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Maps and Charts Name:______________________________________ Maps and Charts Lab He had bought a large map representing the sea, without the least vestige of land And the crew were much pleased when they found it to be, a map they could all understand - Lewis Carroll, The Hunting of the Snark Map Projections: All maps and charts produce some degree of distortion when transferring the Earth's spherical surface to a flat piece of paper or computer screen. The ways that we deal with this distortion give us various types of map projections. Depending on the type of projection used, there may be distortion of distance, direction, shape and/or area. One type of projection may distort distances but correctly maintain directions, whereas another type may distort shape but maintain correct area. The type of information we need from a map determines which type of projection we might use. Below are two common projections among the many that exist. Can you tell what sort of distortion occurs with each projection? 1 Map Locations The latitude-longitude system is the standard system that we use to locate places on the Earth’s surface. The system uses a grid of intersecting east-west (latitude) and north-south (longitude) lines. Any point on Earth can be identified by the intersection of a line of latitude and a line of longitude. Lines of latitude: • also called “parallels” • equator = 0° latitude • increase N and S of the equator • range 0° to 90°N or 90°S Lines of longitude: • also called “meridians” • Prime Meridian = 0° longitude • increase E and W of the P.M. • range 0° to 180° E or W The main lines of latitude and longitude are given in degrees (°) because they represent angular distances. A single degree of latitude covers about 69 miles (60 nautical miles). To locate places more precisely, one degree can be divided into 60 minutes (‘), and one minute can be divided into 60 seconds (‘’). From Trujillo and Thurman, Essentials of Oceanography A nautical mile, by definition, is the distance represented by one minute of latitude, and it equals about 1.15 statute miles (the miles that we use on land), or 6076 feet. By convention, latitude is listed first and longitude second. Here, for example, is how you would write the latitude and longitude of Oceanside Harbor: 33°12’N, 117°24’W. Verbally, you would say that the harbor is located at “33 degrees 12 minutes north, 117 degrees 24 minutes west.” Map Scales A scale allows us to translate the distance on a map or chart to the true distance on the Earth’s surface. You’re probably familiar with the bar scales shown on many road maps. In oceanography, a useful bar scale comes from the fact that one minute of latitude is equal to one nautical mile. Therefore, you can use the black-and-white latitude minute bars on a nautical chart to figure out true distances in nautical miles. Another form of map scale is the ratio scale, which relates map distance to true distance as a ratio, written, for example, as 1:24,000 or 1:100,000. For instance, if the ratio scale of a map is 1:100,000, this means that one inch measured on the map equals 100,000 inches actual distance, or one centimeter on the map equals 100,000 centimeters actual distance, etc. Steps for calculating true distances from map distances using the ratio scale: • Using a ruler, measure the map distance between two points in inches or centimeters. • Multiply that value by the ratio given on the ratio scale to get the actual number of inches or centimeters (it will be a large number!). • Convert that value to a unit that is meaningful over large distances, such as nautical miles. 2 Contour Lines Surface features on maps and charts are indicated by contour lines. A contour line is a line drawn on a map through all points having equal elevation, depth, or other value (such as temperature or salinity). Successive contour lines have units of separation called contour intervals. Contour intervals are always consistent round- number values. For example, on a chart with a 50-fathom contour interval, the contour lines will be labeled 50 fathoms, 100 fathoms, 150 fathoms, etc. Contour lines have the following characteristics: 1. Contour lines cannot join, split, or cross one another. 2. Closely spaced contour lines represent steep slopes; widely spaced lines represent gentle slopes. 3. In the area of a valley or canyon, contour lines form a “V” shape where the base of the “V” points up the valley or submarine canyon. Contour maps that show depths below sea level are called bathymetric maps. In constructing bathymetric maps, first the depths of water are determined by soundings at many places. The soundings are then plotted on a map, and contour lines are then drawn connecting all points of equal depth. Exercises and Questions On the bathymetric map below, the soundings are in feet below sea level. Draw the 100-foot and 200- foot contour lines, and label each contour line with the correct depth. The bathymetric map on the next page shows the region of the Santa Barbara Channel and the Channel Islands west of Los Angeles. The soundings in fathoms below sea level (1 fathom = 6 feet). 1. Draw in the 50-fathom contour line off the mainland and around the islands. The 50-fathom line marks the edge of the continental shelf in this area. 2. Then contour the map at 100-fathom intervals (100, 200 300 fathoms etc.), continuing to the greatest depth shown. 3. Label each contour line with the correct depth. 3 4 Approaches to San Diego Bay Nautical Chart This is a standard nautical chart used for navigation near San Diego. It shows depth soundings and contour lines to portray depths and features of the seabed. It also shows other information, some of which will become familiar to you as you answer the questions below. 1. What type of map projection is used here? (See the explanation key area.) 2. What is the ratio scale of this nautical chart? (fill in the blanks) _______:________________ 3. Based on the ratio scale above, how many nautical miles (nm) does one inch on the map represent? This is called the map's verbal scale. (12 inches = 1.0 foot; 6076 feet = 1.0 nm.) 4. Based on your answer above, what is the distance in nm from Point Loma to the northern tip of North Coronado Island off the coast of Mexico? (Note: This is not Coronado Island at San Diego Bay, but an island west of the coast of Mexico.) Measure the distance in inches, and convert to nm. 5. What is the distance in nm from the northern tip of North Coronado Island, Mexico, to the closest corner (southeast corner) of the Explosives Dumping Area? Measure the distance in inches, and convert to nm. 6. What units are used to indicate depth on this chart? What is the greatest depth shown? 7. What is the latitude and longitude of Point La Jolla, to the nearest minute? (Be sure to write N, S, E or W with your numbers.) 8. What is the latitude and longitude of the bull ring on the U.S.-Mexico border, to the nearest minute? (Be sure to write N, S, E or W with your numbers.) 5 The following questions involve the contour lines shown on this nautical chart. Begin with the sets of clear plastic plates. Directions • Sort the ten plastic plates into A group and B group (all plates are labeled A or B). • Work with your lab partners to stack the five A group plates in an order that makes sense for the contour lines. Your arrangement must obey basic rules of contour lines and look realistic, like an underwater mountain, ridge, or valley. • Find the locations on the Approaches to San Diego Bay nautical chart that match the contour lines for the A group and the B group plates, positioning the five stacked plates at those locations so that the contour lines on the plates match the contour lines on the map. • Have your instructor check the correctness of your stack order and your locations for A group and B group before answering the questions. A group 9. The contour lines for this location represent what geographic feature? (circle the best answer): submarine canyon seamount (submarine mountain) submarine basin 10. The deepest contour line represented by the stacked plates at location A is __________________ (write the depth and the unit; look along the contour line on the map for a round number indicating depth in that unit) 11. The shallowest contour line represented by the stacked plates is ______________________ (write the depth and the unit) 12. Which side (choose a compass direction: N, NE, E, SE, S, SW, W, NW) of the geographic feature at this location has the steepest slopes, and how can you tell? 13. Which side (choose a compass direction: N, NE, E, SE, S, SW, W, NW) of the geographic feature at this location has the gentlest slopes, and how can you tell? B group 14. The contour lines for location B represent what geographic feature? (circle the best answer): submarine canyon seamount (submarine mountain) submarine basin 15. The deepest contour line represented by the stacked plates is ________________________ (write the depth and the unit) 16. The shallowest contour line represented by the stacked plates is ______________________ (write the depth and the unit) 17. How do the contour lines at this location illustrate the Rule of V’s? (If you don’t understand this, go back to the “Contour Lines” section at the beginning of the exercise and read about “V’s”.) 6 San Diego to Santa Rosa Island Nautical Chart This is a standard nautical chart used for navigation along the Southern California coast between Santa Monica and San Diego and around the offshore islands.
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