Google is making it more difficult for advertisers to have an independent view of the data generated from ad buys in its ecosystem.

In a note to partners sent Friday and obtained by AdExchanger, Google said it will no longer let buyers use the DoubleClick ID when leveraging its data transfer service. The service allows marketers to pull data out of DoubleClick Campaign Manager (DCM) for cross-platform reporting and measurement.

The DoubleClick ID pulls together data from the company’s various ad and consumer-facing products around a unique user ID associated with the DoubleClick cookie. Previously, buyers could use data transfer to pull log files of impressions served on DCM and measure them against impressions served from other ad servers across the web.

As of May 25th, the same day the EU’s GDPR goes into effect, the DoubleClick ID will no longer be available for data transfers on YouTube impressions and those recorded by the DCM ad server and the DoubleClick Bid Manager (DBM) DSP. Those IDs also won’t be available for DBM first in the EU, and eventually globally. Google will also remove encrypted cookie IDs, IP addresses and user list names from data transfer for all bids in Google Ad Exchange.

Google gave the following comment: "We’re making these changes as part of our ongoing commitment to user privacy. Ad reporting is an important part of the digital ecosystem, and we are committed to partnering with advertisers and partners to help refine strategies, including investing heavily in the expansion of Ads Data Hub."

For buyers, stripping out the DoubleClick ID cuts off visibility to user activity within the DoubleClick ecosystem.

“The DoubleClick ID is so powerful because it ties their entire ecosystem together,” a source with knowledge of the change told AdExchanger. “That’s why they’ve been able to so aggressively expand on the software side and why they’re the DSP, ad server and site-side analytics tool of record for the majority of enterprises. It’s a major leg up they have over other point solutions in the space.”

The change will limit advertisers’ ability to measure the reach and frequency of Google campaigns against other platforms by limiting any measurement using the DoubleClick ID to Google’s own Ads Data Hub

In its note to advertisers, Google has included that the DoubleClick ID, tied to sensitive information like user search histories, could violate the strict data privacy requirements of GDPR. But for marketers, the changes make common analyses, like attribution, reach and frequency, difficult or impossible to do, said Ari Paparo, CEO of Beeswax.

“You can't build your own conversion attribution if you don't have user IDs,” he said. “And you can't calculate unique reach or session depth. Google's stance to remove this data globally is very conservative, and will disadvantage them versus more open DSPs.”

“They're ending interoperability,” said Adam Heimlich, SVP of programmatic at Horizon Media. “If you base your audience strategy on your own partner data, the ability to translate it into Google and back out is going away.”

That’s a big deal for enterprise clients, who increasingly want to own their own audience strategies. But Google could be banking on the fact that the long tail of advertisers will buy into its entire stack and use the DoubleClick ID as the default understanding of their audience, Heimlich said.

“That’s what the battle is: whose user ID is going to be the basis for targeting going forward?” he said. “Google is setting up the processes that will eventually make it easy for businesses to use a stack based on Google IDs they don’t own. Big clients will be reluctant to commit out of concern that Google could raise the price for that.”

This isn’t the first time Google has tried to get marketers on Ads Data Hub – and deeper inside the walls of its ecosystem – this month. A few weeks ago, Google suspended third-party ad serving in the EU on YouTube citing concerns over GDPR compliance. Google also said this month that a plan announced in January to discontinue third-party pixel tracking on YouTube will come into effect under GDPR.

Update: This story has been updated to reflect that the DoubleClick ID is not associated with PII like names and addresses but the DoubleClick cookie.