According to the invention, a field mobile terminal, such as a PDA, is adapted to a use. The mobile terminal is provided with data, which is divided into several parts. Each part concerns data connected to a certain area, and the data of each part has been encrypted by using a specific encryption key or keys. The provision of data is a preliminary step for adapting the mobile terminal and it may be done in an office of a public utility or a corresponding organization. A decryption key (or keys) is location specific. The decryption keys are in a special server. When a decryption of a part (or parts) is needed, the server finds out the location of the mobile terminal. This is done by asking for the location information from a special location information application in a communication network. The application utilizes the location information of the mobile network, which is updated in the wireless network system, such as in a GSM network. Alternatively, the location information of the mobile terminal may be delivered to the server automatically. A method according to the invention provides that the location service utilizes the location information on the mobile terminal, which location information is within the knowledge of the network.