idea vs dead reckoning

idea and dead reckoning both are nouns.

Word	Noun	Adjective	Verb	Adverb
idea	Yes	No	No	No
dead reckoning	Yes	No	No	No

As nouns, dead reckoning is a hyponym of idea; that is, dead reckoning is a word with a more specific, narrower meaning than idea:

idea (noun)	dead reckoning (noun)
the content of cognition; the main thing you are thinking about	navigation without the aid of celestial observations
your intention; what you intend to do	an estimate based on little or no information
an approximate calculation of quantity or degree or worth
a personal view
(music) melodic subject of a musical composition