How do I read my chart scales? - SailNet Community
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post #1 of 12 Old 05-27-2010 Thread Starter
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How do I read my chart scales?

So I'm having some difficulty trying to determine exactly how to use the scale on my charts...here is an (edited) picture of exactly what I'm referring to:



When I'm trying to figure Nautical mileage, is my 1 mile between Blue dot and Yellow dot? Blue and Green? Green and Yellow?

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post #2 of 12 Old 05-27-2010
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Green to blue and green to yellow appear to be the same distance and = 1 NM.

The scale of your chart is 1:40 000 as indicated, but if all else fails 1 minute of latitude = 1 NM - that's the scale on the left (and right) margin of the chart but never on the top/bottom margins.
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post #3 of 12 Old 05-27-2010
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Don't use the scale to measure nautical miles. Use the latitude marks at the side of the chart. A nautical mile (one minute of arc) varies in length (feet) depending on the latitude so you should always use the latitude marks.

A nautical mile varies in length from 6040 feet at the equator to 6108 feet at the pole. The mean length is 6080 feet and this is the standard nautical mile.

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post #4 of 12 Old 05-27-2010
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Hays,

The scale of this chart is for a somewhat small area. Therefore the longer NM.

On your example, blue to red, and red to green are each 1/2 NM, and each of them is divided into 1/10 NM. Green to yellow is 1 NM.

So, dependent on what you are using to measure NM, and a protractor works as well as anything, you have a scale that that can be translated. If something is 5.7 NM, you will move the protractor adjusted at 1NM five times and then adjust it to the balance of your distance and measure against the 1/10 scale. Clear as Mud?
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post #5 of 12 Old 05-27-2010
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Vasco— The scale on a chart should be reasonably accurate over its entire area...

As Mawn has pointed out—the green dot is the same distance from the yellow or blue dots and can be used to determine distances. The reason for the side to the left of the green dot is so you can read/measure tenths of a mile.

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post #6 of 12 Old 05-27-2010
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Hays--In your effort to learn to sail it would be wise to take some of the Cost Guard Auxillary classes and particularly, in this case, the class on coastal piloting and navigation. You might also amble over to the West Marine store (there's no shortage of those in the Bay Area) and pick up one of the books on the subject.

At the scale of the Chart's you're using for sailing in the Bay, using the distance scales on the bottom corner of the chart (which I guess are chart-kits) is perfectly adaquate for measuring purposes. The "10ths" scale on the left side of the scales is to allow you to step off distances with a divider. First set to a whole measure of miles (nautical or statute) which is walked off along a course line and then, the last distance measured on the 10th's scale. Knot meters/logs and GPS devices are normally graduated in knots or nautical miles hence the nautical mile scale is normally the most useful for your speed/distance/time computations. For the sake of convenience, remember that a 10th of a mile is about 200 yards.

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post #7 of 12 Old 05-27-2010
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Quote:
Originally Posted by Vasco View Post
Don't use the scale to measure nautical miles. Use the latitude marks at the side of the chart. A nautical mile (one minute of arc) varies in length (feet) depending on the latitude so you should always use the latitude marks.

A nautical mile varies in length from 6040 feet at the equator to 6108 feet at the pole. The mean length is 6080 feet and this is the standard nautical mile.
Agreed about using the latitude scale. The scale shown by the OP is probably based on the scale shown in the title block which is based on a parallel of latitude which might not actually be on the chart. As an example the 1:80,000 chart that I use to teach coastal navigation is accurate at 49 30' N which not actually on the chart.

A nautical mile is defined as 1852 meters.

Quote:
INTERNATIONAL NAUTICAL MILE. The international nautical mile is equal to 1852 metres (6076.1 feet). For practical purposes, the sea mile is used for expressing distances. A sea mile is the length of one minute of arc, measured along the meridian, in the latitude of the position.

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Last edited by jackdale; 05-27-2010 at 10:57 AM.
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post #8 of 12 Old 05-27-2010
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or take an American Sailing Association or USSAILING Coastal Navigation class.

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post #9 of 12 Old 05-27-2010
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If you are going to do coastal work, read a book, read your charts, and have a good GPS. If they all agree, go sailing. If you are crossing a very large bay or ocean do more.

First....go sailing!
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post #10 of 12 Old 05-28-2010 Thread Starter
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Thanks a lot guys! Definitely a lot of help, and I do plan on taking classes...just a matter of time and money!

No more my own boat... dying to get back on the water :-(
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