Plotting

The final exam for this module has a total of 10 multiple choice questions. You must score a 90% to pass, which means you can’t miss more than 1 question.

Coastal charts of U.S. waters are published by the National Ocean Services (NOS). Charts have a Chart Title, which usually contains the name of the prominent navigational area covered by the chart.  It will state what type of projection the chart is, the scale of the chart, and the unit of the soundings (feet, fathoms, or meters).  The soundings on the East Coast of the U.S. are recorded as mean lower low water based for normal weather conditions. Mean lower low water is the 19-year average of the lowest low water of each tidal day. This is not the lowest that the water can be, but it should prevent vessels from running aground based on this information. However, extreme weather conditions should be taken into consideration when using chart information. The heights on a chart are measured in mean high water. This measurement should assist vessels clearing bridges and overhead cables.

A nautical chart is a flat graphic representation of a navigational portion of the earth’s surface. In this course you will be using a training chart, which is not used for actual navigation. “TR” after a chart number indicates that it is a training chart. A chart shows the depth of water by numerous soundings. Sometimes depths that are equal are joined with a line to form a contour. A contour line represents equal depths in an area. A break in a contour line with a number in the break shows that the points on that contour line will be that specific depth. Depths of water may be in feet, meters, or fathoms. This information as well as the distance representation is found in the scale of a chart. It is extremely important for the mariner to know the scale measurements for his or her nautical chart. Deeper navigable will be white while shallower depths are shown in a different color, such as blue. Charts also show land areas, shorelines, and topographical features. Land areas are often shown using a different color, such as brown (buff). The most important information on a chart are lights, buoys and other aids to navigation and are depicted in symbols. These symbols have information about what the mariner would observed both during the day and at night. Charts include information on bridges, cables, wrecks, and other information of interest to mariners. Charts assist the navigator with arriving safely at a destination and in avoiding dangers. Today’s mariners tend to rely on their GPS, but a chart should always be available as a backup in the event that the GPS goes out. Local charts are required on certified boats.

Charts are Mercator Projections, which provide equal expansion in all directions, thus flattening out the spherical earth.

Latitude Lines are parallel, running east and west around the earth. They are measured north and south (ladder-up scale i.e. the rungs of a ladder), with the equator being the zero mark. The North Pole would be 90 degrees North and the South Pole would be 90 degrees South. Latitude is measured in degrees, minutes, and seconds. 1 degree of latitude is equal to 60 minutes. 1 minute of latitude is equal to 1 nautical mile. This is where the term “a mile a minute” comes from. On a chart, latitude lines can be used to measure distances, but never longitude, except at the equator.

Longitude Lines run north and south but are measured east and west. Since they are widest at the equator and come together at the poles, these lines are not parallel.  The Prime Meridian, which goes through Greenwich, England, is zero degrees. These lines go from 0-180 degrees, with 180 degrees being the exact opposite side of the earth from the 0 degree line. Any point on a nautical chart can be expressed using latitude and longitude, with the latitude number always being given first.

An important part of any chart is the scale, which determines distances on that specific chart. The scale of a nautical chart is the ratio of the distance on the chart to the actual distance on the surface of the earth. For example, for a scale of 1:5,000, one unit on the chart is equal to 5,000 units on the surface of the earth. Chart scale will be different on each chart, so it is important for the mariner to be familiar with it. Charts are oriented with North at the top.  Any point on a chart can be expressed by a latitude number and longitude number. Identical latitude scale runs on both sides of a chart, while identical longitude scale runs along the top and bottom of a chart.

Nautical charts have one or more compass roses printed on them.  Since charts are oriented with North at the top, direction is measured as a straight line from the center point of the circle to a number on the compass rose. There are two rings, the outer ring is in true; the inner ring is in magnetic.  Both rings have an arrow pointing due North. True North is the North Pole, whereas the Earth’s magnetic forces cause magnetic compass needles to point to Magnetic North Pole. Variation is the difference between true north and magnetic north. Since the variation changes with geographic location, the variation for an area is always found in the center of the compass rose on a chart as well as any annual change. Variation is listed as either East or West, and it is important to include that information along with the numerical value.