A guidance system for a lateral move irrigation machine of the type having an elongate boom mounted on a plurality of motor driven towers which drive the boom along the field in a straight line path. A plurality of check points in the field are established by a series of magnetic and metallic elements placed in rows along the path of travel of the towers. The towers carry gradiometers or proximity switches which are sensitive to the elements and which deactivate the drive motors of the towers when the check points are reached. When all of the towers have reached the check points, the boom is properly aligned, and override circuits then activate the drive motors to resume movement of the machine.