Here's why:
* Common nouns are general names for people, places, things, or ideas. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns are specific names for people, places, things, or ideas. They are always capitalized.
Since "elevation" refers to the height of something in a general sense, it's a common noun.
For example:
* "The elevation of the mountain is 10,000 feet."
* "The elevation of the airplane is 30,000 feet."