A substantial change broke news in the market this morning between Nielsen Holdings & Abritron Ratings. Nielsen has agreed to purchase the the radio ratings company for $1.26 billion dollars. NY Times reports the offer of $48 per share in cash for each outstanding share of Abritron represents a premium of 26% to Abritron’s closing price on Monday.

The deal will unite two of the best-known measurement companies in the television and radio industries. More than 50 years after their creation, the Nielsen ratings still help determine the fate of many American TV shows. And Arbitron has been one of the top radio ratings watchers for decades.

Both companies have been steadily remaking themselves in the Internet age, adding ways to track online and mobile programming to their offerings. In March,Nielsen announced the introduction of “cross-platform campaign ratings” that tracks advertising both on television and online.

And Arbitron has sought to capture newer forms of listening with its Portable People Meter, a pagerlike device that monitors codes embedded into both traditional radio and online broadcasts. [AlLindstrom]