Today, much of it is by GPS. We have very accurate satellite elevation maps of the entire world now.

But historically, elevations were determined through geometry (or its more calculation-friendly cousin, trigonometry). You start with some known points (say, sea level at the coast) and measure angles to new points from a distance at several of those points. That establishes the new point’s positions in 3D space. Then you use those new points to survey another set. And so on. This lets you form a mesh of triangles that can cover great distances. [Here’s a map of one such historical survey](https://upload.wikimedia.org/wikipedia/commons/6/64/1922_Index_of_Great_Trigonometrical_Survey_of_India.jpg), which was used to determine the height of the Himalayas and the fact that Mount Everest is the highest point on Earth.

Triangulation. Theodolites work in the vertical as well as the horizontal. Nowadays you can use GPS to augment traditional surveying methods. Most national mapping organisations have a datum level against which altitudes are measured. Britain’s is the port of Newlyn, Cornwall and is based on the average of sea level measurements over a six year period about a hundred years ago.

https://www.ordnancesurvey.co.uk/newsroom/news/100-years-of-ordnance-datum-newlyn

The traditional way of mapping uses triangulation.

If we start with two points that are a known distance apart (such as two points on the coast at sea level), we can use those to determine a third point using a device called a theodolite. This uses a scope to aim at our chosen point, and then takes a bearing off of a compass, and an angle of elevation from a level.

If we have two known points, and a bearing and angle of elevation to a third, we can start drawing triangles. With the distance between two points, and the angle from each of those to a third, using a bit of trigonometry we can figure out the distance to that third point. If we know the distance to that point and the vertical angle, we can do another sum to calculate the height.

We can then use any two of our three known points to work out a fourth, then take any two of our four points to work out a fifth and so on.

It isn’t quick, but we can slowly use these techniques to map out an area completely, including marking the elevations of any point within that area.

While every country will have accomplished this slightly differently, for anyone in the UK, you can still find trig points dotted all over the place – a concrete post with a mounting point on top for a theodolite, typically on notable points like hilltops, and all within visible range of at least two more trig points. These are the points used by the Ordnance Survey in their originally mapping of Great Britain (and are all marked on said maps).

In more modern times we have moved on to more modern techniques to update our mapping – satellite systems like the US GPS, Russian GLONASS and European Galileo satellite navigation systems can provide very accurate positioning, and have been being used to update maps and refine the information we have.

In terms of altitude finding, there are also other simple systems that use measurements like atmospheric pressure and temperature variations to give altitudes (often used in devices like watches), though these are of much poorer accuracies and do need regular calibration.

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