A datum line, such as the leading or trailing edge of an aerofoil component, is identified on a surface mesh of the component for use in a simulation. The method includes the steps of: defining a reference coordinate system including a direction X, which may be a general flow direction over the component; defining a span variable that varies along the component in a forwards marching direction, for example corresponding to the root to tip direction of the aerofoil; selecting a first frontal node on the datum line; using a Pareto front algorithm to define the datum line at an extremity of the surface mesh with respect to the X direction, the Pareto front algorithm evaluating a criterion for dominated nodes and operating on a Pareto variable to select successive connected frontal nodes that form the datum line.