Topographic map – terrain, elevation colours and contour lines anywhere

A topographic map is the map that shows the shape of the land: where the ground rises and falls, how steep a slope is, where a valley runs and how high a summit stands. Printed topo maps from national mapping agencies do this beautifully, but only for their own country, and online map apps rarely show contour lines at all. This map works the same way everywhere. Pan and zoom to any place and you see the relief as light and shadow, coloured from green lowlands through tan hills to grey high mountains. From zoom level 11, about a 30 km wide view, contour lines are traced for exactly what is on the screen, at an interval that suits the relief in view, and labelled with their height on every index contour. A click on the map gives the height of that spot in metres and feet. Hikers use it to read a route before they go, cyclists to see where the climbs are, students to learn how contours describe a landscape, and anyone house hunting to see whether a plot lies on a slope or in a hollow. The contour lines in view download as GeoJSON or KML, and the contour generator makes DXF and shapefiles for CAD and GIS.

How to use it

1. Search for a town, city or postcode, or pan and zoom the map to the place you want to see.
2. Zoom in until contour lines appear; they are traced for the part of the map on your screen.
3. Keep the automatic contour interval, or choose 1 to 200 metres or feet; every fifth line is a thicker index contour with its height.
4. Click anywhere on the map to read the height above sea level there, and drag the marker to read another spot.
5. Turn the elevation colours or the terrain shading off if you want a plainer map.
6. Download the contours in view as GeoJSON or KML, or copy the share link to come back to the same view.

How accurate is it?

The elevation data under the map

The shading, the colours, the contours and the clicked heights all come from Terrain Tiles, an open worldwide elevation dataset that takes the best public source for each region: lidar models in England and much of the United States, national models of 5–10 m in Austria, Norway, Australia and New Zealand, EU-DEM at 25 m across most of Europe and SRTM at about 30 m nearly everywhere else. A detail smaller than the source resolution, such as a dyke, a sunken lane or a small quarry, is softened or missing.

How the contour lines are drawn

For the view on your screen, the tiles at the matching zoom level are read into a grid of at most 320 cells across, never finer than one pixel of the data. The grid is smoothed lightly to remove the pixel steps of the data, and each contour height is traced with marching squares and linear interpolation between grid points. The interval is chosen from the relief in view to give about 25 lines, so it can change as you pan from a plain into the mountains. A fixed interval that would give more than 250 lines in a large view is shown as every second or fourth line, and the panel says so.

How exact a height is

A clicked height is read from the finest tiles at that spot and is usually within a metre on lidar, and within 3–10 m on EU-DEM and SRTM, worse on steep slopes and in dense forest where radar partly hit the canopy. Heights are above mean sea level (the geoid), like the heights on printed maps, not the ellipsoidal height a GPS receiver shows before correction.

The colours and the shading

The colours are a fixed scale from 0 to 4,500 m, so the same colour means the same height everywhere and the sea stays as on the background map. The shading assumes light from the north-west, as on most printed maps, so slopes facing south-east look dark even in full sun; it shows shape, not sunlight.

Good to know

Contours that disagree with a printed map

National topographic maps draw contours from their own survey data and often smooth or edit them by hand around cliffs and buildings. Where the open data is coarser, the lines here are smoother and can lie tens of metres sideways from the printed ones on a gentle slope, while the height itself differs by only a few metres.

Flat land and water

On very flat land the automatic interval drops to 1 m or less, and the lines then show the noise of the data as much as the land. Lakes are flat in the data, so their shores may get a ragged contour. In the sea the data holds either the sea floor or 0 m, depending on the source.

Not for navigation or building work

The map does not show paths, cliffs, scree or hazards the way a hiking map does, and the data can be years old. Carry a proper map on the mountain, and use a survey or your national elevation model for a building permit, earthworks or drainage.