left field vs piece of ground

left field and piece of ground both are nouns.

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

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

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

left field (noun)	piece of ground (noun)
the fielding position of the player on a baseball team who is expected to field balls in the left third of the outfield (looking from home plate)	an extended area of land
the piece of ground in the outfield on the catcher's left