Contour lines are the lines on a map that join points of the same height. Drawn at a fixed height step, they turn a flat sheet into a readable picture of hills, valleys and slopes. They are also the best-known member of a larger family: isolines, lines through points of equal value, which map air pressure, temperature, rainfall and population density the same way.
This guide explains how to read contour lines, what the contour interval tells you, how contours are computed from data, and how other isolines and isopleth maps work. Travel-time isochrones, a special case built from road networks, have their own guide: what is an isochrone.
What contour lines are
A contour line connects points at the same elevation above a reference surface, usually mean sea level. A contour map draws a set of these lines at regular height steps, so the spacing and shape of the lines show how the ground rises and falls.
Contour lines are one kind of isoline, a line on a map along which a continuous variable has a constant value. The name comes from the Greek isos, meaning equal, and each variable has its own term built the same way, as the table further down shows.
How to read contour lines
The same rules apply to every contour map. The USGS guide to topographic map symbols states them this way:
- Lines never cross. Each line joins points of one height, so two lines of different height cannot meet. Where the ground is vertical, maps switch to a cliff symbol.
- Spacing shows the slope. Contours close together mean a steep slope. Widely spaced contours, or none at all, mean the ground is nearly level.
- The interval is constant. The height difference between neighbouring lines is the same across the map, and on USGS maps it is printed in the margin.
- Index contours are heavier. Every few lines, a wider index contour carries a printed elevation so you can orient yourself. The thinner intermediate contours in between add detail.
- Closed loops mark a top or a hollow. A closed contour usually marks a hill or summit. Depression contours, drawn with small inward ticks, mark a hollow.
The shape of the lines tells you the landform. Contours that bend into a V or U pointing uphill mark a valley, and streams run along the point of the V. Contours that bend into a V or U pointing downhill mark a ridge or spur. Two sets of closed rings facing each other in an hourglass shape mark a saddle, the low point between two summits.
From line spacing to slope
Spacing becomes a number once you know the scale and the interval. The slope is the interval divided by the ground distance between two lines. We computed it for the two most common combinations of national map series:
| Line spacing on the map | 1:25,000, 10 m interval | 1:50,000, 20 m interval | Slope |
|---|---|---|---|
| 1 mm | 25 m on the ground | 50 m on the ground | 40% (21.8°) |
| 2 mm | 50 m | 100 m | 20% (11.3°) |
| 5 mm | 125 m | 250 m | 8% (4.6°) |
| 10 mm | 250 m | 500 m | 4% (2.3°) |
Both combinations give the same slope for the same spacing on paper, because the interval doubles along with the ground distance. So on either sheet, contours 1 mm apart mean ground steeper than most roads, and contours 1 cm apart mean a gentle rise. The calculation and its output are in scripts/seo/blog-keyword-research/what-is-an-isoline-map-slope.mjs.
To check a single point instead of reading it off the lines, use our elevation tool. It returns the height of any address or map point and can draw the elevation profile along a path.
The contour interval
The interval sets the level of detail. The USGS guide notes that a map of flat terrain may use a contour interval of 10 feet or less, while mountainous areas may use 100 feet or more. Metric national series follow the same logic: a 10 m interval on 1:25,000 sheets and a 20 m interval on 1:50,000 sheets are common, often with an index contour every fifth line.
A smaller interval is not always better. In steep terrain it packs the lines so tightly that they merge, and in flat terrain a large interval can leave whole areas without a single line. Some maps add dashed supplementary contours at half the interval where the ground is too flat for the regular lines to show anything.
Other isolines
Contours are the most common isoline, but the same idea maps any quantity that changes smoothly across space:
| Isoline | Connects points of equal | Where you see it |
|---|---|---|
| Contour line | Elevation | Topographic and hiking maps |
| Isobath (bathymetric contour) | Water depth | Nautical charts |
| Isobar | Air pressure | Weather charts |
| Isotherm | Temperature | Climate and weather maps |
| Isohyet | Precipitation over a period | Rainfall and hydrology maps |
| Isochrone | Travel time from an origin | Catchment and delivery analysis |
The reading rules carry over. On a weather chart, tightly packed isobars mean a strong pressure gradient and stronger wind. NOAA's JetStream lesson on surface pressure maps draws isobars every 4 millibars, starting from 1000 millibars, after each station's pressure has been reduced to sea level so that stations at different altitudes can be compared.
Isopleth maps
Cartographers split isoline maps into two kinds, depending on where the values come from.
Isometric lines map a value that exists at every point. You can stand on a spot and measure its elevation or its temperature. Contours, isobars and isotherms are all isometric.
Isopleths map a value that only exists for an area. Population density is the standard example: a single point has no density, so you divide each district's population by its area, assign the result to a point inside the district, and interpolate lines between those points. The geographer John K. Wright proposed this split in 1944, and the terminology stuck in cartography textbooks.
An isopleth map shows the same kind of data as a choropleth map, rates per area, but draws it as a smooth surface instead of colouring each district in one flat tone. That makes the pattern easier to see across district borders. It also invents a gradient inside each district that the data does not contain, so use it only when the phenomenon really changes gradually, such as rainfall or commuting density, and not for values that jump at a border, such as tax rates.
In everyday use, and in many school geography courses, "isopleth map" is simply used as another name for any isoline map. When you read the term, check whether the source means the strict sense or the general one.
How isolines are made
An isoline map starts from measurements, and measurements rarely sit on a neat grid. Weather stations, survey points and depth soundings are scattered. Producing the lines takes two steps.
1. Interpolate a surface. The scattered values are estimated onto a regular grid. Common methods include inverse distance weighting, where nearby points count more than distant ones, and linear interpolation over a triangulation of the points. GDAL's gdal_grid implements these, including inverse distance to a power, nearest neighbour, moving average and linear interpolation over a Delaunay triangulation. Elevation often skips this step, because digital elevation models are already gridded rasters.
2. Trace the lines. A contouring algorithm then walks the grid. The standard method for two-dimensional grids is marching squares. For each cell it checks which corners lie above and below the chosen value, and places a line segment through the cell where the value is crossed. The segments join into continuous lines. GDAL's gdal_contour does this for a raster elevation model at a fixed interval and writes the result as vector lines.
Interpolation is an estimate. Where measurements are sparse, the lines between them are a model of the surface, and the map is only as reliable as the data behind it. For an isopleth map, the choice of the point inside each district also shifts the lines, so the result depends on the district boundaries as well as on the data.
Isolines and isochrones
An isochrone is an isoline of equal travel time, but it is built differently. Instead of interpolating measurements, a routing engine expands outward from an origin across the road network and marks how long it takes to reach each place. The boundary at 10, 20 or 30 minutes becomes the isochrone. Because of this, an isochrone depends on the travel mode, the road network and the starting point, while a contour depends only on the terrain.
If the question is "where can I reach in 15 minutes", read what is an isochrone. If the question is how high, how steep, how warm or how wet, you need an isoline map of the kind described here.
Related reading
- Types of maps: where contour and isoline maps sit among reference and thematic map types. Topographic maps, which combine contours with water, roads and place names, are one of the reference types covered there.
- Choropleth map: the usual choice when the data belongs to areas, such as regions or postcodes, and should stay tied to their borders.
- Elevation tool: look up the height of a point or the profile of a route.
Frequently Asked Questions
What are contour lines?
Contour lines are lines on a map that join points of the same elevation above a reference level, usually mean sea level. Drawn at a fixed height step, they show the shape of the land on a flat sheet: close lines mean steep ground, wide spacing means gentle ground. Contour lines are the best-known kind of isoline, the general term for any line of equal value.
How do you read contour lines?
Find the contour interval in the legend, follow a heavier index contour to its printed height, then count the thinner lines up or down to the point you need. Closely spaced lines mean a steep slope. Closed rings mark a summit, contours bending into a V that points uphill mark a valley, and a V pointing downhill marks a ridge. Contours never cross, because each line has one height.
What is a contour interval?
The contour interval is the height difference between two neighbouring contour lines. It is constant across one map and printed in the margin. Flat terrain gets a small interval, mountains a large one. Many national 1:25,000 series use 10 m, and many 1:50,000 series use 20 m.
What is an isopleth map?
An isopleth map is an isoline map of a value that cannot be measured at a single point, such as population density. The value is calculated for areas, for example people per square kilometre per district, assigned to a point inside each area, and then interpolated into lines. Maps of values that exist at every point, such as elevation or temperature, use isometric lines instead. In everyday use, many textbooks treat isopleth map as a synonym for isoline map.
What is the difference between an isoline and an isochrone?
Isoline is the general term for any line of equal value. An isochrone is the specific isoline of equal travel time from an origin, computed over a road network instead of a grid of measurements. Our separate guide to isochrones covers that case.
