Contour Interval Consistency on Maps

A contour interval is a fundamental concept in surveying and mapping, specifically for topographic maps. It represents the vertical distance or difference in elevation between two consecutive contour lines on a map. Contour lines connect points of equal elevation above a given datum (usually mean sea level).

Map Contour Interval Rule

When creating a topographic map, the contour interval within the limits of that specific map must be kept constant. This is a critical rule to ensure the map is accurate, consistent, and easily interpretable by anyone using it.

Here's why keeping the contour interval constant is essential:

• Uniform Representation: A constant contour interval ensures that the vertical scale of the map is uniform across the entire area it covers. This means that every unit of vertical change (e.g., 5 meters or 10 feet) is represented by the same amount of space between contour lines, making it easy to understand the terrain.
• Accurate Interpretation of Slope: The spacing of contour lines directly indicates the steepness of the terrain. If contour lines are close together, the slope is steep; if they are far apart, the slope is gentle. This interpretation is only reliable if the contour interval remains consistent throughout the map. If the interval varied, comparing slopes in different parts of the map would be misleading or impossible without constantly checking the interval value.
• Readability and Usability: For engineers, planners, hikers, and anyone else using a topographic map, consistency makes the map much easier to read and use. They can quickly grasp elevation changes and landforms without having to re-adjust their understanding of the vertical scale.
• Facilitates Calculations: Many calculations, such as determining gradients, volumes of earthwork, or lines of sight, rely on a known and constant contour interval. Varying the interval would complicate or invalidate these calculations.

Analysis of Options

Let's consider why other options are not correct:

• May be kept constant: While technically possible, this option implies it's a choice, not a necessity. For a functional and accurate map, it's a requirement.
• May not be kept constant: If the contour interval is not kept constant, the map would become highly confusing and unreliable. Different parts of the map would represent elevation changes using different scales, leading to misinterpretations of the terrain.
• May vary according to the configuration: Allowing the contour interval to vary based on the terrain's "configuration" would defeat the purpose of a consistent vertical scale. For example, using a smaller interval in flat areas and a larger one in steep areas might seem to save lines, but it would distort the visual representation of slope and make the map much harder to interpret consistently. To show detail in flat areas, a smaller interval is chosen for the *entire* map, or supplementary contours (half-interval contours) might be used, but the primary interval remains constant.

Therefore, for a topographic map to be a reliable and useful tool, the contour interval within its limits must be kept constant.