Method for marking sports field by placing stripes of paint according to a predefined plan via a spraying nozzle on a levelled playing surface such as grass, gravel, ice, artificial grass or synthetic material field, which method comprises the steps of: - adding the paint in portions in a reservoir, - moving a robot unit along the stripes according to the predefined plan, which robot unit brings along the reservoir for paint and a pump for pumping paint from the reservoir to the spraying nozzle, - the unit receiving signals from a number of GPS sending units and hereby the unit continuously calculates its current position on the sports field, - the unit using the current position on the sports field for calculating a set of control signals for two or more movable elements for propulsion of the unit and for controlling the application of paint, - forming stripes during the unit's movement by pumping paint from the reservoir to the spraying nozzle and applicating said paint to the playing surface in accordance to the control signals, - choosing replaceable containers which each comprise: - an inner flexible bag arranged in an outer rigid framing element consisting of a fixed part of the robot unit, or - at least one inner flexible bag which is arranged in a replaceable outer rigid framing element, - connecting the replaceable containers to the pump by bringing a connecting pipe on the outside of the framing element in fluid connection with the paint in the flexible bag, - passing the paint through the connection pipe to the pump, - pressurizing the paint in the pump, - directing the paint from the pump under pump pressure in a nozzle pipe to the spraying nozzle, - releasing the paint during spreading, - replacing the replaceable containers, characterized in that the method further comprises the steps of: - adjusting a distance between two screens (21, 22) arranged on each side of the spraying nozzle, at least one of which is an adjustable screen, wherein the adjusting if performed by use of an actuator, which is connected to the control box and is controlled by control signals dependent on the unit's position on the playing surface, such that a uniform stripe width can be achieved. - adding the paint in portions in a reservoir, - moving a robot unit along the stripes according to the predefined plan, which robot unit brings along the reservoir for paint and a pump for pumping paint from the reservoir to the spraying nozzle, - the unit receiving signals from a number of GPS sending units and hereby the unit continuously calculates its current position on the sports field, - the unit using the current position on the sports field for calculating a set of control signals for two or more movable elements for propulsion of the unit and for controlling the application of paint, - forming stripes during the unit's movement by pumping paint from the reservoir to the spraying nozzle and applicating said paint to the playing surface in accordance to the control signals, - choosing replaceable containers which each comprise: - an inner flexible bag arranged in an outer rigid framing element consisting of a fixed part of the robot unit, or - at least one inner flexible bag which is arranged in a replaceable outer rigid framing element, - connecting the replaceable containers to the pump by bringing a connecting pipe on the outside of the framing element in fluid connection with the paint in the flexible bag, - passing the paint through the connection pipe to the pump, - pressurizing the paint in the pump, - directing the paint from the pump under pump pressure in a nozzle pipe to the spraying nozzle, - releasing the paint during spreading, - replacing the replaceable containers, characterized in that the method further comprises the steps of: - adjusting a distance between two screens (21, 22) arranged on each side of the spraying nozzle, at least one of which is an adjustable screen, wherein the adjusting if performed by use of an actuator, which is connected to the control box and is controlled by control signals dependent on the unit's position on the playing surface, such that a uniform stripe width can be achieved. - an inner flexible bag arranged in an outer rigid framing element consisting of a fixed part of the robot unit, or - at least one inner flexible bag which is arranged in a replaceable outer rigid framing element, - connecting the replaceable containers to the pump by bringing a connecting pipe on the outside of the framing element in fluid connection with the paint in the flexible bag, - passing the paint through the connection pipe to the pump, - pressurizing the paint in the pump, - directing the paint from the pump under pump pressure in a nozzle pipe to the spraying nozzle, - releasing the paint during spreading, - replacing the replaceable containers, - adjusting a distance between two screens (21, 22) arranged on each side of the spraying nozzle, at least one of which is an adjustable screen, wherein the adjusting if performed by use of an actuator, which is connected to the control box and is controlled by control signals dependent on the unit's position on the playing surface, such that a uniform stripe width can be achieved.