Garmin Engineering gives us some answers:
Let's start with the "measured LESS than the actual distance side of the issue.
- If you are traveling less than about 2.2 mph (3.5KPH), and you
have an "SA Era" unit such as the G-12, G-48, G-II, G-III and others of
similar age, then your GPS will likely not record the
distance traveled into the log. This is because below that speed, the
normal perturbations of the satellite position coupled with the
reception of multipath signals would likely have added enough erroneous
distance to cause a significant distance traveled OVER
measurement. A different algorithm is used in units produced
AFTER the SA era, but even in these, you can lose some
distance if you walk very slowly.
- Another item to consider is the Power save mode. This can cause a shortcutting of corners that will also result in a shorter distance logged.
Now let's discuss the "measured MORE than the actual distance traveled" side of the question.
- These cases are more rare. Logging LONGER distances traveled can
occur due to multipath. This is especially true where a) you have
buildings, mountains, trees or other objects, shadowing one or more of
the satellite signals. This can result in reflected signals
periodically traveling further than the direct signal and causing your
GPS to think it has moved (sometimes a thousand feet or more) when
actually the motion was much less. This would be much more likely in
cases where the GPS had only 3 or 4 satellites locked.
- Another possibility depending upon the Users speed is how the unit will continue to "dead reckon" when reception is lost. The unit will assume that you are continuing on the same heading at the same speed until reception is regained or 30 seconds has elapsed. At this point the unit states reception is lost. However,
the user's travel distance may not have been so far if the user slowed
down during the dead reckoning interval. Still, the distance that
is "traveled during this dead reckoning" is also added tothe trip distance.