If I do a relatively long East/West leg (yesterday was Biggin to Exeter, for example) if find that the line on SD is significantly displaced from that on the plumbed in GPSs (530/430/Pilot IIIc) to the extent that there is probably over a mile between them at the mid point. Thus, one can have a significant fly right on one and fly left on the other.

Tim, I quite understand your reasoning. But you could show the course to steer at the current location. That course would update as you fly along, naturally. But at any point in time, when you view the PLOG, you'd see the appropriate course at this time.

-- Chris.