A system for mapping the distribution of residue material in an environment in which one or more agricultural machines are operable, the system comprising: a sensing arrangement comprising one or more sensors which can be mounted or otherwise coupled to an agricultural machine operating within the environment; and one or more controllers, configured, in use, to: receive sensor data from the one or more sensors indicative of residue material spread by a spreader tool of the agricultural machine; determine, from the sensor data, a local distribution of residue material associated with the spreader tool; and update a map of a global distribution of residue material across the environment in dependence on the local material distribution determined from the sensor data and the location of the agricultural machine; wherein updating the map of the global distribution comprises assigning one of a plurality of categories to a sub-region of the environment in dependence on the determined local distribution at the location of the particular sub-region; and characterized in that the plurality of categories include: (i) a region corresponding to standing crop (i.e. unharvested crop) but where material has been spread into, for example as the agricultural machine passed along an adjacent row in the environment; and/or (ii) a region corresponding to an area of harvested crop where residue material has been spread more than once, for example as the agricultural machine passes along an adjacent row in the environment. a sensing arrangement comprising one or more sensors which can be mounted or otherwise coupled to an agricultural machine operating within the environment; and one or more controllers, configured, in use, to: receive sensor data from the one or more sensors indicative of residue material spread by a spreader tool of the agricultural machine; determine, from the sensor data, a local distribution of residue material associated with the spreader tool; and update a map of a global distribution of residue material across the environment in dependence on the local material distribution determined from the sensor data and the location of the agricultural machine; wherein updating the map of the global distribution comprises assigning one of a plurality of categories to a sub-region of the environment in dependence on the determined local distribution at the location of the particular sub-region; and characterized in that the plurality of categories include: (i) a region corresponding to standing crop (i.e. unharvested crop) but where material has been spread into, for example as the agricultural machine passed along an adjacent row in the environment; and/or (ii) a region corresponding to an area of harvested crop where residue material has been spread more than once, for example as the agricultural machine passes along an adjacent row in the environment. receive sensor data from the one or more sensors indicative of residue material spread by a spreader tool of the agricultural machine; determine, from the sensor data, a local distribution of residue material associated with the spreader tool; and update a map of a global distribution of residue material across the environment in dependence on the local material distribution determined from the sensor data and the location of the agricultural machine; wherein updating the map of the global distribution comprises assigning one of a plurality of categories to a sub-region of the environment in dependence on the determined local distribution at the location of the particular sub-region; and characterized in that the plurality of categories include: (i) a region corresponding to standing crop (i.e. unharvested crop) but where material has been spread into, for example as the agricultural machine passed along an adjacent row in the environment; and/or (ii) a region corresponding to an area of harvested crop where residue material has been spread more than once, for example as the agricultural machine passes along an adjacent row in the environment. (i) a region corresponding to standing crop (i.e. unharvested crop) but where material has been spread into, for example as the agricultural machine passed along an adjacent row in the environment; and/or (ii) a region corresponding to an area of harvested crop where residue material has been spread more than once, for example as the agricultural machine passes along an adjacent row in the environment.