The choice of contour vertical interval on a map is primarily determined by the characteristics of the terrain and the specific purpose for which the map is created.
The contour interval, which is the vertical distance or difference in elevation between successive contour lines on a map, is a crucial element in depicting the shape and slope of the land. Selecting the appropriate interval ensures that the map is both informative and easy to read.
According to the provided reference, the contour interval is selected based on:
- The characteristics of the terrain being mapped
- The purpose of the map
Let's delve into these key factors.
Terrain Characteristics
The nature of the landscape plays a significant role in deciding the contour interval.
- Steep or Rugged Terrain: Areas with dramatic elevation changes require a smaller contour interval. This allows more contour lines to be drawn closer together, accurately representing the steep slopes and intricate details of the topography. Using a large interval in such areas would result in very few lines, making the map difficult to interpret and obscuring important elevation variations.
- Flatter Areas: In contrast, relatively flat or gently sloping terrain can effectively use a larger contour interval. Since elevation changes are minimal, a larger interval will still show the overall slope without cluttering the map with too many lines. A small interval in flat areas might produce an excessive number of lines that are very far apart, offering little useful information about the subtle changes in elevation.
Example:
| Terrain Type | Recommended Contour Interval | Reason |
|---|---|---|
| Mountains (Steep) | Smaller (e.g., 10-20 feet) | Shows rapid elevation changes accurately |
| Plains (Flat) | Larger (e.g., 50-100 feet) | Avoids clutter, shows general slope |
| Rolling Hills | Moderate (e.g., 25-50 feet) | Balances detail and readability for moderate slopes |
Purpose of the Map
The intended use of the map is equally important in selecting the contour interval. Different applications require different levels of detail.
- Detailed Engineering or Planning Maps: Maps used for site analysis, construction planning, or detailed resource management often require a small contour interval (e.g., 1, 2, or 5 feet). High precision is needed to understand subtle variations in slope and elevation for design purposes.
- General Purpose or Navigation Maps: Maps intended for hiking, general navigation, or large-scale regional planning typically use a larger contour interval (e.g., 20, 50, or 100 feet). These maps prioritize readability and ease of use over fine-grained detail. The goal is to show the general form of the land and major elevation changes.
- Small-Scale Maps: Maps covering very large areas (small scale) generally use very large contour intervals (e.g., 100, 200, or 500 feet) to avoid the map becoming overly dense with lines.
Practical Considerations:
- Map Scale: The scale of the map (the ratio of distance on the map to the corresponding distance on the ground) is closely related to both terrain and purpose. A large-scale map showing a small area in detail can support a smaller contour interval, while a small-scale map covering a large area typically requires a larger interval.
- Readability: The chosen interval must ensure the map remains clear and readable. Too small an interval in rugged terrain or on a small-scale map can lead to contour lines being too close together, making the map look cluttered.
In summary, determining the optimal contour vertical interval is a balance between depicting the terrain accurately and ensuring the map is suitable for its intended use and scale, while maintaining readability.
