observation vs mensuration

observation and mensuration both are nouns.

Word	Noun	Adjective	Verb	Adverb
observation	Yes	No	No	No
mensuration	Yes	No	No	No

As nouns, mensuration is a hypernym of observation; that is, mensuration is a word with a broader meaning than observation:

observation: the act of making and recording a measurement
mensuration: the act or process of assigning numbers to phenomena according to a rule

Other hypernyms of observation include measure, measurement, measuring.

observation (noun)	mensuration (noun)
the act of observing; taking a patient look	the act or process of assigning numbers to phenomena according to a rule
the act of making and recording a measurement
the act of noticing or paying attention
facts learned by observing
a remark expressing careful consideration