How to Read Navigation Charts

Written by will charpentier
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If you plan to leave the comparative safety of your local lake's familiar waters, you'll use a chart to find your way. A navigation chart--it's more than a simple map--is covered with information on the area it encompasses: depth of water, warnings about pipelines and ageing ordinance on the sea bottom, numbers, lines and strange navigational symbols. It also has keys and guides to those symbols and instructions for those lines and numbers; a few minutes of reading will tell you all you need to know.

Skill level:

Things you need

  • Navigation chart
  • Parallel ruler
  • Blue pencil

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  1. 1

    Find the scale on the chart in the upper left corner. This is a number, like "1:2,160,000" or "1:80,000." The larger the second number, the smaller the scale, because on the chart whose scale is 1: 2,160,000, each inch on the chart equals 2,160,000 inches on the ground--29.62 nautical miles. The level of detail is smaller than a chart where each inch equals 80,000 inches--1.09 miles--on the ground.

  2. 2

    Buy and keep a copy of NOS (National Ocean Service) Chart 1 next to your chart table. Chart 1 is a booklet that shows and explains all of the symbols and abbreviations used on nautical charts, including the symbols for rocks, submerged obstructions, reefs, wrecks, power lines and the other things that stick up from the bottom and out of the water. The symbols for channels, buoys and day beacons are shown as well.

  3. 3

    Find locations on the chart by using their latitude and longitude. The numbers printed in bold type along the top and bottom of the chart are meridians of longitude; the bold numbers along the sides are parallels of latitude. Draw a vertical line between a location and the number above it or below it on the edge of the chart to find its longitude, and draw a horizontal line though the same location to the number on the edge of the chart to its left or right to find its latitude.

  4. 4

    Look at the numbers in the "water" parts of the chart. These numbers tell you the depth at that location or, more specifically, whether or not you'll "run out of water" at that spot.

    On the bottom of the chart, in large letters, you'll see the words, "SOUNDINGS IN ...," followed by the unit of depth used on the chart. Most charts in the United States will say, "Soundings in Fathoms," "Soundings in Fathoms and Feet" or "Soundings in Meters." Since a fathom is 6 ft., a depth of 3 fathoms equals 18 ft. If the chart says, "Soundings in Fathoms and Feet," a depth of "12" isn't 12 fathoms; it's 1 fathom and 2 feet (the first numbers are fathoms, the final number is always less than 6, since it's part of a 6-foot fathom). Soundings in meters can be quickly converted to feet, by dividing by 3: the math isn't accurate, since a meter is slightly more than 3 ft. long, but it gives a quick, conservative estimate of the depth in feet.

    In the upper right corner of the chart, there's a notation that the depths shown are the depth of the water at Mean Lower Low Water (MLLW), the lowest average level, so these are the average minimum depths. If the tide table show a minus sign (like, "-1") that means that you need to subtract that number from the chart's water depth.

    Similar depths are connected by dark blue Contour Lines. If there's no depth shown at a particular location, you can glance along the contour lines nearest the location until you see a number for the depth. If the contour line north of the location traces to a "6" and the contour line south of the location traces to a "7," then the depth lies between 6 and 7of whatever unit is used for the Soundings on the chart.

  5. 5

    Buoys and day marks are shown on the chart to mark approaches and safe channels. Charts indicate prominent landmarks, like a hill, a water tower or even some buildings to aid the mariner in determining his position and orientation.

Tips and warnings

  • Do your navigational work before you set sail.
  • When entering a new port, never hesitate to use your marine VHF radio to ask for "local knowledge."

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