Hundreds of workers at the national mapping agency Ordnance Survey (OS) are staging a strike in a row over pay.
More than 400 members of the Public and Commercial Services union and Prospect walked out on Tuesday for 12 hours.
The unions claim staff's pay expectations are not being met but OS bosses said the most recent pay offer is "fair and realistic".
The strike affects the government agency's head office in Southampton as well as other offices across the UK.
Ben Middleton, Prospect negotiator, said: "Members now feel they have no other avenue open to them other than strike action.
"OS management has failed to respond to their reasonable expectations to have pay protected against inflation and for staff to progress to the rate for the job in a realistic timeframe.
"OS have stubbornly refused to acknowledge that staff who have patiently borne a succession of poor pay offers feel that patience has run out."
Nadine Grant, head of human resources at OS, said: "We have worked very hard with trade union representatives to reach agreement on our latest pay offer.
"It is a high priority for us, but we can only fund what we can afford. Our offer is fair, realistic and affordable.
"We are disappointed with the outcome of a ballot of union members but the fact is that those who voted to strike are one-third of our workforce."