Rory Cellan-Jones holding mobile phones in Cardiff
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Mapping Britain's 3G mobile data networks: Cardiff

A BBC crowd-sourcing survey to map the UK's mobile phone networks has revealed the parts of Britain with good and bad 3G mobile coverage.

Users of more than 44,000 phones downloaded an app to measure data speeds over a month.

Rory Cellan-Jones began a train journey from Cardiff to London by meeting Epitiro's Jon Curley, who helped design the application and the interactive map which lets people check reception at specific postcodes.

Go to next video: Mapping Britain's 3G: Oxfordshire