What if you had a 1:80 scale model of the Eiffel Tower. The model stands 4 meters tall. How could you find the height of the actual Eiffel Tower? After completing this Concept, you'll be able to use indirect measurements to solve scale problems like this one.
Watch This
CK-12 Foundation: 0312S Scale and Indirect Measurement (H264)
For some more advanced ratio problems and applications, watch the Khan Academy video at http://www.youtube.com/watch?v=PASSD2OcU0c .
Guidance
One place where ratios are often used is in making maps. The scale of a map describes the relationship between distances on a map and the corresponding distances on the earth's surface. These measurements are expressed as a fraction or a ratio.
So far we have only written ratios as fractions, but outside of mathematics books, ratios are often written as two numbers separated by a colon (:). For example, instead of , we would write 2:3.
Ratios written this way are used to express the relationship between a map and the area it represents. For example, a map with a scale of 1:1000 would be a map where one unit of measurement (such as a centimeter) on the map would represent 1000 of the same unit (1000 centimeters, or 10 meters) in real life.
Example A
Anne is visiting a friend in London, and is using the map below to navigate from Fleet Street to Borough Road. She is using a 1:100,000 scale map, where 1 cm on the map represents 1 km in real life. Using a ruler, she measures the distance on the map as 8.8 cm. How far is the real distance from the start of her journey to the end?
Solution
The scale is the ratio of distance on the map to the corresponding distance in real life. Written as a fraction, it is . We can also write an equivalent ratio for the distance Anne measures on the map and the distance in real life that she is trying to find: . Setting these two ratios equal gives us our proportion: . Then we can cross multiply to get .
That’s how many centimeters it is from Fleet Street to Borough Road; now we need to convert to kilometers. There are 100000 cm in a km, so we have to divide our answer by 100000.
The distance from Fleet Street to Borough Road is 8.8 km.
In this problem, we could have just used our intuition: the scale tells us that any number of cm on the map is equal to the same number of km in real life. But not all maps have a scale this simple. You’ll usually need to refer to the map scale to convert between measurements on the map and distances in real life!
Example B
Antonio is drawing a map of his school for a project in math. He has drawn out the following map of the school buildings and the surrounding area
He is trying to determine the scale of his figure. He knows that the distance from the point marked A on the baseball diamond to the point marked B on the athletics track is 183 meters. Use the dimensions marked on the drawing to determine the scale of his map.
Solution
We know that the real-life distance is 183 m, and the scale is the ratio .
To find the distance on the map, we use Pythagoras’ Theorem: , where and are the horizontal and vertical lengths and is the diagonal between points and .
So the distance on the map is about 16.12 cm. The distance in real life is 183 m, which is 18300 cm. Now we can divide:
The scale of Antonio’s map is approximately 1:1100.
Another visual use of ratio and proportion is in scale drawings . Scale drawings (often called plans ) are used extensively by architects. The equations governing scale are the same as for maps; the scale of a drawing is the ratio .
Example C
Oscar is trying to make a scale drawing of the Titanic, which he knows was 883 ft long. He would like his drawing to be at a 1:500 scale. How many inches long does his sheet of paper need to be?
Solution
We can reason intuitively that since the scale is 1:500, the paper must be long. Converting to inches means the length is .
Oscar’s paper should be at least 22 inches long.
Watch this video for help with the Examples above.
CK-12 Foundation: Scale and Indirect Measurement
Vocabulary
- A ratio is a way to compare two numbers, measurements or quantities by dividing one number by the other and expressing the answer as a fraction.
- A proportion is formed when two ratios are set equal to each other.
- Cross multiplication is useful for solving equations in the form of proportions. To cross multiply, multiply the bottom of each ratio by the top of the other ratio and set them equal. For instance, cross multiplying results in .
- Scale is a proportion that relates map distance to real life distance.
Guided Practice
The Rose Bowl stadium in Pasadena, California measures 880 feet from north to south and 695 feet from east to west. A scale diagram of the stadium is to be made. If 1 inch represents 100 feet, what would be the dimensions of the stadium drawn on a sheet of paper? Will it fit on a standard inch sheet of paper?
Solution
Instead of using a proportion, we can simply use the following equation: (distance on diagram) = (distance in real life) (scale). (We can derive this from the fact that .)
Plugging in, we get
The scale diagram will be . It will fit on a standard sheet of paper.
Practice
- A restaurant serves 100 people per day and takes in $908. If the restaurant were to serve 250 people per day, how much money would it take in?
- The highest mountain in Canada is Mount Yukon. It is the size of Ben Nevis, the highest peak in Scotland. Mount Elbert in Colorado is the highest peak in the Rocky Mountains. Mount Elbert is the height of Ben Nevis and the size of Mont Blanc in France. Mont Blanc is 4800 meters high. How high is Mount Yukon?
- At a large high school it is estimated that two out of every three students have a cell phone, and one in five of all students have a cell phone that is one year old or less. Out of the students who own a cell phone, what proportion owns a phone that is more than one year old?
For 4-6, suppose a map of Ratio City has a scale of 1:1,000,000, where 1 centimeter on the map represents 10 kilometers in real life. Use that scale to determine the real-life distances in kilometers.
- The distance on the map between city hall and high school is 1.2cm.
- The distance on the map between city hall and the main library is 0.6cm.
- The distance on the map between the main library and the high school is 0.4cm.
For 7-10, use the map in Example A. Using the scale printed on the map, determine the distances (rounded to the nearest half km) between:
- Points 1 and 4
- Points 22 and 25
- Points 18 and 13
- Tower Bridge and London Bridge