Reading nautical charts is not the same as reading a map. It’s a bit more complicated because a map only shows you how to get from one point to another using roads while a nautical chart has a lot more information you can use to navigate safely on the open sea. Knowing how to read nautical charts is a skill that you must have if you enjoy spending time on your boat. Here are the things to know to be able to read nautical charts with ease.
Scale of the Nautical Charts
The scale of the chart you’re reading is the first thing you need to know when reading nautical charts. You can encounter a large scale chart like a harbor chart that is very detailed, or you can choose a small scale chart that covers a much bigger area on the earth’s surface. Large scale charts are usually between 1:20,000 and 1:40,000. This means that 1 nautical mile is about 4” of the 1:20,000 chart and 1.8” of the 1:40,000 chart.
To find out the scale of the chart you need to look for the designation on the chart which is a fraction(1/20,000). If the number after the slash is higher, that chart shows a larger area in a smaller scale which means it will be less detailed.
The water depth is indicated by numbers that are printed on the water areas of the chart. However, the essential part of chart reading is knowing which unit of measurement is used for creating the chart. It can be feet, meters or fathoms and it’s crucial for your safety. You can see the unit used on the face of the chart because it’s usually shown in a large print to avoid confusion. If the map uses fathoms and there is a number on the map like 04 it means the water at this spot is zero fathoms and 4 feet deep. So the first number shows the fathoms (1 fathom = 6 feet) and the second number shows the feet.
Aside from numerical chart symbols, water depths can be represented with contour lines. If you are reading a small scale chart you may encounter the 10-fathom contour line which means that the water depth within this line is not deeper than 60 feet. Larger scale charts have contour lines too but most of them are 3, 2, or 1 fathom lines. Large scale charts also have 3 feet contour lines and they have blue tinted areas so you can quickly identify the shoal water.
Symbols and Abbreviations
Nautical charts also incorporate other symbols and abbreviations to mark the location of various obstructions such as rocks, wrecks, anchorages, lighthouses, buoys, channels, reefs, submerged obstructions and tide rips. Most of the symbols used for them are intuitive and you’ll understand what the symbol means at first glance.
Nautical charts also include aids to navigation. You can find small triangles on a nautical chart which means there is a red daybeacon. If you see a magenta flare symbol, it means there is a lighted marker. The chart may have buildings, towers and hills marked if those locations may be of importance to the navigator.
The compass rose is an indicator for true north – the outer circle and magnetic north – the inner circle. If you encounter a group of three concentric circles on the chart you can identify it as a compass rose. The more common compass people use is a magnetic one so the inner circles will be the ones of greater importance. Those circles are usually marked with degrees between 0 and 360.
Plotting A Course
When you’re first learning how to plot a course, it might be intimidating, but if you get the feel of it, it’s not that tough. You’ll need a few tools including a pencil, a few dividers and a parallel rule.
To start plotting a course you will need 2 points on the map. Starting point (A) and ending point (B). When you have these 2 points, draw a straight line between them using the parallel rule. You want to know the distance and direction from point A to point B.
To learn the direction and distance you will need to align one leg of the parallel rule to the line you drew from point A to point B. Holding that leg pressed on the chart, move the other leg until the outside edge of the leg is at the very center of the cross in the closest compass rose. The point where the edge of the parallel rule meets the center of the magnetic compass rose is your magnetic course from point A to point B. This is your steering course.
Measuring distance on a nautical chart is a bit easier. For this you will need to place one divider on point A and another on point B. Without moving the position of your dividers, place the points of the dividers on the latitude scale. Count the minutes on the lines of latitude scale between the dividers and you will get the distance in nautical miles because one minute equals one nautical mile.
Learning how to read nautical charts may be intimidating for beginners but it’s an essential skill you must have if you want to safely navigate your boat from one point to another. Even though it seems difficult at first glance, the symbols and other markings are quite intuitive and it doesn’t take long for people to get the hang of it and start plotting their course to enjoy their days off with plenty of fun activities.