Using a scale ruler on a map involves measuring the distance between two points on the map and then using the ruler's scale to convert that measurement into the actual ground distance.
Here's a step-by-step guide:
-
Identify the Scale: Find the map's scale. This is usually located in the map's legend or near the border. The scale can be represented as a ratio (e.g., 1:25,000), a representative fraction (e.g., 1/25,000), or a graphical scale (a bar divided into segments representing ground distances).
-
Choose the Appropriate Scale on the Ruler: A scale ruler typically has multiple scales printed on it. Select the scale that matches the map's scale. For instance, if your map scale is 1:25,000, find the 1:25,000 scale on the ruler. Note: some rulers may not have the exact scale and require estimation.
-
Measure the Distance on the Map: Place the scale ruler along the straight line connecting the two points you want to measure. Align the '0' mark of the chosen scale with one of the points.
-
Read the Distance: Read the value on the ruler where it intersects the second point. This measurement represents the ground distance in the units indicated on the scale ruler (e.g., meters, kilometers, miles, feet). If the point falls between markings, estimate the distance to the nearest reasonable fraction.
-
Account for Scale Unit: Ensure you understand the units of measurement on the scale ruler. For example, a 1:25,000 scale may represent 1 centimeter on the map as 25,000 centimeters (or 250 meters) on the ground.
Example:
- You measure the distance between two points on a map using a scale ruler with a 1:25,000 scale.
- The measurement on the ruler is 5 centimeters.
- Since the scale is 1:25,000, this means 1 cm on the map equals 25,000 cm (or 250 meters) on the ground.
- Therefore, 5 cm on the map represents 5 * 250 meters = 1250 meters (or 1.25 kilometers) on the ground.
Important Considerations:
- Straight Lines: Scale rulers are most accurate for measuring straight-line distances. For curved lines, use a map measurer (opisometer) or divide the curve into smaller, straight segments and measure each segment individually.
- Map Distortion: Be aware that maps can have distortion, especially over large areas. The scale ruler will provide the most accurate measurement in the immediate vicinity of where it's applied.
- Ruler Accuracy: Ensure the scale ruler is accurate and undamaged.
In summary, using a scale ruler involves matching the ruler's scale to the map's scale, measuring the map distance, and then converting that measurement into the corresponding ground distance using the scale ratio.
