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December 1, 2001
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Learn to "clap a 90."

Crawford
Often while performing construction layout activities, a quick, approximate 90-degree angle is needed to obtain a perpendicular line to your layout work. Perhaps you are setting clearing stakes perpendicular to the centerline of a road—and a rough 90 is all that is needed. For a quick perpendicular, it isn’t practical to set up an instrument for this type of 90. Experienced field personnel use two quick methods to establish this 90. One is a simple process of using your anatomy and the other is using a simple prism device. Let’s take a look at how each method is performed.

Five steps to clapping a 90.

The Wingding 90 or Clapping a 90

A method that has been used since the beginning of time is commonly called the “wingding,” or also sometimes called “clapping a 90.” This process consists of simply using your arms to measure the 90 degrees. Hopefully, your arms will be about the same length. If they aren’t, your rough 90 will not be very accurate. This process is illustrated and explained in the following five steps.

Step 1
At the point on a line where a 90-degree angle is needed, carefully position yourself so that you are standing exactly on the line.

Step 2
With your feet aligned 90 degrees to the line, hold one arm out and sight down the line, moving your arm until it is pointed exactly on the line. Hold it steady on the line.

Step 3
Rotate your head to look in the opposite direction and raise your other arm and sight onto the line, moving this arm until it is pointing exactly on the line. Be careful not to move your other arm.

Step 4
Rotate your head right and left, confirming that both arms are pointed on line.

Step 5
Close your eyes and quickly bring your arms together in front of you. Match up your fingertips, open your eyes, and sight over top of your hands. You are looking at a point that is in a direction approximately 90 degrees from the line you are standing on.

With this simple process, you can affirmatively answer the simple question, “Do you ‘wingding’?”

The Double Right-Angle Prism 90

If a perpendicular line that is generally more accurate than the wingding is needed, the double right-angle prism is often the tool of choice. It is used when measuring cross-sections or setting slope stakes. This double right-angle prism consists of two pentagonal prisms for line of sight to the right and left. Using a double right-angle prism isn’t difficult. The following illustrates how to use the double right-angle prism for establishing lines perpendicular to a centerline.

Step 1
Attach a plumb bob and string to the bottom of the double right-angle prism. Hold the prism horizontally and exactly over the point on the centerline where a perpendicular is needed.

Step 2
Open up the protective lens cover on the prism. Standing with your feet perpendicular to the line and looking directly at the exposed glass, peer at the prisms.

Step 3
Look at the top prism and observe features to your right. If you have stakes on the line, they will be in view. Then, look at the bottom prism and observe features (stakes) to your left.

Step 4
Move yourself forward or backward from the line and/or rotate the prism until the images in the top prism and the bottom prism are directly in line (vertically) with each other.

Step 5
When the top and bottom images are in a vertical line, communicate to your rod holder to move left or right until the rod being held is directly in line with the other images. When all three images are in a vertical line, the rod being held is perpendicular to the line. Tell your rod holder to mark the point.

Thus, you have completed the double right-angle 90.

Summary

Obviously, a 90-degree angle from either of the methods shown isn’t exact; however, when a rough 90 is needed, either of these methods will suffice. Field personnel should be adept at these and other methods of quick layout techniques.

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