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Understanding Slope and How it is Measured

Understanding Slope Header

Maybe you're wanting to figure the slope of a drive way, a sanitary sewer pipe, a parking lot, a handicap accessible ramp, or maybe even the slope of a ditch. Also from time to time, you may hear people mention that something is at a percent slope and you wonder exactly what that means. If you take something like a ruler, pencil or stick and lay it on your desk, that's zero percent slope. Now, hold the object straight up. Straight up is 100 percent slope. Now you're getting the idea right? A 45-degree angle of the pencil results in a 50 percent slope. Lower it some more and you can easily get the idea of what 10% slope looks like, then 5 and even 2% slope.

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Many times slope plays a much more important role in design and in our lives in general than most people ever realize. The Americans with Disabilities Act (ADA) sets and enforces regulations with design criteria such as the slope of parking lots, sidewalks, and ramps. Sanitary sewer lines are usually installed at a 2% slope so that the water doesn't outrun the solids resulting in a clogged sewer line. Roads and highways are designed such that rainwater runs off yet is not sloped too steep making driving dangerous. There's literally thousands of examples where slope plays into our daily lives.

Slope is found often in the world of Civil Engineering, Design, Landscaping, and Construction. One must be able to fully understand what slope means and how it works. While we can talk and think about slope, we also need to know how to calculate and work with it.

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Slope as a Percentage

How do you calculate the percentage grade of a slope? Slope can be calculated as a percentage which is calculated in much the same way as the gradient. Convert the rise and run to the same units and then divide the rise by the run. Multiply this number by 100 and you have the percentage slope. For instance, 3" rise divided by 36" run = .083 x 100 = an 8.3% slope.

Gaining a basic understanding of how to measure slope is important since you can then apply that knowledge to many other situations. Let's start by thinking about using wooden stakes, a good strong string, a string level, and a pocket tape measure to discover slope of the ground. The string level can be used to tie a string off in a level manner from stake to stake. Then using the tape measure to measure down from level the distance to a point on each end of the string. Then you take the distance from point to point along the string. Taking the difference in the two elevations you measured earlier, divide that figure by the length of the string you measured. From here you do the math to calculate slope as a percentage.

Now that you have this basic understanding of how to measure slope, you can then see how a transit or builders level could be used to measure down from level. The transit level sights a level line (invisible since its a sightline) to the grade rod (also known as a level rod) at one point along the distance you are measuring slope from. Then a reading is taken from level to the second spot. Once you know the distance from the two measured spots, and the distance from level on both spots, you can then do the math to calculate slope.

Other tools used for measuring slope would be laser levels, inclinometers, and slope meters.

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