This guest Boat Rat comes to us courtesy of Captain John of SkipperTips.com.
Did you know that you can use depth curves for safer sailing navigation anywhere in the world? Or that a tried and proven method to find your way home in pea-soup fog has been used for well over 100 years?
Look over any nautical chart or chart plotter software and you will note curved lines or enclosed circles. These depth contours show areas of equal depth. Check along the line or circle for a break that shows a number (see illustrations 1 and 2).
Contour Line Secrets
Depths are written in numbers divisible by 6. For example, the line closest to shore might be 6 feet, the next line 12 feet, the next 18 feet, and so forth (illustration 1). But never assume that contour lines follow a consecutive pattern of 6 foot increments.
For example, in illustration 2, the contour lines jump from 18 feet to 30 feet to 36 feet. Study your navigational chart to decipher the depth information.
Contour Circle Secrets
Some charted contour circles show depths in breaks along the circle (illustration 3). Like contour lines, depths may not be consecutive between adjacent contour circles. Notice that the contour circles show 18 feet, then 30 feet and then 60 feet!
Sometimes you will see contour circles without depths shown in a break along the circle (illustration 4). Use this easy method to find the depth of any contour circle:
* Look for the lowest depth just outside the circle (highlighted in illustration).
* Find the highest depth just inside the circle (highlighted in illustration).
* Determine what number–divisible by 6–would lie between the two.
In the illustration, you see 19 feet of water just outside and 16 feet just inside the circle. This indicates an 18 foot contour circle.
For more depth chart symbols every skipper needs to know, including five ways to use contour lines or contour circles, click here.