plumb line vs lead line

plumb line and lead line both are nouns.

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

As nouns, lead line is a hyponym of plumb line; that is, lead line is a word with a more specific, narrower meaning than plumb line:

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
lead line: (nautical) plumb line for determining depth

Other hyponyms of plumb line include sounding line, plumb rule.

plumb line (noun)	lead line (noun)
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	(nautical) plumb line for determining depth