A system for data sharing between guidance devices (100, 102, 104) of roving agricultural vehicles (12, 14, 16), the system comprising: a central server having: a communications module for receiving upload guidance data and for transmitting download guidance data; and a database for storing the upload guidance data; a first guidance device of a first agricultural vehicle, the first guidance device having a communication module for wireless transmission of upload guidance data to the central server, the upload guidance data including coverage data of area covered by the first agricultural vehicle whilst operating; and a second guidance device of a second agricultural vehicle, the second guidance device having a communication module for wireless receipt of download guidance data from the central server; wherein the central server is configured so that at least part of the download guidance data transmitted to the second guidance device comprises data generated by the sever using at least part of the upload guidance data received from the first guidance device stored in the database of the central server; and wherein the central server is configured to receive instructions, directed to one of the first guidance device or the second guidance device, and transmit the instructions as download guidance data to the one of the first guidance device or the second guidance device. a central server having: a communications module for receiving upload guidance data and for transmitting download guidance data; and a database for storing the upload guidance data; a first guidance device of a first agricultural vehicle, the first guidance device having a communication module for wireless transmission of upload guidance data to the central server, the upload guidance data including coverage data of area covered by the first agricultural vehicle whilst operating; and a second guidance device of a second agricultural vehicle, the second guidance device having a communication module for wireless receipt of download guidance data from the central server; wherein the central server is configured so that at least part of the download guidance data transmitted to the second guidance device comprises data generated by the sever using at least part of the upload guidance data received from the first guidance device stored in the database of the central server; and wherein the central server is configured to receive instructions, directed to one of the first guidance device or the second guidance device, and transmit the instructions as download guidance data to the one of the first guidance device or the second guidance device. a communications module for receiving upload guidance data and for transmitting download guidance data; and a database for storing the upload guidance data; a first guidance device of a first agricultural vehicle, the first guidance device having a communication module for wireless transmission of upload guidance data to the central server, the upload guidance data including coverage data of area covered by the first agricultural vehicle whilst operating; and a second guidance device of a second agricultural vehicle, the second guidance device having a communication module for wireless receipt of download guidance data from the central server; wherein the central server is configured so that at least part of the download guidance data transmitted to the second guidance device comprises data generated by the sever using at least part of the upload guidance data received from the first guidance device stored in the database of the central server; and wherein the central server is configured to receive instructions, directed to one of the first guidance device or the second guidance device, and transmit the instructions as download guidance data to the one of the first guidance device or the second guidance device.