That answer, like many things RF related, depends on number of factors.
For example:
-Are your particular lines HVDC or KV AC lines?
-Copper core steel, fiber core steel, or steel core?
-Powerline Ethernet present on the lines?
-Are you near a crossover point?
-Are you near a switching point?
-What is the distance between phase carrying lines? Phase to ground line distance ?
Each one of these can affect the local RF environment. Personally I'd be more concerned with the effect on the compass. GPS' packet and topology is specifically designed to reject common error sources. Assuming a quality receiver of course.