Contours to DEM Converter: Turn Contour Lines Into an Elevation Grid

What a DEM Is

A digital elevation model (DEM) is a grid of heights. The land is cut into square cells, and each cell holds one height above sea level. Contour lines show the same land as lines of equal height, which is easy for people to read. A DEM is easier for computers: GIS programs need it to draw hillshade, measure slope and aspect, trace where water flows, check what can be seen from a point and build 3D views.

What This Contours to DEM Tool Does

This contours to digital elevation model online tool needs one thing from you: a file of contour lines, each with its height. It reads the lines, puts points along them, joins all points into triangles (TIN) or weighs the nearest points (IDW), and works out a height for every cell of a regular grid. It then shades the grid, works out the figures and makes the downloads. Nothing is built until you press Generate DEM; a “Creating DEM” wheel then shows for about 10 seconds while the grid is made in a background worker, and the DEM appears on the map.

Free and Private: Your File Stays in Your Browser

Contours to DEM is free here: there is no account, email or payment, and no upload. If you looked for a contours to digital elevation model online free download, there is nothing to install either, because the tool runs in your browser. The file is opened in your browser, the DEM is built there, and the GeoTIFF, ASCII grid, PNG and PDF are made on your own device. This matters for survey and client data that should not leave your computer.

Uploading a File or Trying a Sample

Press Choose file or drop a file on the dashed box. The tool reads GeoJSON, KML, KMZ, zipped Shapefiles and DXF (see the table of formats below). To try it first, press one of the Stone Mountain samples: 44 contour lines at 10 m around Stone Mountain in Georgia, made with our Contour Map Generator, once as GeoJSON in latitude and longitude and once as DXF in local metres.

The Height Field and the Units

Each contour line needs a height. The tool looks for it in the attributes (names such as elevation, elev, height, contour, level, z or value come first), in the Z value of the vertices and, for DXF, in the elevation of each polyline or a number in its layer name. Only sources with at least two different values are offered. Choose the right one under Height from, and set Heights in the file to metres or feet. For DXF, also set the map units of the drawing. The m and ft buttons at the top choose the units of the results and the downloads.

Method and Cell Size

TIN (triangles) is the default and fits contour data best: every cell takes its height from the triangle of line points around it. IDW (distance weighted) takes the weighted mean of the 12 nearest points. The cell size is set to about 500 cells across the data and rounded to a common value; type your own to change it, or press Auto to go back. The grid is kept under 4 million cells for TIN and 1.5 million for IDW, so a phone can handle it. When the settings are ready, press Generate DEM. If you change a setting afterwards, the DEM on screen stays until you press Generate DEM again, and a note reminds you that it was made with the old settings.

Reading the Preview, the Figures and the Chart

The DEM appears over the map, coloured from green (low) to brown (high) and shaded as if lit from the north-west, with your contour lines on top. The Layers box in the top-right corner of the map turns the basemap, the DEM colours, the hillshade and the contour lines on or off, so you can see the shading alone, the colours alone, or compare the grid with your lines. The yellow box gives the size of the grid and the height range; the cards give the lowest and highest point, the relief, the mean elevation, the grid, the area covered, the mean slope and the share of flat triangles (for IDW, the number of points used). The chart shows how much of the area lies at each height, and the Grid details table lists what a GIS user needs: cell size, extent, coordinate system and the no-data value (-9999).

Downloads and the PDF Report

The GeoTIFF button gives a contours to DEM GeoTIFF download: one band of 32-bit floating point heights with its position, ready for QGIS, ArcGIS, Global Mapper and GDAL. ASCII Grid saves the same grid as plain text (Esri .asc), which almost any GIS and many CAD and modelling programs open. PNG heightmap saves a 16-bit grayscale image where black is the lowest point and white the highest; the height range is in the file name. Download PDF report saves a report with the maplity.com letterhead, the figures, the height chart, the DEM image with the lines and the method.

TIN: Triangles Between the Lines

A TIN (triangulated irregular network) joins all points on the lines into triangles. The tool uses a Delaunay triangulation, which makes the triangles as even as possible: no point lies inside the circle through the three corners of any triangle. Inside each triangle the ground is a flat, sloping plane through its three corners, so a cell between a 100 m line and a 110 m line gets a height that changes evenly between them. Before triangulating, the tool adds points along long straight parts of the lines, about one cell apart, so the triangles follow the lines and do not cut across them.

IDW: Weighing the Nearest Points

IDW (inverse distance weighting) gives each cell the weighted mean of the 12 nearest points on the lines, with a weight of one over the distance squared. Near points count much more than far ones. The surface is smooth and has no triangle edges, but because all points on one line have the same height, IDW tends to make small steps along each line and rounded, bulging shapes between them.

TIN or IDW: Which to Choose

For the best contours to digital elevation model online result from contour lines, keep TIN: in the round trip of 200 places further down, TIN was closer to the true ground in 200 of them. The table compares the two on the same data.

Choosing a Cell Size

The cell size sets how fine the grid is. A common rule for DEMs made from maps is a cell of about 0.5 mm at the map’s scale: finer cells do not add real detail, because the contours hold no information between the lines. The table gives this rule for common scales, with the contour interval such maps usually have.

Halving the cell size makes the grid four times bigger. If your lines are far apart, a large cell gives the same surface in a much lighter file; if they are close together, as in a detailed site survey, a small cell keeps the shape of each line.

Known Artifacts and How to Spot Them

Flat triangles appear where all three corners of a triangle are on the same line: inside the last line around a hilltop, on valley floors and on wide terraces. The tool counts them and shows their share of the area. Straight edges run along triangle sides where the lines are far apart. Steps along the lines are typical of IDW. Near the outside of the data, triangles get long and thin, and cells beyond the outermost points are left empty. A spike or pit that stands out in the hillshade usually means one line has a wrong height.

1. Digitised paper maps

Many places only have contours from old topographic maps or survey plans. Once the lines are digitised with their heights, this tool turns them into a DEM that modern GIS tools can use for slope, hillshade and 3D views.

2. From CAD survey drawings to GIS

Surveyors and engineers often deliver contours in DXF. Converting them to a GeoTIFF or ASCII grid lets planners, landscape architects and GIS teams use the same ground model without CAD software.

3. Site design and earthworks

A DEM of a plot is the starting point for volume work: compare it with a design surface to estimate cut and fill, check drainage falls, and plan platforms, roads and retaining walls.

4. Flood and drainage studies

Flow direction, flow accumulation and flood extent models need a grid, not lines. A DEM made from local contours often has more detail than global elevation data in small, flat areas.

5. Hillshade and relief maps

A hillshade made from the DEM makes the shape of the land easy to see under a map. Designers and cartographers use it for posters, guide maps and reports.

6. 3D models and printing

The contours to DEM PNG heightmap (16-bit) goes straight into Blender, Unity or Unreal as a displacement map or terrain, and the GeoTIFF into tools that make 3D prints of the land.

7. Game levels and simulations

Game designers and simulation teams build real terrain from contours of a real place, then export the heightmap at the size their engine needs.

8. Solar and wind site checks

Slope and aspect from the DEM show which parts of a site face the sun and how much grading the panel rows need, and where turbines would sit on exposed ground.

9. Teaching GIS and geography

Students see how a few lines become a full surface, compare TIN and IDW, and learn why the result has flat tops and straight edges. The sample files make a ready classroom example.

10. Checking contour data

Building a DEM is a quick quality check: spikes, holes and stepped surfaces show lines with a wrong height, a missing height or a duplicate line.

1. From Latitude and Longitude to a Flat Plane

Distances must be in metres, so lines in latitude and longitude are first moved to a flat plane around the centre of the data (φ0, λ0):

Over a few tens of kilometres this is accurate to well under 0.1%. DXF and other local files are used in their own units.

2. Delaunay Triangles and Barycentric Weights

The Delaunay triangulation joins the points so that no point lies inside the circumcircle of any triangle. For a cell centre P in the triangle with corners A, B and C, the barycentric weights say how close P is to each corner, and the height is their weighted sum:

3. Inverse Distance Weighting

IDW uses the 12 nearest points, found with a grid of buckets so not every point has to be tested:

4. Hillshade and Slope

The preview uses Horn’s method on each 3 × 3 block of cells (a b c / d e f / g h i), with the sun at 315° and 45° above the horizon:

The mean slope in the cards uses central differences on the grid, in percent, averaged over all covered cells.

5. Placing the Grid on the Earth

The GeoTIFF stores the top-left corner and the cell size. For latitude and longitude input the cells are square on the ground, so in degrees they are wider than tall:

The ASCII grid writes the same corner with GDAL’s dx and dy lines when the cells are not square in degrees.

The Downloads in Detail

The GeoTIFF is a single-band, uncompressed TIFF of 32-bit floats with the GeoTIFF tags for the pixel size, the tie point and the coordinate system (EPSG:4326, or none for local data) and a GDAL no-data value of -9999. The contours to DEM ASCII grid (.asc) follows the Esri format with an NODATA_value line. The PNG heightmap is a 16-bit grayscale PNG, scaled from the lowest height (0) to the highest (65,535); empty cells are black. All three are written by Maplity’s own code in your browser.

Software Libraries

The triangulation uses Delaunator with robust-predicates for exact geometry tests. Shapefiles are read by shpjs, which also reprojects them with proj4 using the .prj; KML is converted by @tmcw/togeojson and KMZ is unzipped by JSZip. The DXF reader, the gridding, the IDW search, the hillshade and the file writers are Maplity’s own code. The map is Leaflet 1.9 with Esri, CARTO and OpenStreetMap basemaps, the PDF is made by jsPDF, and the page runs on Next.js and React.

Limits

A DEM from contours knows only the heights on the lines. Features smaller than the contour interval, such as banks, ditches, kerbs and small mounds, are missing, and the ground between two lines is assumed to slope evenly. Very large files are thinned to 400 thousand points and the grid is kept under 4 million cells, so for a whole country use a desktop GIS. For construction, legal boundaries and flood design, use a land survey or national lidar data.

How the Test Was Made

We took the same 200 famous places as on our Contour Map Generator and Elevation Profile Viewer pages. For each place, the heights of the 1 km² square came from AWS Terrain Tiles (zoom 15, light smoothing), and the Contour Map Generator’s own engine drew contour lines at its automatic interval. This converter then rebuilt a DEM from those lines alone, by TIN and by IDW, with one cell per terrain pixel, and every cell was compared with the true height. RMSE is the root mean square error in metres; within ½ interval is the share of the ground that came back within half a contour interval of the truth.

Over the 200 places with enough lines (9,755 contour lines in all), the typical TIN error was 18% of the contour interval and 95.4% of the ground came back within half an interval; for IDW the figures were 29% and 90.6%. TIN was the closer of the two in 200 places. Relative to the interval, the weakest results were at places by the water, such as Elbphilharmonie, Waikiki Beach, Giant's Causeway: there the square holds flat water surfaces and sea-floor depths with only a few lines far apart, so much of the ground lies between lines. The best were on steep, even mountain sides such as Matterhorn summit, where many close lines describe the shape well. The error grows with the interval, so rough ground with a 50 m interval has errors of several metres, while gentle ground at 1 or 2 m comes back within a few decimetres.

DEM From Contours: Results for 100 Famous Places in Europe

DEM From Contours: Results for 100 Famous Places in the USA

More Free Terrain Tools

• Contour Map Generator: the reverse of this tool, contour lines from the elevation of any place, as GeoJSON, KML or DXF.
• Elevation Profile Viewer: an elevation chart along a route of up to 5 points.
• Slope and Gradient Calculator: the grade of a path in percent, degrees and a 1:x ratio.
• Map Elevation Finder: click one place and get its elevation.