longness vs elongation

longness and elongation both are nouns.

Word	Noun	Adjective	Verb	Adverb
longness	Yes	No	No	No
elongation	Yes	No	No	No

As nouns, elongation is a hyponym of longness; that is, elongation is a word with a more specific, narrower meaning than longness:

Other hyponyms of longness include extension, lengthiness, prolongation.

longness (noun)	elongation (noun)
the property of being of long spatial extent	the act of lengthening something
duration as an extension	an addition to the length of something
	the quality of being elongated