throughout; over a period of; spanning
Used after a noun indicating a time period, range, or area to express that something continues or extends across that entire span. Often follows words like 長期間, 数年に, 全般に, etc.
See also: 渡る
This research was conducted over a period of ten years.
The meeting continued for three hours.
に渡って stresses that something continues unbroken across an entire span, while にかけて marks a rough range from one point to another without implying full coverage.
に渡って focuses on the extent or duration something spans, while を通して can mean the same but also commonly means 'via/by means of' a channel or person.
Standard kanji spelling; 渡る means 'to extend over'.
Alternate kanji form using 亘 (archaic kanji for 'spanning'); less common.
Derived from the te-form of the verb 渡る (わたる, 'to extend over, to span'), combined with the particle に. The expression grammaticalized into a compound particle meaning 'throughout' or 'over a period of'.