Contour Mapping
Part of Surveying
How to represent three-dimensional terrain on a flat map using contour lines that show elevation.
Why This Matters
A flat map shows where things are horizontally but tells you nothing about hills, valleys, or slopes. For most practical purposes — routing a road, placing a building, designing drainage, assessing flood risk — you need to know the terrain’s shape in three dimensions. Contour mapping is the technique that compresses that 3D information onto a 2D surface.
A contour line connects all points at the same elevation. Where contours are widely spaced, the slope is gentle. Where they crowd together, the slope is steep. A closed set of contours indicates a hill or depression. V-shaped contours pointing uphill indicate a valley; V-shapes pointing downhill indicate a ridge. Once you can read a contour map, you can mentally reconstruct the landscape from the paper and make decisions about water flow, construction difficulty, and site selection without walking every meter of ground.
In a rebuilding context, contour maps are essential for watershed management, choosing settlement sites above flood level, routing canals and roads with minimal earthwork, and defending territory. Making them requires only a level instrument, measuring rods, a way to measure horizontal distances, and the patience to collect enough elevation points.
The Grid Survey Method
The simplest approach to contour mapping is to measure elevations at a regular grid of points across the area, then interpolate contour lines between the points.
Setting up the grid:
- Establish two perpendicular baselines along the edges of your survey area.
- Mark points at regular intervals along each baseline — every 10 meters for a detailed map, every 50 meters for a rough overview.
- Using right-angle construction (a 3-4-5 triangle or a cross-staff), run lines from each baseline point perpendicular to the baseline, through the survey area.
- Mark grid intersections with stakes.
Measuring elevations: Set up your level instrument at a central point where it can see as many grid stakes as possible. Take a backsight on a benchmark to establish the height of instrument (HI). Then sight to a rod held at each grid stake and record the rod reading. Each point’s elevation = HI − rod reading.
Move the instrument as needed to cover the full grid, always connecting each new setup to the previous one through a turning point with known elevation.
Recording: Use a table with grid rows and columns, entering the elevation at each intersection. Label the grid with letter rows (A, B, C…) and numbered columns (1, 2, 3…) so each point has a unique identifier: A-1, A-2, B-1, B-2, etc.
The Spot Height Method
For larger areas where a complete grid is impractical, the spot height method is more efficient. You measure elevations at key terrain features — ridge lines, valley bottoms, saddles, hilltops — rather than at a regular grid.
Walk the area and identify the characteristic shapes: where ridges change direction, where valleys fork, where slopes change angle. Measure the elevation at each of these points. On your map, plot each point with its elevation written beside it.
Contour lines are then drawn by interpolation: between two adjacent points, estimate where each contour elevation falls based on the elevation difference and distance. If point A is at 42 meters and point B is at 48 meters, and they are 60 meters apart, then the 44-meter contour falls at (44-42)/(48-42) × 60 = 20 meters from A. The 46-meter contour falls at (46-42)/(48-42) × 60 = 40 meters from A.
The spot height method is faster and requires fewer measurements, but the resulting contour map is less precise between measurements. It works best when combined with a good knowledge of the terrain’s shape from walking it.
Contour Interval Selection
The contour interval is the elevation difference between successive contour lines. Choosing the right interval is important: too small an interval produces a cluttered map with indistinct information; too large an interval misses important details.
Guidelines:
| Terrain Type | Suitable Interval |
|---|---|
| Flat, low relief (< 5m variation) | 0.5 m or 1 m |
| Gently rolling | 1 m or 2 m |
| Hilly, moderate relief | 5 m |
| Mountainous | 10 m or 20 m |
For construction site planning, 0.5 m or 1 m intervals are needed to design drainage and foundations. For landscape overview and road routing, 5 m or 10 m is sufficient.
Once you choose an interval, index contours (every 5th or 10th contour) are drawn heavier and labeled with their elevation. Intermediate contours are drawn lighter and not labeled, but can be read from their position between the indexed lines.
Drawing Contour Lines
With elevation data collected, you draw contours on your base map by interpolation.
Method:
- On your map paper, mark all your surveyed points with their elevations.
- Choose your first contour to draw. Find all pairs of adjacent points where this elevation falls between them. Mark the interpolated position on the line between each pair.
- Connect these marks in a smooth curve. The curve should follow the terrain shape you know from walking the ground — valleys produce V-shapes pointing uphill, ridges produce V-shapes pointing downhill.
- Repeat for each contour elevation.
Rules contours must obey:
- Contours never cross each other (except at vertical cliffs, which are shown as a special symbol)
- Contours never branch or fork
- Every contour that enters the map must exit, or must close on itself within the map
- Closely spaced contours mean steep slopes
- Widely spaced contours mean gentle slopes
Smoothing: Real contours are smooth curves, not jagged lines through interpolated points. Use your knowledge of the terrain to draw natural-looking curves. A valley bottom should have a rounded V, not a sharp angle. A hilltop should have rounded, roughly circular contours.
Calculating Slope and Drainage
Once you have a contour map, you can extract quantitative information from it.
Slope calculation: Measure the horizontal distance between two contour lines on the map (using your map scale to convert map distance to ground distance). Slope = contour interval / horizontal distance. Express as a ratio (1 in 20), a percentage (5%), or an angle (arctan(1/20) = 2.9°).
Drainage analysis: Water flows perpendicular to contour lines, from high to low. To find drainage paths, draw arrows perpendicular to contours at intervals across the map, pointing downhill. Trace these arrows to see where water from various parts of the terrain collects.
Valley lines (thalwegs) are the paths along which water collects. Find them by identifying where contours form V-shapes pointing uphill — the valley bottom follows the tips of these V-shapes.
Ridge lines (divides) are boundaries between adjacent drainage basins. Find them by identifying where contours form V-shapes pointing downhill — the ridge follows the tips of these V-shapes.
Watershed delineation: A watershed is the entire area that drains to a single outlet point. Start at the outlet point and trace the ridge lines in both directions until they meet. All the land enclosed within the ridge line boundary drains to your outlet. Watershed area, combined with rainfall data, allows you to calculate the runoff volume a drainage structure must handle.
Reading Terrain Features from Contour Maps
| Feature | Contour Pattern |
|---|---|
| Hill | Closed, roughly circular contours, elevation increasing toward center |
| Valley | V-shaped contours opening downhill, with V-tips pointing uphill |
| Ridge | V-shaped contours with V-tips pointing downhill |
| Saddle (pass) | Two closed hills close together, with contours bowing outward between them |
| Cliff | Contours very closely spaced or touching |
| Terrace | Widely spaced contours interrupted by a band of closely spaced ones |
| Depression | Closed contours with tick marks on the downhill side |
Practical Surveying: The Radial Method
For a small survey area where you need a quick contour sketch, the radial method works from a single instrument station:
- Set up your level instrument at a central point on high ground where you can see most of the area.
- Sight to a rod at a distant point. Read the rod and compute the ground elevation.
- Measure the horizontal angle from your baseline to this point.
- Measure the horizontal distance from your station to the point (by pacing, triangulation, or chaining).
- Plot each point on your map using the angle and distance, and label its elevation.
- Repeat for 50-100 points around the area.
- Draw contours by interpolation.
This method is faster than grid surveying but less systematic. It is best for roughly circular areas where the instrument can see most of the terrain from one central position.
Supplement with Field Sketches
The best contour maps combine instrument readings with careful field observation. Walk the terrain before mapping it, sketching ridges, valleys, and terrain breaks. This sketch guides your interpolation and helps you draw natural-looking contours rather than geometrically interpolated but unrealistic curves.