Contour Mapping

Part of Cartography & Surveying

How to represent three-dimensional terrain on a flat map using contour lines, and how to read elevation information from those lines.

Why This Matters

A flat map without elevation information is like a house plan without walls marked. It tells you distances and directions but nothing about the shape of the land — and shape determines almost everything that matters in practice. Where will water flow? Where is it safe to build? Where will defenders have the advantage? Which routes are passable with loaded animals?

Contour maps answer all of these questions by encoding the third dimension (height) into a system of lines, each representing a constant elevation. Once you can read contour lines, you can visualize hills, valleys, ridges, and slopes from paper alone. Once you can draw them, you can communicate complex terrain information to people who have never visited the area.

For a rebuilding community managing land, water, and defense, contour mapping is the foundation of all serious planning. Even crude, approximate contour maps are vastly more useful than plans that ignore elevation entirely.

What Contour Lines Mean

A contour line connects all points at the same elevation above a reference level (usually sea level or an agreed local datum). On a standard map:

• Closely spaced contours indicate steep slopes — the elevation changes rapidly over short horizontal distance.
• Widely spaced contours indicate gentle slopes or flat ground.
• Concentric closed loops indicate a hill (loops getting smaller toward the center) or a depression (same pattern but marked with small ticks pointing inward).
• V-shapes pointing uphill indicate valleys or stream channels — water flows through the V.
• V-shapes pointing downhill indicate ridges or spurs.
• Contour lines never cross — two different elevations cannot occupy the same point.

The contour interval is the elevation difference between adjacent contour lines. On detailed maps of gentle terrain, the interval might be 1 or 2 meters. On regional maps of mountainous areas, 50 or 100 meters. Choose an interval appropriate to your purpose: too large and detail is lost; too small and the map becomes cluttered with lines.

Index contours: Every fifth contour line is typically drawn heavier and labeled with its elevation. This makes reading easier — you count up or down from the nearest labeled line.

Field Methods for Collecting Elevation Data

Hand level and rod: The simplest precise instrument. A hand level is a small tube with a bubble level, allowing you to sight horizontally. To measure a height difference: stand at point A, hold the level at eye height, and sight to a rod or measuring pole held at point B. Read the height where the horizontal line of sight intersects the rod. This reading, adjusted for the eye height above the ground, gives the elevation difference between the two stations. Accurate to a few centimeters per setup.

Clinometer method: Use a clinometer to measure the slope angle between two points, then calculate the elevation change using: height difference = distance × tan(slope angle). For a 10° slope over 50 m: 50 × tan(10°) ≈ 50 × 0.176 ≈ 8.8 m rise. Accurate to within a few percent on well-measured angles.

Water level: Fill a long hose (10–20 m) with water. The water surface at both ends is always at the same elevation — this is the principle of communicating vessels. Mark or read the water level at each end while the hose connects two points; the difference in ground-to-water-surface height at each end gives the elevation difference. Simple, cheap, accurate, but slow.

Altimeter: A barometric altimeter measures air pressure and converts it to elevation. Accurate to within a few meters when the weather is stable (pressure changes with weather as well as elevation). Useful for regional reconnaissance but less reliable for detailed mapping.

Systematic Survey Approach

To map contours for a defined area, plan a systematic grid or cross-section survey:

Grid method:

1. Lay out a regular grid over the area, spacing determined by desired detail (10 m grid for detailed work, 50–100 m for regional).
2. Measure the elevation at each grid intersection.
3. Plot the grid on paper. Interpolate between measured points to find where each contour line crosses the grid lines.
4. Connect the crossing points smoothly for each elevation.

Cross-section method:

1. Run a series of parallel transect lines across the area (north-south or along slope direction).
2. Measure elevation at regular intervals along each transect.
3. Plot profiles (elevation vs. distance) for each transect.
4. Find where each target elevation occurs along each profile.
5. Connect matching elevation points across profiles.

Spot height method (for rough maps): Rather than full gridding, survey key points: hilltops, valley bottoms, ridge saddles, stream confluences, obvious slope breaks. Mark these with elevation on a base map. Then sketch contours that smoothly connect points of equal elevation, guided by your field observations of how the terrain flows.

Drawing and Interpolating Contours

Raw field data gives you a scatter of elevation measurements. Turning this into a contour map requires interpolation — estimating where the exact contour line passes between your measurement points.

Linear interpolation: Assume the slope between two adjacent measurement points is uniform. If point A is at 45 m and point B is at 52 m, and they are 100 m apart, the 50 m contour is (50-45)/(52-45) = 5/7 of the way from A to B, or about 71 m from A.

Smooth curves: Contour lines in nature rarely follow sharp angles. Draw smooth, flowing curves. They should respect the terrain features you observed in the field: if there is a stream, contours must form a V pointing upstream at the crossing; if there is a distinct ridge, contours should form a V pointing away from the ridge crest.

Field checking: After drawing your first draft, go into the field and walk along a few of your contour lines. You should be maintaining roughly constant elevation as you walk. If you find yourself climbing and descending while following the line, adjust it.

Reading Contour Maps for Practical Decisions

Slope gradient: Count how many contour lines cross a given distance. Multiply by the contour interval to find total elevation change. Divide by horizontal distance to get the gradient. Example: 5 contour lines of 5 m interval over 100 m horizontal = 25 m rise over 100 m = 25% slope.

Watershed boundaries: The ridgeline separating two drainage areas follows the highest contour points between valleys. Trace the path that always moves away from valley V-shapes on both sides. This boundary determines where rain runoff flows — critical for water source planning and flood risk assessment.

Siting decisions: Avoid building in closed low areas (surrounded by higher contours on all sides) — these collect water. Choose locations on gentle slopes that drain away from the building. Roads should follow contours as much as possible to minimize grading and maintain consistent slope.

Irrigation planning: Water flows perpendicular to contour lines and downhill. An irrigation channel that follows a contour carries water across a hillside without losing elevation, distributing it to lower fields by gentle control. Map this before digging — a mistake in contour interpretation can send water the wrong direction.

Contour maps reward the effort of their creation many times over. A single good map of an area can guide a century of decisions.