A system was developed for the delineation of distance intervals in front of an observer during seabird surveys. Delineation is based on a coordinates system (x.y) with the vanishing point on the horizon being the origin (0:0), within a vertical plane in front of the observer. Coordinates of parallel interval limits can be calculated using y = H x/d_p (H = height of the observer's eyes; d_p = perpendicular distance between transect centerline and interval limit), and coordinates of radial interval limits using (w_p = distance between the observer and the vertical plane in direction of interval limit; d_c = observer's distance to the interval limit). The use of the delineation system will help reduce observer-induced variation in seabird at-sea surveys.