How is elevation measured? Just went through the I5 grapevine portion and saw a road marker saying 3000ft elevation.
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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.
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