30 is illustrated in greater detail in fig2 in the preferred form of the invention the beacon is positioned and oriented by land surveying techniques at a selected known location with respect to the planned path 20 and exit location 22 the beacon may consist of a turntable 50 upon which a solenoid 52 is mounted the turntable is mounted on a base 54 to rotate about an axis 56 made vertical by adjusting the lengths of base support legs 58 60 62 to thereby make the solenoid 52 horizontal at all azimuthal orientations as the turntable rotates any convenient method such as the use of a spirit level may be utilized for this purpose the turntable can be set in two orientations perpendicular to each other by a pair of pins 64 and 66 at diametrically opposite locations on the turntable that fit into two pairs of holes 68 and 70 in an orientation ring 72 mounted on base 54 this ring can be rotated about the turntable axis 56 and clamped at any orientation by clamps 74 and 76 after the base 54 is leveled the orientation of ring 72 is set by loosening clamping screws 78 and 80 on clamps 74 and 76 rotating the solenoid 52 while sighting along its axis 82 to make it point toward a surveyed reference location such as location 84 in fig1 and tightening the clamp screws the axes and the location of the beacon can thus be.