UPDATE, 11:00 AM: Nielsen just weighed in on today’s news. The measurement company says it “is focused on measuring video however consumers access it, and we are well positioned to continue providing market-trusted, independent measurement to the industry. Our accredited, census-based methodology remains the gold standard for TV audience measurement. We continue to collaborate with our clients to ensure that the solutions we deliver reflect the realities of the market.”

PREVIOUS, 9:50 AM: Look out, Nielsen: Shares in Rentrak are up 11.3% mid-day — and Nielsen’s down 5.1% — after Rentrak unveiled a multifaceted agreement with global ad giant WPP that could provide a major boost to its effort to dominate the ratings business. Rentrak will pay $98M in stock for its top competitor in TV data, the U.S. business of WPP’s measurement firm Kantar Media. WPP’s GroupM, the country’s largest ad buyer, says it will use Rentrak’s national and local TV info to help clients. And WPP will buy $56M of Rentrak stock, lifting its ownership stake to 16.7% with the option to buy as much as 20% in the open market.

This is “a transformational transaction which cements Rentrak’s as the industry’s census-level currency for TV measurement and positions it well to displace Nielsen over the next few years as the primary currency for TV ratings,” Brean Capital’s Todd Mitchell says. Wunderlich Securities’ Matthew Harrigan also likes the deal: “As multi-platform viewing grows, big data is more requisite for targeting ads and assessing viewership. This data is available on a real time basis with aggregation for privacy and scaling issues. Rentrak is now focused on better integration with ad agency systems to support more use of its tools.”

GroupM Global Chairman Irwin Gotlieb used the occasion to take a shot at Nielsen’s on audience samples. “Television measurement needs to move toward census-based methodology,” he says. “Our agencies are doing such refined targeting and segmentation, and that work can only be supported by census data. It is our hope that we can act as catalysts in moving the industry toward greater data reliability and accountability.”

The ad buyer vows to use Rentrak data for sales across national and local markets — including for TV’s all-important auto sales — as well as for on-demand and digital platforms. Today’s announcement specifically targets Nielsen’s weak spots, “more than 200 networks that are not currently measured by the legacy sample currency, as well as in local markets where today’s diary measurement has been less than stable.”

Still, it will be difficult to unseat Nielsen. GroupM probably considers Rentrak data “not as currency” for ad deals but as “incremental data around behavior and other spaces,” RBC Capital Markets’ David Bank says. He acknowledges that Nielsen’s ratings “may not be perfect; its sample and demo in an increasingly fragmented Video market where marketers want to buy more targeted audiences and Media operators want to fulfill that is sometimes called into question. But its data set is the Industry standard and [Nielsen]  is moving to address its sample (e.g. adding broadband only homes) and offer unified solutions across traditional digital screens.”