Pipeline following sensor arrangement

A method comprising:

• placing an electromagnetic signal on an underground line;
• translating a mobile frame comprising an attached outrigger frame on the ground such that the outrigger frame is overlying a length of the underground line;
• while the mobile frame is translating on the ground, detecting the signal, simultaneously, at three locations on the outrigger frame, wherein the three locations are disposed along a line;
• while the mobile frame is translating on the ground, detecting, with an orientation sensor, the angle of the line on which the three locations are disposed relative to a level plane as the mobile frame translates on the ground;
• using each of the detected signals along with the detected level and location, determining an estimated position of the underground line;
• from the estimated position, determining a predicted location of the underground line and an estimated error value in the predicted location.