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Jim Sexton 11-28-2004 07:00 PM

All About Charts
 
<HTML><TABLE cellSpacing=0 cellPadding=0 align=right border=0><TBODY><TR><TD width=8></TD><TD vAlign=top align=left><IMG src="http://www.sailnet.com/images/content/authors/sexton/chart1_title.jpg"></TD></TR><TR><TD colSpan=2 height=8></TD></TR></TBODY></TABLE><P>The mark of a professional mariner is in attention to details. A good navigator can use a chart to navigate successfully. A professional navigator can also do this, but additionally he knows and understands every detail of the chart he is using. You can tell the difference between the two by observing how each studies the chart before using it.</P><P>Everything you see on a chart has meaning and represents a shorthand way of conveying important information. Knowledge of the chart symbols enable understanding of what you see. Here is how to become a professional.</P><P>The first thing you want to do before using any chart is to carefully read the borders of the chart and the information posted under the chart's Title Block. Here is what you can expect to see and what it means: </P><P><B>Soundings</B>. At the top and bottom of the chart you will see the unit of measurement used for the soundings in large block letters on this chart. SOUNDINGS IN FATHOMS, indicates that soundings are in fathoms and fractions of fathoms. One fathom is six feet. SOUNDINGS IN FATHOMS AND FEET, indicates the soundings are in fathoms with subscript in feet. For spot depths, the fathom will be the larger number while the fraction of fathoms or feet will be the smaller subscript number. For example, you might see a spot depth on the chart of 4<FONT size=-2>2</FONT>. This indicates a depth of 4 fathoms and, if the soundings are in fathoms and feet, the small 2 indicates two feet, so in this case the depth is 4 fathoms plus 2 feet or 26 feet. SOUNDINGS IN METERS, indicates that the soundings on this chart are in meters and decimeters. The larger number on the spot depths are in meters while the smaller number is tenths of a meter. Use the depth conversion scale provided to convert between meters, feet and fathoms. </P><P>All depths indicated on charts are reckoned from a selected level of water called the chart sounding datum. For NOAA charts the datum is usually Mean Lower Low Water. This is the average height of the lower low waters of each tidal day over a 19-year period. Older charts use Mean Low Water and foreign charts can use other tidal datums. You must understand the soundings and tidal datum for the chart you are using and take into consideration the height of tide and the tidal range to avoid grounding. You also must be aware that there will be certain times of the year or weather conditions which can make the actual depth less than that depicted on the chart. </P><P><B>Scale</B>. The scale of a chart is the ratio of a given distance on a chart to the actual distance it represents on the earth. For example a 1:40,000 or 1/40,000 scale means that one unit on the chart, represents 40,000 of the same units on the surface of the earth. A chart covering a relatively large area is called a small scale chart and one covering a relatively small area is called a large scale chart. Since the terms are relative, there is no sharp division between the two. Thus a chart of scale 1:40,000 is large scale when compared with a chart of 1:100,000, but small scale when compared with one of 1:20,000. Think of the scales as fractions and you will always remember the difference between them. For example, 1/2 of a pie is larger than 1/4 of a pie. The chart with the larger denominator in the fraction indicates the one that is smaller in scale in relation to the other one. Remember that as the scale decreases, the amount of detail shown also decreases. Large scale charts show large amounts of detail, while small scale charts show small amounts of detail. Small scale charts are used for planning and off shore navigation. Larger scaled charts are used for coastal navigation or entering and anchoring in harbors and small waterways. </P><TABLE cellSpacing=0 cellPadding=0 align=right border=0><TBODY><TR><TD width=8></TD><TD vAlign=top align=left><IMG src="http://www.sailnet.com/images/content/authors/sexton/chart2.gif"></TD></TR><TR><TD colSpan=2 height=8></TD></TR></TBODY></TABLE><P><B>Heights</B>. The shoreline shown on charts is generally based on mean high water. A light's height is usually computed from mean sea level. The heights of overhanging obstructions (i.e. bridges and power cables) are usually reckoned from mean high water. A high water reference gives you the minimum clearance. Heights and elevations on land are reckoned from mean sea level. </P><P><B>Datum Note</B>. Most charts produced by DMA and NOS will show a datum note in the Title Block. The note may say "World Geodetic System 1972 (WGS-72)", or World Geodetic System 1984 (WGS-84)", or World Geodetic System (WGS)." A datum note for a chart for which satellite positions can be plotted without correction will read: "Positions obtained from satellite navigation systems referenced to the world Geodetic System 1984 (WGS-84) can be plotted directly on this chart." It is vitally important that your GPS be set to use the datum of the chart you are using. By now the majority of NOAA charts have been converted from NAD 27 to WGS-84 or WGS. However, if you find yourself using an NAD 27 chart, you must set the GPS to read NAD 27. This way the GPS coordinates will agree with the latitude and longitude scale on your chart. If you don't set the GPS you can expect to have position errors of 10 to 110, (33-360 feet), meters, depending on where you are in the U.S. See your GPS manual on how to set this up. </P><P><B>Type of projection</B>. Most of the time you will be using a Mercator projection. If not, make sure you understand the correct usage for that type chart. For a complete understanding of chart projections see "The American Practical Navigator" by Bowditch. </P><P><B>Chart dates and Edition</B>. NOAA charts have two dates. At the top center of the chart is the date of the first edition of the chart. In the lower left corner is the current edition number and date. This date shows the latest date through which the Notices to Mariners were applied to the chart. Subsequent changes will be printed in the Notice to Mariners. In order to be legal, you must have the latest Notices to Mariners corrections added to your chart. </P><TABLE cellSpacing=0 cellPadding=0 align=right border=0><TBODY><TR><TD width=8></TD><TD vAlign=top align=left><IMG src="http://www.sailnet.com/images/content/authors/sexton/10052Kchart.gif"></TD></TR><TR><TD colSpan=2 height=8></TD></TR></TBODY></TABLE><P><B>Notes and warnings</B>. Read all notes and warnings posted in the Title Block area and check for applicability to your voyage. </P><P><B>Other misc. information</B>. Depth conversion scale, logarithmic speed scale and distance scales will all be in the margins of the chart. Take the time to note their location and understand how to use them. All of this information is also available on raster scanned electronic charts, since they are a photograph of the paper chart. Many of these details are also available through the Properties menu selection for both raster and vector charts. How to get there will vary between software programs. Make sure that you know the scale of each chart that you are using as the active chart. As you zoom in, you may change charts as you go from a smaller scale chart to a larger scaled one and it is not always apparent. When you get to the largest scale chart available, then you may start to magnify the chart (over zooming.) You must stay aware of the scale and any over zooming so you don't become confused. For example on a 1:80,000 chart, one inch equals 1.1 NM. When you zoom in, this relationship changes to where 2 inches might equal 1.1 NM. </P><P>All vector charts are WGS-84 calibrated so they are directly compatible with your GPS. However, some raster charts may be NAD 27 datum, or in foreign waters something entirely different. It is important that you know the datum of the electronic chart you are using and set your GPS accordingly. </P></HTML>


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