center field vs piece of ground

center field and piece of ground both are nouns.

Word	Noun	Adjective	Verb	Adverb
center field	Yes	No	No	No
piece of ground	Yes	No	No	No

As nouns, piece of ground is a hypernym of center field; that is, piece of ground is a word with a broader meaning than center field:

center field: the piece of ground in the outfield directly ahead of the catcher
piece of ground: an extended area of land

Other hypernyms of center field include parcel, parcel of land, piece of land, tract.

center field (noun)	piece of ground (noun)
the fielding position of the player on a baseball team who is expected to field balls in the central third of the outfield	an extended area of land
the piece of ground in the outfield directly ahead of the catcher