Straight line of travel means you do not turn left or right. Different from heading continuously east. In the former, walking straight on the curved surface cre

Straight line of travel means you do not turn left or right. Different from heading continuously east. In the former, walking straight on the curved surface creates a curved path. In the latter, the compass needle will shift a bit as you travel, so you will be turning a bit to stay on the due easterly course. To your point, this a straight line relative to the traveller while staying on the surface of the globe (which is the premise).