GRAPHICAL METHOD. The graphical method is used if the height of a tide is needed for different times during a day. This method assumes that the rise and fall of tides conforms to a simple "cosine" curve. If the tide follows a cosine curve, the graph can provide an accurate estimate of tide heights between high and low water; however, if the tide does not follow the cosine curve, the tide estimates will be off. To determine tides using the graphical method, use the following example:

EXAMPLE: High tide occurs at 0012 and is 11.3 feet; low tide occurs at 0638 and is 2,0 feet

1. Using graph paper, setup a time scale and a height scale. See figure 6-6-6.

2. Plot the time and height of high water and low water, and connect the two points with a straight line. See figure 6-6-7.

3. Divide the line into quarters as shown in figure 6-6-8.

4. At the quarter point closest to the high-water point, draw a vertical line equal to one-tenth of the tides range. In our example, the range is 13 feet; 1/10 of 13 is equal to 1.3 feet. Because this point is nearer the high tide, draw this line up, toward the top of the graph (B in figure 6-6-9). Repeat this procedure for the

quarter point closest to the low-water point but draw the line toward the bottom of the graph (D in figure 6-6-9).

5. Draw a smooth cosine curve connecting points A, B, C, D, and E, as shown in figure 6-6-10.  

The graph is now complete and ready to use. To find the height of the tide at any time, simply follow the time upward until you intersect the cosine curve. From this point move horizontally to the height scale on the left and read the height.

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