An arc-second is one 3,600th of a degree, a tiny angle used to measure position on Earth. A grid with spacing of 15 arc-seconds has a value every few hundred metres at the equator.
Key takeaways
- An arc-second is 1/3,600 of a degree of latitude or longitude, so it is a unit of angle, not of distance.
- The grid behind this map has a value every 15 arc-seconds, which is a few hundred metres at the equator.
- A cell shrinks east to west as you move towards the poles, because lines of longitude meet there.
- Smaller arc-second spacing means a sharper grid, but it can show detail only where the data exist.
How arc-seconds work
Angles are divided like time. A circle has 360 degrees, a degree has 60 arc-minutes, and an arc-minute has 60 arc-seconds. So one arc-second is 1/3,600 of a degree. On Earth, a degree of latitude is the same distance everywhere, so an arc-second of latitude is always about the same length. A degree of longitude is widest at the equator and shrinks to nothing at the poles, so an arc-second of longitude shrinks too.
Global map grids use these units because they describe positions on a globe. The GEBCO_2026 Grid has its values on a 15 arc-second interval, in 43,200 rows and 86,400 columns, which comes to 3,732,480,000 data points [1]. In this site we call that 15 arc-seconds. Each value refers to the centre of its cell [1].
The homepage section on accuracy converts that into distance: under half a kilometre across at the equator. That is the best the grid can resolve. It cannot show a seamount or a canyon narrower than a cell, and it is smoothed at the scale of a cell.
Spacing is not the same as truth. A fine grid can be built from coarse data. In the deep ocean, much of the grid is built from satellite-derived estimates [1], which cannot show anything much smaller than a few kilometres. The numbers are laid out at a fine spacing, but the real information behind some of them is blurrier. See digital elevation model for how such grids are used, and satellite-derived bathymetry for where the blur comes from.
An example from our data
The map you drag around is drawn from tiles that are coarser than the full grid, so a tap on the map can read slightly different from the figure on a page. For Challenger Deep, the full grid reads 10,927 m, while the deepest single cell anywhere, in the Mariana Trench, reads 10,931 m.
Both numbers are values in cells of 15 arc-seconds, from the GEBCO_2026 release. A published depth for the same place, taken by a ship over a few metres of seabed, can differ from the grid by tens of metres, because the grid averages the seabed over a whole cell.
The thing people get wrong
The common mistake is to think an arc-second is a distance, such as "15 arc-seconds is 15 seconds of travel". It is an angle. The distance it covers depends on where you are, which is why the same grid has cells that are narrower in Norway than in Ecuador.
The second mistake is to treat finer spacing as proof of better data. A 15 arc-second cell in a surveyed harbour and one in the middle of the Pacific are the same size, but the harbour cell is a measurement and the Pacific one may be an estimate.
Arc-second vs metre
| Arc-second | Metre | |
|---|---|---|
| What it measures | An angle on the globe | A distance |
| Does it change with latitude? | The angle does not, the ground distance does | No |
| Used for | Grid spacing of global maps | Depths and heights |
Frequently asked questions
How many metres is an arc-second?
A few tens of metres of north-south distance. East to west it is the same at the equator and shrinks towards the poles. A grid of 15 arc-seconds is therefore under half a kilometre on a side at the equator.
Is a smaller number of arc-seconds better?
It means finer spacing, so more potential detail. But detail only appears where the data behind the grid are good enough.
Why not use metres instead?
Global grids are laid out in degrees of latitude and longitude, because that is how positions on a sphere are stated. Metres change with latitude, arc-seconds do not.
Sources
- GEBCO_2026 Grid, GEBCO.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).