Contour lines and contour intervals: how to read and make topographic maps
Key takeaways
- Contour lines join points of equal elevation, turning a flat map into a readable picture of terrain.
- The contour interval is the vertical step between lines and is chosen to suit map scale, terrain and project needs.
- Index, intermediate and depression contours are the three types most maps use, with supplementary lines added on flat ground.
- Contours from a survey are interpolated from a surface model, so their quality depends on point density, breaklines and height accuracy.
- A digital elevation model stores heights as a grid; contours are one way of displaying that elevation data.
What are contour lines on a topographic map?
Contour lines are imaginary lines drawn on a map that connect points of equal elevation above a reference level, usually mean sea level or a national height datum. They let a two-dimensional map show the three-dimensional shape of the land.
What is a topographic map? It is a detailed map that shows both natural and built features together with relief, and contours are its defining element. Imagine slicing a hill with horizontal planes at equal height steps: the outline of each slice, projected onto the map, is a contour.
The purpose of a topographic map is to let engineers, planners and field crews judge slope, drainage, visibility and earthwork before they visit a site. In surveying, the same idea appears on site plans produced by a topographic survey, usually at intervals of 0.25 to 1 m.
What is a contour interval and how do you calculate it?
The contour interval is the constant vertical distance between two adjacent contour lines on a map. It is normally printed in the map legend, but you can calculate it from the labelled lines.
- Find two labelled index contours next to each other, for example 100 m and 150 m.
- Subtract the lower value from the higher value: 150 − 100 = 50 m.
- Count the spaces between those two index lines, not the lines themselves. With four intermediate lines there are five spaces.
- Divide the height difference by the number of spaces: 50 ÷ 5 = 10 m contour interval.
The right map contour interval depends on scale and terrain. The values below are typical, not fixed rules:
| Map or plan type | Typical scale | Typical contour interval |
|---|---|---|
| Engineering site plan | 1:200 – 1:500 | 0.1 – 0.5 m |
| Detailed topographic survey | 1:500 – 1:1,000 | 0.25 – 1 m |
| Regional planning map | 1:10,000 | 2 – 5 m |
| National topographic series | 1:25,000 – 1:50,000 | 5 – 20 m (often 10 m) |
| Mountain or small-scale maps | 1:100,000 and smaller | 20 – 100 m |
A small interval shows subtle grading and drainage on flat sites; a large interval keeps steep terrain readable without lines merging into a solid mass.
What are the three types of contour lines?
The three types of contour lines on most maps are index, intermediate and depression contours. Supplementary contours are a fourth type used where ground is very flat.
Index contours
Index contour lines are drawn thicker and carry the elevation label. Usually every fifth line is an index contour, which makes heights quick to read.
Intermediate contours
Intermediate contour lines are the thinner, unlabelled lines between index contours. You find their height by counting intervals up or down from the nearest index line.
Depression contours
On a contour map, a depression such as a pit or sinkhole is shown by closed contours with short tick marks, or hachures, pointing downhill into the hollow. Without the ticks, a closed loop would be read as a hilltop.
Supplementary contours
Supplementary contours are dashed lines at half the normal interval. They show detail on flat ground, such as floodplains, where standard lines would be far apart.
How to read contour maps: rules and landform examples
Reading a contour map comes down to a few rules for contour lines and recognising the patterns they form. Once you know them, valleys, ridges and slopes stand out quickly.
- Contours never cross. A point cannot have two elevations; the only exception is an overhanging cliff.
- Every contour closes on itself, either within the map or beyond its edge.
- Close contour lines indicate steep slopes; widely spaced lines indicate gentle ground; evenly spaced lines mean a uniform slope.
- In a valley, contours form a V or U pointing upstream, towards higher ground. Streams cross contours at right angles.
- On a ridge or spur, the V points downhill, away from the high ground.
- Concentric closed loops mark a hilltop, unless they carry depression ticks.
- A saddle appears as an hourglass shape between two hilltops.
Gradient on a topographic map
To find the slope between two points, divide the height difference by the horizontal distance. If two points are 20 m apart in height and 400 m apart on the ground, the gradient is 20 ÷ 400 = 0.05, which is 5 % or 1 in 20.
To find elevation on a topographic map at a point between lines, interpolate in proportion to its position: a point halfway between the 40 m and 50 m contours lies at roughly 45 m.
Digital elevation model (DEM), DTM and DSM explained
A digital elevation model (DEM) is a raster grid in which each cell stores a height value. It is the digital elevation data from which modern contours, slope maps and colour elevation maps are generated.
| Term | What it represents | Typical use |
|---|---|---|
| DEM | General term for gridded elevation data | Mapping, hydrology, visualisation |
| DTM (digital terrain model) | Bare ground, with vegetation and buildings removed | Contours, earthworks, drainage |
| DSM (digital surface model) | Top surface, including trees and roofs | Line-of-sight, solar, urban models |
| TIN (triangulated irregular network) | Surface built from measured points and breaklines | Survey contours, volumes |
Free global elevation datasets derived from satellite missions typically have cell sizes of about 30 m and vertical errors of several metres, which suits regional studies and a global elevation map, but not site design. National mapping agencies often publish airborne lidar DEMs at 1 m or finer. For a building plot or drainage design, a measured survey remains the reliable source of site elevation.
How do you make a contour map from survey data?
Surveyors make a contour map by measuring enough spot heights to describe the ground, building a surface from them and interpolating contour lines at the chosen interval. Good contours depend more on field technique than on software settings.
- Set up control. Establish benchmarks with known elevation, tied to the national datum, using GNSS or levelling.
- Collect spot heights. Measure a grid or feature-based pattern of points. Typical spacing is 5–20 m on uniform ground and tighter where slope changes.
- Record breaklines. Measure tops and toes of banks, kerbs, ditch lines and ridges as continuous strings so the surface does not cut across them.
- Build the surface. Software triangulates points and breaklines into a TIN or grid DEM.
- Interpolate contours. Lines are threaded through the surface at the interval selected, with index contours every fifth line.
- Check and edit. Compare contours with field photos and check points, then smooth or correct artefacts.
- Label and publish. Add index labels, depression ticks and the datum note.
For a contour survey on open ground, a GNSS RTK rover such as the compact E1 Lite GNSS receiver (about 870 g, IMU tilt) collects points quickly. Under trees or near buildings, a total station such as the TS1000, with 2 mm + 2 ppm prism accuracy, fills gaps. Where millions of points are needed, a terrestrial laser scanner captures dense surfaces; see point cloud processing for how those clouds become terrain models.
GPS elevation and how to find the elevation of a property
GNSS receivers measure height above a mathematical ellipsoid, not above sea level. A geoid model converts that ellipsoidal height into the orthometric elevation used on maps.
This is why GPS elevation on a phone can differ from a map value by several metres, while a survey receiver applies the correct geoid and datum. Vertical GNSS accuracy is typically one and a half to two times worse than horizontal, so RTK heights are usually in the 1.5–3 cm range under good conditions. When centimetre heights matter, surveyors check against a benchmark elevation or run a level loop.
To find the elevation of a property, owners can read approximate values from a topographic map or public elevation data. For flood elevation certificates, drainage design or a floodplain elevation question, a surveyor measures from a known benchmark to give a certified result.
Choosing equipment for a land contour survey
The best elevation measuring equipment depends on site size, cover and the level of detail required. Most firms combine two or more methods.
| Method | Typical vertical accuracy | Point density | Best for |
|---|---|---|---|
| GNSS RTK rover | 1.5–3 cm | Hundreds to thousands of points per day | Open sites, roads, farmland |
| Total station | Millimetres to 1 cm | Hundreds of points per day | Built-up areas, under canopy |
| Terrestrial laser scanner | Millimetres to 1 cm at short range | Millions of points per setup | Complex surfaces, structures |
| Drone photogrammetry or lidar | Centimetres, with ground control | Very high over large areas | Large open sites; see drone surveying with RTK |
Mars Geosystems supplies GNSS receivers, total stations and laser scanners with an MOQ of one unit and OEM options. To match instruments to your contour work, send your project details through the quote page.
Frequently asked questions
What do contour lines represent on a topographic map?
Contour lines represent lines of equal elevation. Every point on one line sits at the same height above the reference datum, usually mean sea level. Their spacing shows steepness, their shapes show landforms such as valleys, ridges and hills, and their labels let you read the height of any point by counting intervals from the nearest index contour.
What do close contour lines indicate?
Close contour lines indicate steep ground, because the elevation changes by one full contour interval over a short horizontal distance. Lines that merge almost into one band indicate a cliff. Widely spaced contour lines indicate gentle or flat terrain, and evenly spaced lines indicate a slope with a constant gradient.
How do you find the contour interval on a map?
Pick two neighbouring labelled index contours, subtract the lower elevation from the higher one, then divide the result by the number of spaces between them. For example, 200 m and 100 m index lines with four thinner lines between them give five spaces, so the contour interval is 100 ÷ 5 = 20 m.
What is an index contour?
An index contour is a thicker, labelled contour line that appears at regular steps, usually every fifth line. It gives a reference elevation so readers can count up or down to find the height of intermediate contour lines. On a map with a 10 m interval, index contour lines typically fall at every 50 m.
How is a contour line on a topographic map drawn from survey data?
Surveyors measure spot heights and breaklines with GNSS, total stations or scanners, then software builds a triangulated surface model. Contour lines are interpolated through that surface where it crosses each chosen height. The surveyor then checks the lines against the field, edits artefacts, adds index labels and notes the height datum.
What is the difference between a digital elevation model and a contour map?
A digital elevation model stores elevation data as a grid of height values, which computers can analyse for slope, drainage and volumes. A contour map is a graphical display of that same surface as lines of equal height. Contours are easy for people to read, while DEMs are better for calculations and 3D visualisation.
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