Contour Line

A contour line (or isoline) is an imaginary line connecting all points with equal elevation above sea level. Contour lines are the primary means of representing terrain in topographic plans and maps.

Types of contour lines:

• Index contours - main lines corresponding to full elevation intervals
• Emphasized contours - every fifth or tenth contour, drawn thicker
• Half-interval contours - additional lines between index contours
• Supplementary contours - drawn in flat terrain areas for detail

Contour interval:

Contour interval is the elevation difference between adjacent contour lines. It can be:

• 0.25 m - highly detailed plans
• 0.5 m - standard topographic plans (1:500)
• 1 m - medium detail
• 2.5 m or 5 m - smaller scale maps

What can be determined from contour lines?

• Elevation - reading contour labels
• Slope - closer contours mean steeper slope
• Terrain form - depressions, hills, valleys
• Water flow direction - from higher to lower contours
• Slope aspect - orientation to cardinal directions

Contour line drawing rules:

• Contour lines never cross (except overhangs)
• Contour lines are closed curves (or exit plan boundaries)
• Labels are written with top facing uphill
• Slope arrows indicate direction of descent

Digital contour development:

Today, contour lines are generated automatically from digital terrain models (DTM) created from LiDAR, photogrammetry, or tacheometric measurement data. This ensures high precision and speed.