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1 Introduction	9 Altitude Intercept	17 Latitude by Meridian Altitude
2 Corrections to Sextant	10 Using Position Plotting Sheets	18 Latitude and Azimuth by Polaris
3 Altitude Corrections	11 Plotting Lines of Position	19 Running Fixes
4 Time	12 Summary of 1 thru 11	20 Time of Sunrise Sunset
5 Finding GHA and Declination	13 Finding Deviation or Gyro Error	21 Star Identification/Selecting for Fix
6 Assumed Position and Local Hour Angle	14 Finding Azimuth by 229	22 Time Tick Problems
7 Computed Altitude and Azimuth	15 Amplitudes	23 Deviation Table Construction
8 Interpolation	16 Time of Local Apparent Noon	24 Sample Final Exam

Position Plotting Sheets 

  After you've chosen an assumed position and figured the azimuth and altitude intercept, you'll be ready to plot the line of position on the chart. But you won't have a regular chart.

  When you're offshore in deep water, out of sight and radar range of land there's no use for a chart showing the details in a scale large enough to allow accurate plotting, so the government doesn't print them.

  Instead, there are different kinds of position plotting sheets. These are Mercator charts of small areas made up so that enough of them to navigate around the world would fit in one pocket of your briefcase and cost less than a dollar. To get this convenience, you have to do a little finishing work.

  Remember: this is a Mercator chart. The spacing of the meridians in proportion to the spacing of the parallels depends on the latitude. Near the poles, the meridians would be much closer together than the parallels. At the equator, the distance between the lines of latitude would be equal to the distance between the lines of longitude. The point is, with this type plotting sheet, you must choose the sheet which has your latitude on it.

  You can't substitute. When you have the sheet with the right latitudes on it, turn it the right way up (for north or south latitude) and label the meridians to suit your problem.

 Common mistakes and how to avoid them.

 Labeling longitudes backward:  most of us are used to working in west longitude, so if the middle meridian is labeled 75˚, we call the next one west (left) 76˚. In east longitude, it goes the other way. Put the meridian labels at the top and bottom of every meridian and include the name (E or W) every time. This will force you to think about it so you won't label or read them backwards. Draw an arrow in the margin in the direction that longitude increases.

  Reading the latitudes backwards. you've spent most of your life in north latitude, and are used to seeing latitude increase as you go north. If the problem puts you in south latitude, you have to change your habits. If you set the sheet up for a problem in south latitude, put the S next to each parallel on both sides and draw an arrow pointing south to remind yourself

  Reading directions 180˚ out:  if you're in north latitude, the numbers on the inside of the rose apply. In south latitude, you use the outside numbers. North is always at the top, east to the right, etc. But your eye wants to read the number that's right side up. It's very common for beginners to write down 90˚ when it was 270˚ and so on. Solution: decide which numbers you're supposed to use, and strike the other set out. (Don't mark them out with a great black blob. Your plot may pass through that place and it would be hard to see.)

  A common question and the answer: how do i know how to label the meridians? Where do i start? It doesn't matter so long as your problem will fit on the sheet. In any of the problems in this book, or on the test, the problem will fit on the sheet if you label the

  Center meridian with the degree of longitude given as a starting position in the problem.

  Later on you may get fancy and rig your sheet so the plot comes out in the clear space and doesn't lap over the compass rose. It just makes it easier to read. If it ever happens that your plot starts running off the edge of the paper, just start over after relabeling the longitudes to make it fit. You need the practice anyway.

  

Now the other plotting sheet

   View image

    This type can be used anywhere a Mercator chart can be used. You could navigate around the world with one copy if you erased carefully. Don't. They come in pads of 100 and the cost is minimal.

    You label the parallels of latitude to fit your problem.

    Notice the middle latitude on this following example turned out to be 28˚N. That controls the spacing of the meridians.

    Notice the scale of degrees on the outside of the right half of the rose, reading from 0˚ on the middle latitude to 70˚ up and down. Those are not directions. They are used to find two points to draw your first homemade meridian through. Once the first one is established, any others you may need are spaced same distance as the middle one is from the first one you draw. The latitude scale works the same as any other, only it's in the middle instead of on the left and right margins. The longitude scale in the southeast corner is funny looking and takes a little practice. Since the middle latitude established the width of one degree of longitude, it naturally controls the minutes.

  This universal plotting sheet is the most complicated one in print. If you can handle it, you can manage any of them. There are some plotting sheets printed complete with all the labels, ready to use. If the examiner gives you one of those, it will save you a little work. But look out! An examiner may give you a plotting sheet with latitudes that don't match the problem. He may tell you to simply change the latitude numbers to fit the problem. Don't do it!

   If you work on the wrong plotting sheet, your work could be absolutely correct but your answers won't match the coast guard's because of the difference in latitude scales. The universal plotting sheet is reproduced and explained here. View image

PLOTTING LINES OF POSITION

 

Last edited on 25-Sep-2009 04:19:03 -0500