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A chart is a map of sea and land combined. A chart shows the water, the shoreline, and a few landmarks (places on land in known locations that are easily visible from the water). Modern-day charts have a lot of data on depth of water as well. In several places on a chart you will find a compass rose. This gives the directions of true north, south, east, and west. It also gives magnetic directions as well. (Magnetic direction is a little different from true direction. A compass shows magnetic direction.)
Charts include a grid of horizontal and vertical lines known as latitude and longitude. It is the lines of latitude and longitude that are used for navigation.
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Dividers Dividers are used to accurately transfer distance data from the edges of the chart to any other place on the chart. Looks a lot like the kind of compass that you draw a circle with, doesn't it? In this lesson you can use a drawing compass as dividers if you'd like. There are many other tools used in navigation, but these are enough to get us started. (back to top) |
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So let's use our new tools to find the the position of a ship on Earth. First we need a "fix". We'll use one sent to us by radio by a captain, who used a GPS receiver to plot the position of the vessel. (Click on GPS to go to the glossary and find out what it is!)
Latitude = 37° 30' North, Longitude = 122° 29' West.
The North shows us that he is referring to lines of latitude that are north of the equator. The West shows us that he is referring to lines of longitude that are west of Greenwich, England.
We now need a chart of the approximate area the ship is in. (back to top)
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Using the chart (shown above) and dividers we can plot the position of the Lady on that day. Notice that the chart is north side up on your monitor? Now, take a look at the top and right side of the chart. Those numbers (on what looks like a ruler) are the lines of latitude and longitude, listed in degrees and minutes. Take your dividers (if you don't have a set of dividers, use a couple of pieces of paper with straight edges). Now, starting on the right side of the chart, find 38°. Did you find it in the upper right hand corner? Notice how 50', 40', and 30' are listed below it? Those are the minutes of latitude counting up from 37° (off of the chart).
Remember the captain sent:"Latitude = 37° 30' North", right? So if we count up from the 37° off the chart to 30', there it is! See it, on the lower right hand side of the chart? Now lay the edge of one piece of paper horizontally across your monitor resting on that 30' mark. Take note of where on the chart that imaginary line falls. The Lady Washington is somewhere on that line, but where?
Mason also sent: "Longitude = 122° 29' West." Look at the top right hand corner of the chart. See the 122°? That is it, 122° of longitude! Now count over to the left until you reach 29'. See how there are 10 little black and white horizontal lines between 20' and 30'? Use these to count to 29'. Take your second piece of paper and lay it on your monitor with one edge running vertically from 29' straight down the screen. Where the two pieces of paper intersect (meet) each other is the position of the ship. Neat, huh?
You should have found that the ship is presently off the Northern shore of Half Moon Bay. Congratulations - you have just plotted a position!