sounding line vs plumb line

sounding line and plumb line both are nouns.

Word	Noun	Adjective	Verb	Adverb
sounding line	Yes	No	No	No
plumb line	Yes	No	No	No

As nouns, plumb line is a hypernym of sounding line; that is, plumb line is a word with a broader meaning than sounding line:

sounding line: (nautical) plumb line for determining depth
plumb line: a cord from which a metal weight is suspended pointing directly to the earth's center of gravity; used to determine the vertical from a given point

Other hypernyms of sounding line include perpendicular.

sounding line (noun)	plumb line (noun)
(nautical) plumb line for determining depth	a cord from which a metal weight is suspended pointing directly to the earth's center of gravity; used to determine the vertical from a given point