Richard decided to spend his vacation backpacking to see how far from sea levels he could make it. The term underline in the sentence above can BEST be described as

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Richard decided to spend his vacation backpacking to see how far from sea levels he could make it. The term underlined in the sentence above can BEST be described as "a reference point for measuring elevation."

That is why Richard has decided to take his backpack and start walking and climb the mountain. The sea level is the point of reference to measure the elevation of a place, a town, city, or region. There are places at sea level such as the coasts of places located at high elevations such as Denver, Colorado.

If you want to practice resistance sports, the best way to do it is in high elevation places, such as Colorado. Indeed, in Colorado Springs there are the headquarters and training facilities of the US Olympic teams.