure, layout and construction of a nautical chart. This publication covers basic topographic and hydrographic features, aids to navigation, chart symbols, miscellaneous other charting features and supplemental information. Available at http://www.starpath.com/catalog/books/1873.htm
Chapman's Book of Piloting: Because it's a great reference for navigating, this book should be in your library. Available at Amazon.com: http://www.amazon.com/exec/obidos/tg/detail/-/0688168906/104-2656074-5443945?v=glance
Actual Charts: The Office of Coast Survey (OCS) produces nautical charts for United States waters. OCS is an office within the National Oceanic and Atmospheric Administration, NOAA. It is the oldest U.S. scientific organization dating back to Thomas Jefferson who established the office in 1807 to encourage safe and efficient commerce. To order go to NOAA Office of Coastal Survey: http://nauticalcharts.noaa.gov/staff/charts.htm
Here's another website to locate and order nautical maps (NOAA MapFinder): http://mfengine.nos.noaa.gov/scripts/esrimap.dll?Name=Mapserver&cmd=Start
Maptech MapServer--Prints Digital Maps. This is a good site to obtain maps of harbor entrances: http://mapserver.maptech.com/
