LESSON G: NAVIGATIONAL CHARTS

A nautical chart is a kind of map that contains information useful to sailors. As you know, a map gives us a detailed "bird's eye view" of some area of land. A chart gives the same kind of view of the water. The area detailed on a map or chart may be large, like a map of a continent (or even the whole world), or it may be small like a map of your city, your neighborhood, or a chart of your harbor. The smaller the area shown, the greater the detail. Imagine that you want to help a new friend from a different city to find your home. Sending a map of the world probably wouldn't do them much good. A highway map showing the route between your town and your friend's town and a map of your neighborhood are what your friend will need. The differences in size between a world map, a highway map, and a city street map are called differences in "scale" and we have more to say about these differences later.

Because the sailor navigating on the water has to pay attention to different things than someone travelling on land, the nautical chart looks different from a land map. If you take a look at a nautical chart, (click for image) you'll see that a number of different colors are used to portray the various areas. Yellow is land, green marks the shoals (water too shallow for navigating in anything but the smallest boats, like canoes or rowboats), navigable water is shown in blue (the lighter the deeper) and white is used for the deepest water. Notice that the land areas on a nautical chart do not contain many details, only things that might be helpful to people in boats: the contour of the coastline, and prominent landmarks like mountains, church steeples, or towers. Water areas on the nautical chart show much more information than water areas on land maps. The colors of the water areas give a general idea of water depth, but numbers are provided as well for more precise depth readings, which in this case are given in fathoms (one fathom equals six feet). The number 1₃ indicates one fathom plus three feet, or nine feet. The units of measurement for things like depth of water and height of objects above water (offshore rocks, cliffs, lighthouses) will often vary from one chart to the next (meters, feet, fathoms, etc), so it is important to read the fine print on whatever charts you use. Various symbols on the chart show such things as dangers to navigation like submerged rocks and sunken wrecks, and aids to navigation such as lighthouses and buoys.


North, South, East and West (Using the Compass Rose)

You probably know about compass directions and that most (but not all) maps and charts have North at the top, South at the bottom, East on the right side and West on the left. Every nautical chart will also contain at least one "compass rose" (click for image) printed somewhere on its surface. This is a circle three or four inches across with another circle inside of it and a little bullseye or a tiny + exactly in the middle. The outer circle will have a star lined up with the number 0. The star represents the North Star and the 0 is the "true" bearing for true north or geographic north-the location of the North Pole. All the other numbers on the circle stand for other bearings--other possible directions to head on the surface of the Earth.

Instead of using names for all these possible directions, we use numbers. That way we can divide the circle up into 360 degrees and we can divide each degree up into 60 minutes, each minute into "tenths of a minute" and each tenth-of-a-minute up into ten parts (hundredths of a minute) and so on. This gives us an infinite number of possible directions of travel and a very exact way of describing each of these directions.

Question: What would be the bearing for the South Pole? Answer: 180 degrees true (You have heard of naming directions such as "Northeast", "Southwest" and even "Northeast by East", but soon we would run out of names for these directions. We will never run out of numbers, however, because we can always add another decimal place and begin counting again.)

The true bearing for North is 0. The true bearing for East is 90 degrees. Notice that a line from the + in the middle of the compass that connects to East (90 degrees) makes a right angle with the line from the + that would connect to North. The degrees of bearing on a compass rose are the same degrees used in geometry to measure angles.

Question: Using these angles and bearings, what is the bearing, in degrees, for the direction called Northeast--half way between North and East?
Answer: 45 degrees, half way between 0 (north) and 90 (east).

You may have wondered why we called these bearings "true" bearings and why we call North "true" North. Directions don't tell lies! But there is another set of directions that we have to know about. We use a magnetic compass on our boat to help us find our way. The compass does not see the North Star and point toward the North Pole. Instead, the compass lines itself up with the Earth's magnetic field. The compass points to "Magnetic North" which is almost (but not quite) the same as "True North". The second circle on the compass rose, (inside the circle marked with the star) lines "0" up with Magnetic North. There are three facts you need to know about Magnetic North before we go on: Magnetic North is not the same direction as True North, although they are close to the same direction in most regions of the world. The difference between them is called variation. Variation (between Magnetic North and Truth North) is different at different places on Earth. The compass rose on two different charts might show different variations. Because the earth's magnetic field is always shifting, the magnetic North Pole moves around (it is currently located just off Ellef Ringnes Island, in the Northwest Territories of Canada) and so variation at any fixed point on Earth may change slightly from year to year. For instance, the chart of the lower Connecticut River where it meets Long Island Sound says that the variation in 1984 was 15 degrees, 15 minutes West. It says there will be an annual increase of 4 minutes. In the 15 years since 1984, we could expect the variation to have increased 60 (4X15) minutes, which happens to equal one degree. So when we use this chart on Voyage 2001, magnetic North will be 16 degrees, 15 minutes west of True North.

The compass rose helps us navigate. First we figure out our starting point on the chart. Let's call our starting point "Point A". Then we figure out the first leg of our journey and mark it on the chart as "Point B". We draw a line between point A and Point B, making sure the line doesn't cross any land, rocks, or other obstructions and that the water is deep enough for us to pass safely over the bottom. This line shows our course from A to B.

Now that we know our course, we need to find out the compass bearing (direction) we must steer to follow the course we have drawn on the chart. We use the compass rose to find this bearing. But the course will probably not pass directly through the center of the compass rose. We need another tool, called a parallel ruler to make a line through the exact middle of the compass rose. The line has to extend from the + point out to the circle of "magnetic" bearings. If the line on the compass rose is exactly parallel to the line of our course, the reading on the compass rose will be the direction on our compass we must go to follow that course. That is how we find the bearing from point A to point B.


Size or distance on a chart -- "scale"

Any chart or map that we rely on to find our way must have a consistent "scale". This means that the size and shape of something shown must fit exactly the same proportions as the size and shape of the real object. You have seen scale models of cars (such as matchbox cars), trains (model trains), houses (dollhouses) or boats. Dolls are scale models of people.

Nautical charts come in different scales, just as regular maps do. The scale is expressed as a ratio, like 1:50,000. This means that something that's one inch on such a map is actually 50,000 inches long in the real world. A street plan of a city might be of a scale around 1:50,000. It would show all kinds of details you wouldn't be able to see on, say, a map that showed the whole continent of North America, which might have a scale of 1:4,000,000. A street plan is an example of a large-scale map; a map that showed all of North America would be a small-scale map. Here's an easy way to remember the difference: a large-scale map or chart makes things (like streets and parks) look larger (so you can see them), a small-scale map or chart makes things look smaller. Even if you could draw the streets and parks of your hometown on a standard-sized map of North America, they would be drawn so small you wouldn't be able to see them.

Here are charts of the same area in two different scales, Shippigan Island in New Brunswick, Canada. (Note: the charts posted here are for educational purposes only and should not be used for navigation. Navigational charts are updated regularly to reflect the changes that take place in the real world, and you should always use the latest updated charts for the areas in which you'll be sailing.) This is the smaller-scale chart (click here for image). Its scale is 1:350,000. Notice how on this chart the island appears smaller, and includes much less detail than on this larger-scale one (click here for image), whose scale is 1:40,000. This chart shows the water area (called a strait) between the southern end of Shippegan Island and the mainland. What features does it show that are not included in the smaller-scale chart?

In the course of the voyage taken on the Easy Wind, we use both small-scale and large-scale charts. The small-scale ones are useful for getting an overview, for mapping out where we think we'll be in a day's or a week's time, and the large-scale ones are useful for navigating in and out of harbors, or sailing through a group of small islands. The large-scale charts show us the more of the details we need, like hazards and aids to navigation, than small-scale ones can.

Remember the course from A to B that we plotted in our discussion of the compass rose? We learned how to find the bearing or direction to steer to go from A to B. The next thing we have to know to navigate between these two points is how far away B is and how long it will take us to get there. Using a ruler we find the distance on the chart between A and B in inches and fractions of an inch. We then compare those inches with something on the chart that tells us the scale. You will find a long line (5 or 6 inches long) that says "Nautical Miles" or "Scale of Miles." Measure the distance from A to B along those lines to figure out how far apart these two points actually are. Once you know this distance, find out how fast the boat is moving. You can get this from an instrument on the boat, like a speedometer on your car, which gives your speed. You calculate how long it will take you to get to point B by dividing the number of miles you have to go by the time it takes to go one mile. Add that time to the time on your watch or ship's clock and you will know when you will arrive at point B.


Longitude and Latitude (How coordinates make life a lot easier aboard Easy Wind)

So you have learned to navigate using your chart, your compass rose, your compass and your chart's scale. You are heading for point B, somewhere at sea. You know what course to steer and how long it will take you to get there. How do you know when you reach point B. What if there is not a big pole sticking out of the ocean saying "This is point B" on it? How do we describe a point on the earth's surface that's on the ocean--no street signs, no numbers, no zip code?

Answer: we need to use coordinates: Latitude and Longitude to pinpoint places on a chart and describe exactly where we are.

Along the very edges of the chart you'll notice lines marked off with numbers, kind of like a ruler. These show the longitude and latitude of the area depicted by the chart. Lines of longitude and latitude are the ones you see making up the grid that's often shown on a globe. The ones that go up and down are longitude lines. (click for image) The ones that go side to side are latitude lines. (click for image) Here's an easy way to remember which is which: think of the lines as marking the cuts that you make to slice up an orange. The pieces you get when you cut up the orange along the lines of LONGitude are LONG and pointy. If you cut along the lines of LATitude you get FAT round disks.

Notice that the circles described by longitude are all the same size, whereas the circles described by latitude vary in size; they get smaller the closer you get to the poles. Any given line of longitude represents what is known as a great circle, the largest possible circle you can draw on a globe. If you were to cut through the globe along any great circle, you would always cut through the exact center of the globe, and cut it into two equal pieces. (Note, however, that this holds true only for a perfect sphere. The earth is not a perfect sphere, so the two pieces of the real Earth formed by a cut through a great circle would not always necessarily be equal halves.) Great circles play an important role in long-distance navigation (including airplane navigation): the shortest distance between any two points on a globe is always a route along a great circle. You can easily find a great-circle route between any two points on a globe with a piece of string: just stretch the string tightly between your fingers, which you press down on the two points. All lines of longitude are great circles, but only one line of latitude is: the equator.

Positions in longitude and latitude are measured in degrees. Lines of latitude are also called parallels (because they're parallel!). The equator is 0 degrees latitude, the North Pole is 90 degrees north latitude, and the South Pole is 90 degrees south latitude. The lines of longitude stretch from the North Pole to the South Pole, and are also called meridians. A meridian would be half of a great circle. The Prime Meridian, 0 degrees longitude, passes through Greenwich, England. Directly opposite it is 180 degrees longitude, which passes mostly through the Pacific Ocean. Going west from Greenwich, degrees of west longitude increase up to 180, and going east of Greenwich, they increase as degrees of east longitude until they meet the same meridian at 180 degrees.

Along the edges of the chart you'll find the scales of longitude and latitude: the longitude scales run along the top and bottom, and the latitude scales run up and down the left and right sides of the chart. You might well be confused at this point: didn't I just say before that longitude runs up and down and latitude side to side? That's true, but the scales used to determine each have to run perpendicular to the lines. In other words, the latitude scale runs up and down because each point on it marks the intersection of a separate side-to-side line of latitude that crosses it there. We're not measuring along a line of latitude here, we're trying to pick the one correct line from an infinite number of other possible lines that lie parallel to it. The same holds true for the longitude scale, only the lines it determines aren't parallel; they're closer together toward the poles and farthest apart at the equator.

The chart's longitude and latitude scales serve two main navigational purposes. First, they allow you to express the position of any point on the chart as a set of numbers, known as coordinates, giving its longitude and latitude. (Conversely, if you happen to know the coordinates of an object, you can look them up on your chart and see where the object is.) Second, the lines on the latitude scales (running up and down the side edges of the chart) can be used to measure distance. The longitude scales (on the top and bottom edges) cannot be used to measure distance, because the distance between each line of longitude gets smaller the closer you get to one of the poles. The distance between degrees of latitude, however, remains constant no matter where you are (because they're parallel) and can therefore be used as a scale to measure distances between objects on your chart. Each degree of latitude or longitude can be further divided into 60 minutes, and each minute is divided into 60 seconds. Nowadays, however, it is more common to divide minutes into hundredths and thousandths, using the decimal system. One minute of latitude is equal to one nautical mile (isn't that handy?). A nautical mile (6080 feet) is somewhat larger than the mile we use to measure distance on land (which measures 5280 feet and is known as a statute mile).

One thing you need to remember when using the latitude scale to measure distances is that even though the degrees of latitude are evenly spaced on the curved surface of the globe, this is not the case on the flat chart. They're still parallel, they're just not evenly spaced. The flat chart's distortion makes it necessary for the distance between each marked degree to increase the closer you get to one of the poles. You can see this if you measure 5 minutes of latitude with a ruler or set of dividers once at the bottom of the chart, and again at the top of the chart. You won't really be able to see the difference on the 1:40,000 scale chart, but on the 1:350,000 it's quite apparent.

Here's how to measure distance using the latitude scale and a set of dividers The compass you use in math class will do in a pinch, or even a paper clip bent into an angle with two equal sides. First mark the starting and ending points of the distance you want to measure and connect them with a line using your straightedge. You should use a pencil, not a pen, and you don't need to draw a solid line - a series of faint dashes is enough. It's OK to mark up the chart a bit, but it's always a good idea to do so as sparingly as possible. For one thing, you don't want to risk confusing yourself with markings made for an earlier measurement, nor do you want to make the lines so heavy as to cover up any important features on your chart. That's why a series of dashes is often better than a solid line. Also, vigorous erasing of a line drawn too heavily can remove important features from the chart. The next step is to open the dividers to a width corresponding to a handy number of minutes (say five) at the place on the latitude scale roughly in the middle of the line you want to measure. Remember: it's not only the scale along the side that gets more distorted the farther north you go - the whole chart "stretches" as well. Then you place one leg of your dividers on the starting point and "walk" your dividers along the line, measuring out intervals of five minutes. When you get to the point where the last interval left is less than five degrees, keep your divider point where it is, write down your total distance so far (say 7 x 5 = 35 miles), and then bring the divider's points closer together to measure the length of the remaining segment. This last segment must be measured on the latitude scale at the same level from which it was taken on the chart (to compensate for the distortion).

The dividers can also be used to determine the precise position of any given point on the chart. Open the dividers from the point in question to the nearest grid line on the chart, then transfer this distance to the scale on the edge of the chart. Do this twice - once for longitude, once for latitude.



On to Lesson H: The GPS -->