A bar scale (also known as a graphic scale) on a map is used to visually represent the relationship between a distance on the map and the corresponding distance on the ground. It allows you to easily measure distances without having to perform calculations using the map's representative fraction (RF) or verbal scale. Here's how to use one:
Steps to Using a Bar Scale
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Locate the Bar Scale: The bar scale is usually found in the map's legend or along one of the map's edges. It typically looks like a small ruler divided into segments, each representing a specific distance on the ground (e.g., kilometers, miles).
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Measure the Distance on the Map: Use a ruler, a piece of paper, or a string to measure the distance between two points on the map.
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Transfer the Measurement to the Bar Scale:
- Ruler Method: Place the ruler along the bar scale. Note where the measured distance on the ruler aligns with the bar scale markings. Read the corresponding ground distance directly from the bar scale.
- Paper Method: Mark the two points of the measured distance on the edge of a piece of paper. Place the paper's edge along the bar scale, aligning one mark with the zero point on the bar scale (if available). Read the corresponding ground distance at the other mark's position on the bar scale.
- String Method: Lay the string along the route between the two points on the map. Mark the string at the start and end points. Then, stretch the string along the bar scale, aligning one mark with zero (if available), and read the corresponding distance on the bar scale.
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Account for Fractional Units: Sometimes, the measured distance won't align perfectly with the whole units on the bar scale. In this case, you may need to estimate the fractional part of a unit. Many bar scales include a smaller, finely divided section for more precise measurements of shorter distances.
Example
Let's say you measure the distance between two towns on a map and find it to be 8 cm. You then place your ruler along the bar scale, and 8 cm corresponds to 2 kilometers on the bar scale. This means the actual distance between the two towns is approximately 2 kilometers.
Advantages of Using a Bar Scale
- Ease of Use: Bar scales are straightforward and require minimal calculation.
- Accuracy: When used carefully, bar scales provide a reasonably accurate estimate of distances.
- Map Size Changes: Bar scales remain accurate even if the map is enlarged or reduced (e.g., photocopied) because they change proportionally with the map. Verbal and ratio scales become inaccurate if the map size changes. As the reference material indicates, bar scales are often the safest choice.
Tips for Accuracy
- Follow Curves: When measuring distances along roads or rivers, use a string or flexible ruler to accurately follow the curves.
- Be Precise: Take your time and be as accurate as possible when measuring on the map and aligning with the bar scale.
- Understand Limitations: Remember that all map measurements are approximations. Bar scales provide a good estimate, but minor inaccuracies can occur.
Using a bar scale is a simple and effective way to determine real-world distances from a map, offering a visual reference that adapts to map size changes.
