playing area vs piece of ground

playing area and piece of ground both are nouns.

Word	Noun	Adjective	Verb	Adverb
playing area	Yes	No	No	No
piece of ground	Yes	No	No	No

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

Other hypernyms of playing area include parcel, parcel of land, piece of land, tract.

playing area (noun)	piece of ground (noun)
a piece of land prepared for playing a game	an extended area of land