Contour Interval – Elevation Difference Between Contours

1. Contour Interval: Definition and Core Concept

A contour interval is the specifically defined vertical difference in elevation between two consecutive contour lines on a map. It is fundamental for representing the three-dimensional configuration of the Earth’s surface on a two-dimensional medium such as a paper or digital map. Each contour line connects points of equal elevation relative to a reference datum (typically mean sea level).

The contour interval is always kept constant for a map to ensure clarity and quantitative accuracy. For example, with a 5-meter interval, every line marks a 5-meter change in elevation from the adjacent line. This constancy is crucial for engineers, planners, and aviators who rely on these intervals to calculate gradients, plan routes, assess drainage, and conduct earthworks.

International standards, such as ICAO Annex 4 for aeronautical charts, prescribe standardized contour intervals (often 50 feet or 15 meters) but allow adjustments based on topography and scale. The contour interval is always indicated in the map legend, in either metric or imperial units.

Smaller contour intervals provide detailed terrain information, while larger ones help avoid clutter on maps covering areas with dramatic relief.

Understanding contour intervals is foundational for professionals in surveying, mapping, and terrain analysis.

2. Contour Lines: Formation, Interpretation, and Map Reading

Contour lines connect points of equal elevation. They are the main method of depicting elevation changes on topographic and aeronautical charts.

• Closely spaced lines: Steep slopes
• Widely spaced lines: Gentle slopes
• Concentric closed contours: Hills (values increase inward) or depressions (values decrease inward, often with hachures)
• V-shaped contours: Point upstream for valleys or downstream for ridges

Index contours are thick and labeled for quick reference. Intermediate contours fill the intervals. On aeronautical and national maps, index contours are usually every 5th or 10th line.

Horizontal distance between contours (horizontal equivalent) varies with slope and is inversely proportional to steepness.

Expert map reading involves analyzing contour patterns and densities to reconstruct the terrain—a critical skill in civil engineering, aviation, and emergency response.

3. Index Contours, Intermediate Contours, and Supplementary Contours

• Index contours: Thick, labeled lines (e.g., every 5th or 10th contour), providing quick elevation reference.
• Intermediate contours: Thinner lines between index contours, marking regular increments.
• Supplementary contours: Used in flat areas to show subtle changes; often dashed or dotted, with an interval smaller than the standard.

Clear distinction of these lines is essential for accurate design and hazard assessment in disciplines like airfield planning and floodplain mapping.

4. Depression Contours, Horizontal Equivalent, and Gradient Calculation

Depression contours mark enclosed areas where elevation decreases inward, shown with hachures pointing inward.

Horizontal equivalent is the ground distance between adjacent contours, variable depending on slope.

Gradient (Slope) is calculated as:

• Ratio: 1 (vertical) : n (horizontal)
• Percentage: (Vertical / Horizontal) x 100
• Degrees: arctangent(Vertical / Horizontal)

Example:

• Contour interval: 5 m
• Horizontal distance: 25 m
• Gradient = 5:25 = 1:5, or 20%, or ≈11.3°

Gradient calculations are vital for drainage, road design, and aviation obstacle clearance.

5. Selection Criteria for Contour Intervals

Contour interval selection depends on:

1. Map Scale: Larger scales need smaller intervals for detail.
2. Terrain Relief: Steep terrain uses larger intervals to avoid clutter.
3. Map Purpose: Engineering needs fine intervals; regional planning uses broader ones.
4. Survey Accuracy: Smaller intervals require denser, more precise data.
5. Regulatory Standards: ICAO, FAA, and others specify minimum intervals.

Contour intervals must always be clearly marked on the map.

6. Contour Interval Calculation: Methodology and Examples

Method 1: Using Known Contour Elevations

1. Identify two adjacent index contours.
2. Subtract lower from higher elevation.
3. Count intermediate lines between.
4. Add one (intervals are between lines).
5. Divide elevation difference by this number.

Formula: [ \text{Contour Interval} = \frac{\text{Elevation difference between index contours}}{\text{Number of intermediate contours} + 1} ]

Example:
Index contours at 100 m and 120 m, 4 intermediate lines:

• 120 - 100 = 20 m
• Number of intervals: 4 + 1 = 5
• Contour interval: 20 m / 5 = 4 m

Method 2: Map Legend

Most maps state the interval in the legend.

Always confirm that intervals are consistent across the map. Variable intervals are rare and must be clearly indicated.

7. Real-World Applications of Contour Intervals in Surveying and Engineering

• Site Planning & Architecture: Guides building placement, drainage, and earthwork.
• Civil Engineering: Critical for route alignment, grading, and cut-and-fill calculations.
• Water Resource Management: Used for flood modeling, watershed delineation, and flow prediction.
• Aviation & Airfield Design: ICAO-mandated contour intervals on obstacle charts ensure safe flight paths.
• Environmental Science: Supports erosion studies, habitat mapping, and conservation.
• Recreation & Safety: Hikers and rescue teams use contour intervals to plan safe routes.

8. Limitations and Sources of Error in Contour Interval Use

• Resolution: Large intervals may miss subtle features; small intervals can clutter the map.
• Survey Accuracy: Inaccurate data leads to errors in interval depiction.
• Vertical Surfaces: Cliffs and overhangs are hard to represent with contours alone.
• Surface Features: Contour maps do not show vegetation, structures, or subsurface data.
• Legal Boundaries: Contours do not represent property lines.
• Generalization: Small-scale maps may smooth or omit minor features.

9. Glossary of Related Terms and Concepts

10. ICAO and Global Standards for Contour Intervals

The International Civil Aviation Organization (ICAO) sets contour interval requirements for aeronautical charts, ensuring global uniformity and safety.

• Obstacle Charts (ICAO Type A/B): Typically 10 meters (or 30 feet), interval and units clearly shown.
• Aerodrome/Heliport Charts: Finer intervals (0.5–2 meters) for airfield grading and obstacle clearance.
• Enroute Charts: Larger intervals (50–100 meters) due to broader coverage and higher flight altitudes.

National agencies (FAA, Ordnance Survey, USGS, etc.) adapt ICAO guidance to local conditions but maintain interoperability.

Standardized intervals and symbology are critical for international aviation, emergency response, and cross-border infrastructure projects.

Summary

A contour interval is a core concept in surveying and mapping, representing the vertical distance between adjacent contour lines. The proper selection and interpretation of contour intervals underpin accurate terrain representation, crucial for engineering, aviation, planning, water management, and outdoor navigation. Mastery of contour intervals aids in safe, efficient, and sustainable decision-making across diverse professional fields.