A method for determining a thickness of a tree trunk in a harvesting head for forestry, said harvesting head comprising a thickness measuring arrangement for measuring a measured thickness of a tree trunk, said thickness measuring arrangement comprising a tree trunk pressing device (3, 4, 7, 8), for pressing said tree trunk in a direction towards a reference surface of the harvesting head, and a first measuring sensor arranged to measure the position of said tree trunk pressing device (3, 4, 7, 8) relative to said reference surface, and a deviation measuring arrangement, for measuring a deviation of said tree trunk from said reference surface, said deviation measuring arrangement comprising a tree trunk surface follower (11, 13; 11') and a second measuring sensor arranged to measure the position of said tree trunk surface follower (11, 13; 11'), wherein the second measuring sensor is a proximity sensor, wherein the tree trunk surface follower is a limbing member (11, 11'), and wherein the limbing member (11, 11') is curved, said method comprising pressing the tree trunk, with said tree trunk pressing device (3, 4, 7, 8), in a direction towards the reference surface of the harvesting head and determining a measured thickness by measuring the position of the tree trunk pressing device (3, 4, 7, 8) by means of the first measuring sensor and relating said position to said reference surface, wherein said method further comprises the steps of determining a deviation of the tree trunk from said reference surface by measuring a position of the curved limbing member (11, 11') by means of the second measuring sensor and relating said position of the curved limbing member (11, 11') to said reference surface, and calculating a corrected thickness (T c) of the tree trunk based on said measured thickness and said deviation, wherein determining the deviation further comprises relating said position of the curved limbing member (11, 11') to said measured thickness, and determining the deviation as a function of said position of the curved limbing member (11, 11'), a curvature of the limbing member (11, 11') and the measured thickness. a thickness measuring arrangement for measuring a measured thickness of a tree trunk, said thickness measuring arrangement comprising a tree trunk pressing device (3, 4, 7, 8), for pressing said tree trunk in a direction towards a reference surface of the harvesting head, and a first measuring sensor arranged to measure the position of said tree trunk pressing device (3, 4, 7, 8) relative to said reference surface, and a deviation measuring arrangement, for measuring a deviation of said tree trunk from said reference surface, said deviation measuring arrangement comprising a tree trunk surface follower (11, 13; 11') and a second measuring sensor arranged to measure the position of said tree trunk surface follower (11, 13; 11'), wherein the second measuring sensor is a proximity sensor, wherein the tree trunk surface follower is a limbing member (11, 11'), and wherein the limbing member (11, 11') is curved, said method comprising pressing the tree trunk, with said tree trunk pressing device (3, 4, 7, 8), in a direction towards the reference surface of the harvesting head and determining a measured thickness by measuring the position of the tree trunk pressing device (3, 4, 7, 8) by means of the first measuring sensor and relating said position to said reference surface, wherein said method further comprises the steps of determining a deviation of the tree trunk from said reference surface by measuring a position of the curved limbing member (11, 11') by means of the second measuring sensor and relating said position of the curved limbing member (11, 11') to said reference surface, and calculating a corrected thickness (T c) of the tree trunk based on said measured thickness and said deviation, wherein determining the deviation further comprises relating said position of the curved limbing member (11, 11') to said measured thickness, and determining the deviation as a function of said position of the curved limbing member (11, 11'), a curvature of the limbing member (11, 11') and the measured thickness.