Using Map Scales and Grid Systems - Third Grade Social Studies
Have you ever wondered how an entire state that stretches hundreds of miles across can fit neatly onto a single page inside your social studies textbook? If cartographers drew maps at their actual real-life size, a map of your town would be as big as the town itself, making it completely impossible to unfold or hold! To solve this problem, mapmakers shrink the real world proportionally using a mathematical tool called a map scale. In addition, they overlay checkerboard grid systems of letters and numbers to help you pinpoint exact locations in seconds.
What Is a Map Scale?
A map scale is the mathematical relationship, or ratio, between a measured distance on a map and the actual corresponding distance on the real surface of the Earth. It tells you how many miles or kilometers in the physical world are represented by a single inch or centimeter on your paper.
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| SAMPLE LINEAR BAR SCALE |
| |
| 0 10 20 30 40 50 Miles |
| |---------|---------|---------|---------|---------| |
| |
| Scale Rule: 1 Inch on the Map = 20 Actual Miles on Earth |
+--------------------------------------------------------------------------+
The Bar Scale (Graphic Scale)
The most common type of scale found on maps for students is the graphic bar scale, which looks like a small printed ruler. The bar is divided into equal segments marked with measurements such as zero, ten, twenty, or fifty miles. By placing a ruler or the straight edge of a piece of paper along the bar scale, you can measure distances between two cities with ease.
How to Measure Distance with the Paper-Edge Trick
You do not even need a plastic ruler to measure distances on a map; you can use a blank strip of scrap paper! First, lay the clean edge of your paper across the map so that it connects City A and City B. Second, make two small pencil tick marks on your paper edge: one at City A and one at City B. Finally, slide your marked paper down to the printed bar scale and align the first tick mark with the zero mark. Read the number where your second tick mark falls, and you have instantly calculated the real-world driving distance in miles!
Large-Scale vs. Small-Scale Maps
The word "scale" can sometimes confuse young geographers because a large-scale map actually shows a smaller land area, while a small-scale map shows a huge land area!
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| COMPARING LARGE-SCALE AND SMALL-SCALE |
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| LARGE-SCALE MAP: Shows a small area with tremendous detail. |
| Example: Map of your school playground (1 inch = 50 feet). |
| You can see swing sets, sidewalks, trees, and basketball hoops! |
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| SMALL-SCALE MAP: Shows a huge area with little individual detail. |
| Example: Map of the entire globe (1 inch = 1,000 miles). |
| You can see continents and oceans, but individual streets are invisible!|
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When to Use a Large-Scale Map
Use a large-scale map when you need to navigate through a localized area, such as a zoo, an amusement park, or your city's downtown shopping district. Because the map zooms in close to the ground, individual buildings, parking lots, and walking trails are clearly visible.
When to Use a Small-Scale Map
Use a small-scale map when you need to view the big picture, such as comparing the sizes of countries, looking at weather fronts moving across North America, or tracing an ocean cruise from California to Hawaii.
Understanding Map Grid Systems
A map grid is a network of intersecting horizontal and vertical lines drawn over a map that creates a pattern of uniform squares. Cartographers label the columns across the top with letters (A, B, C, D) and the rows along the side with numbers (1, 2, 3, 4).
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| AN ALPHANUMERIC MAP GRID |
| |
| A B C D |
| +-----------------+-----------------+-----------------+---------------+
| 1 | [Firehouse] | | [Baseball Park] | |
| +-----------------+-----------------+-----------------+---------------+
| 2 | | [Public Library]| | [City Police] |
| +-----------------+-----------------+-----------------+---------------+
| 3 | [Train Depot] | | [Town Hall] | |
| +-----------------+-----------------+-----------------+---------------+
| 4 | | [School] | | [Post Office] |
| +-----------------+-----------------+-----------------+---------------+
+--------------------------------------------------------------------------+
Finding Coordinates on an Alphanumeric Grid
To find any feature on an alphanumeric grid, you combine its column letter with its row number to produce a grid coordinate. Look at the grid chart above to practice finding locations:
- Where is the Firehouse located? Column A, Row 1 (Coordinate: A1).
- Where is the Baseball Park located? Column C, Row 1 (Coordinate: C1).
- Where is the Public Library located? Column B, Row 2 (Coordinate: B2).
- Where is the Town Hall located? Column C, Row 3 (Coordinate: C3).
- Where is the School located? Column B, Row 4 (Coordinate: B4).
The Index at the Back of an Atlas
In the back of a large atlas, you will find an alphabetical index listing thousands of cities, parks, and rivers. Next to each name is a grid coordinate, such as "Springfield... Map 14, D3." Instead of searching through every square inch of the map with a magnifying glass, you simply look at Column D and Row 3 on page 14 to locate Springfield in seconds!
Chapter Practice Exercises
Question 1: What is a map scale, and why is it impossible to create an accurate map without using some form of scale?
Question 2: Explain the difference between a large-scale map and a small-scale map, and provide one real-life example of when you would use each.
Question 3: How does a paper-edge trick allow you to measure the distance between two towns using a printed graphic bar scale?
Question 4: On an alphanumeric grid map, if the zoo is located at coordinate C4 and the museum is located at coordinate B2, explain how you would locate both attractions.
Solutions and Explanations
Solution 1: A map scale is the mathematical ratio between a distance measured on a map and the actual distance on Earth's surface. Without a scale, a map would have to be drawn at full one-to-one real-world size to be accurate, which would make paper maps hundreds of miles wide and impossible to hold, store, or read.
Solution 2: A large-scale map zooms in on a small physical area, showing high detail such as individual buildings, footpaths, and trees (for example, a map of a city zoo or school campus). A small-scale map zooms out to show an immense region with low detail, such as an entire continent or the whole world, where individual houses and streets cannot be seen.
Solution 3: To use the paper-edge trick, you place a straight piece of blank paper connecting two points on a map, mark each point with a pencil tick mark, and then slide the marked edge against the printed bar scale. Aligning the first mark with zero allows you to read the exact mileage number at the second mark directly off the scale.
Solution 4: To locate the zoo at coordinate C4, you follow Column C down to Row 4 and find the square where they intersect. To locate the museum at coordinate B2, you look across to Column B and down to Row 2, finding the attraction within that specific grid square.