Slope AngleDate: 06/17/2003 at 13:55:13 From: Dee Subject: Degree, percentage of a slope What is the difference between the degree of a slope and the percentage of a slope? Date: 06/17/2003 at 16:15:44 From: Doctor Peterson Subject: Re: Degree, percentage of a slope Hi, Dee. People often use misleading terminology in this area. Let me first state the proper terms and their definitions, and then come back to look at your question. The SLOPE of a line in geometry, or of a slanted surface such as a road or a roof in practical geometry, is the ratio of the "rise" to the "run." This might be presented as a fraction or a decimal: + + + / | /| / | / |6 / |6 / |3 / | / | / | +-------+ +---+ +-----------+ 6 3 9 slope=6/6=1 slope=6/3=2 slope=3/9=1/3 A slope can also be presented as a percentage, which is really just a special fraction whose denominator is 100, as indicated by the "%" symbol. ("Per cent" means "out of 100.") The three slopes above are then 100%, 200%, and 33 1/3 % respectively (obtained by multiplying each slope by 100%). We can also talk about the ANGLE of a slope, which is the angle of the slanted line above the horizontal. This is related to the slope itself by trigonometry: the slope is the tangent of the angle. + + + / | /| / | / |6 / |6 / |3 /45 | /63| /18 | +-------+ +---+ +-----------+ 6 3 9 angle=45 deg angle=63 deg angle=18 deg But here is where a lot of people get confused. We say that an angle is "45 degrees"; we should NOT say, as many do, that "the degree of the angle is 45." That's like saying "the foot of my yard is 100" instead of "the width of my yard is 100 feet." The degree is a unit, not the name of a quantity. What confuses things more is that there is another use of the word "degree" in English that is more vague, but sounds the same; my dictionary defines this as "relative intensity or amount of a quality, attribute, or the like." So we could say "what is the degree of the slope?" meaning merely "how much slope is there?", without referring to the unit or a specific way of measuring the slope. So when we are talking about numerical measurements, we should not use either of the phrases you used, "degree of a slope" or "percentage of a slope." We should instead talk about the "angle of the slope" or the "percentage slope," or something like that. I can accept "degrees of slope," and certainly "a 45-degree slope," because it is clear that the unit is being referred to. Likewise, "percentage of slope" can be understood (but not "the percentage of a slope," which sounds as if you are taking just some part of the slope). Other terms are used in various fields. In talking about roads, we use the terms "grade" or "percent grade"; the angle can then be called the "grade angle." The slope of a roof is called the pitch, and is usually given as a ratio such as 4 inches per foot or 4 in 12 or 4:12, or occasionally as a percentage. Elsewhere, I've seen "gradient" used for "slope," and "inclination" for the angle of slope. A Web search shows many cases where "degree of slope" is used; in some it is clear that they are using it to refer to the angle, but in others it seems just to mean "amount of slope," and either angle or percentage can be used with that term. We probably can't make the whole world use the correct terms; but we can try to do so ourselves, and when they use misleading expressions we can ask for clarification. I hope that answers your question. If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ |
Search the Dr. Math Library: |
[Privacy Policy] [Terms of Use]
Ask Dr. Math^{TM}
© 1994- The Math Forum at NCTM. All rights reserved.
http://mathforum.org/dr.math/