Mark, In GPS terms 300m is unacceptable and Garmin would be facing law suits all over the world if their GPS's were that inaccurate from boat owners running aground and hitting things, at that level of accuracy no one would purchase one.

My guess that there was a possible explanantion or a fault with the unit itself. Classic GPS inaccuracy in hand helds is from the aerial not being in the right position, ie shielded by an object or not being in the right vertical orientation.

The other option is of human error in reading the unit by abbreviating the minutes to only 1 or two digits.

Easy test for the unit though is put it on a fixed object and let it record for about 2 hours, it shouldn't deviate by more than about 10 metres horizontally and about 30 metres vertically. More than that and send it back to be repaired with the trace still in the unit. Its dead obvious when the unit is not doing what it should, the readings dither about all over the place, though the worst I have seen was about 100 metres.