You use a map scale to measure distance by first measuring the distance between two points on the map and then converting that measurement to the real-world distance using the map's scale.
Understanding Map Scales
A map scale represents the relationship between a distance on a map and the corresponding distance on the ground. It can be expressed in three primary ways:
- Verbal Scale: A statement that directly expresses the relationship, like "1 inch equals 1 mile."
- Representative Fraction (RF): A ratio showing the relationship as a fraction, such as 1:63,360 (meaning 1 unit on the map equals 63,360 of the same units on the ground).
- Graphic Scale (Bar Scale): A visual representation of the scale using a line divided into segments that represent ground distances.
Steps to Measure Distance Using a Map Scale
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Identify the Map Scale: Locate the map scale, usually found in the map's legend or border. Determine the type of scale being used (verbal, RF, or graphic).
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Measure the Distance on the Map: Use a ruler or compass to measure the distance between the two points of interest on the map. Ensure you are using the same units of measurement as the map scale (e.g., inches, centimeters). For curved paths, use a string to follow the route and then measure the string.
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Convert the Map Distance to Ground Distance: This is where the type of map scale comes into play:
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Verbal Scale Example: If the scale is "1 inch = 1 mile" and the measured distance on the map is 2.5 inches, then the actual ground distance is 2.5 miles.
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Representative Fraction (RF) Example: If the scale is 1:24,000 and you measured 3 inches on the map, the calculation is: 3 inches * 24,000 = 72,000 inches. Then convert 72,000 inches to a more usable unit like feet (72,000/12 = 6,000 feet) or miles (72,000/63,360 = ~1.14 miles).
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Graphic Scale Example: Place the ruler along the graphic scale, aligning the zero point with your starting point on the map. Read the corresponding ground distance directly from the graphic scale where your ruler ends.
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Example Calculation
Let's say a map has a scale of 1:100,000. You measure the distance between two locations on the map and find it to be 5 centimeters.
- Map scale: 1:100,000
- Measured distance on map: 5 cm
- Calculation: 5 cm * 100,000 = 500,000 cm
- Conversion to kilometers: 500,000 cm / 100,000 cm/km = 5 km
Therefore, the actual distance between the two locations is 5 kilometers.
Tools and Tips for Accuracy
- Ruler: Use a ruler with fine graduations for more precise measurements.
- Map Measurer (Opisometer): A specialized tool for measuring curved lines on maps.
- String: A simple yet effective way to measure winding paths.
- Consider Terrain: Map scales don't account for elevation changes. If you need precise distances over mountainous terrain, use more advanced techniques like GPS or GIS software.
By understanding and correctly applying the map scale, you can accurately determine real-world distances from map measurements.
