A digital elevation model, or DEM, is a grid of numbers in which each cell holds the height of the ground. When it also holds ocean depths, as GEBCO's does, it becomes a model of land and seabed together.
Key takeaways
- A DEM stores terrain as a regular grid with one height value per cell, so a computer can draw it, shade it and measure it.
- The grid behind this map is the GEBCO_2026 release, with one value every 15 arc-seconds.
- Heights above sea level are positive and depths below it are negative, so land and seabed share one scale.
- A DEM is only as detailed as its grid: a narrow trench or sharp peak is smoothed if it is smaller than a cell.
How a digital elevation model works
Imagine laying a net of squares over a landscape and writing down one height in each square. That list of numbers, arranged in rows and columns, is a digital elevation model. A computer can turn it into a shaded picture, a set of contour lines, a 3D view or a measurement of slope.
The GEBCO_2026 Grid is exactly this. It is described as a global terrain model for ocean and land, giving elevation in metres on a 15 arc-second interval grid of 43,200 rows and 86,400 columns [1]. Each value is the elevation at the centre of its cell [1]. Land is above zero and the seabed is below zero, so one number line covers Everest and the Mariana Trench.
Two things control how good a DEM is. The first is the spacing of the cells, called the resolution. The grid behind this map has a value every 15 arc-seconds, explained on the arc-second page. Anything smaller than a cell cannot be seen. The second is where the numbers came from. In a DEM of the ocean, some cells come from ship sonar and some are estimates made from satellite data [1], so two neighbouring cells can have very different reliability.
A DEM is data, not a picture. This map draws it as hillshade coloured by depth, which is one of many ways to show the same numbers. You can read how the colours work in the read the map section of the homepage.
The words "bathymetric grid" and "DEM" overlap. Bathymetry is the depth and shape of the seabed [2]. A bathymetric grid is a DEM that happens to hold underwater depths.
An example from our data
Because the grid joins land and sea, we can ask extreme questions of a single file. The highest cell reads 8,627 m above sea level, and the deepest cell, in the Mariana Trench, reads 10,931 m below it. If you stood the highest ground on the trench floor, its top would still be about 2,300 m under water.
Resolution shows in a famous place. Challenger Deep reads 10,927 m in the full grid. The tiles that the map actually draws are coarser than the full grid, so the figure you see on tapping can be shallower. The accuracy section of the homepage gives the details.
The grid also tells us how much of Earth is under water. 71% of the surface is below sea level in the grid, an area of 362 million km², against 147.9 million km² above it.
The thing people get wrong
A DEM looks like a precise 3D model, so people assume every detail is a measurement. Often it is a smoothed average. A cell stores a single height for a whole square, so a narrow trench or a sharp peak is blurred into its surroundings.
For the ocean, many cells are also estimates. A DEM is a good way to see the big picture of the seabed, and not a substitute for a survey. It must never be used for navigation.
DEM vs hillshade
| Digital elevation model | Hillshade | |
|---|---|---|
| What it is | The data: a height in every cell | A picture made from that data |
| Holds | Numbers in metres | Light and shadow |
| On this map | The grid behind every tap | The shaded relief you see |
Frequently asked questions
Is a DEM the same as a topographic map?
Not quite. A DEM is the raw data: numbers in a grid. A topographic map is a picture drawn from such data, with contour lines, colour or shading.
Does a DEM include the seabed?
Some do. GEBCO's grid covers ocean and land, giving elevation in metres on one grid, so the same file holds mountains and ocean trenches.
Why do negative numbers appear?
Because heights are measured from sea level. Anything below it, such as the seabed, has a negative value.
Sources
- GEBCO_2026 Grid, GEBCO.
- What is bathymetry?, NOAA Ocean Service.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).