Why does my GPS position jump around a lot when I operate my GPS indoors? (or in mountains, or in city canyons, or under tree cover?)
Why does my GPS speed maximum sometimes show 600 miles and hour (or other speed) when I had been going nowhere near that fast?
Why does my tracklog read shorter (or longer) than I know the surveyed trail distance to be?
1) No GPS receiver is designed to be used indoors at any time and
any operation you get is gratituous and likely to have large errors.
2) Multipath results when the direct path to your receiver is blocked (by your body, your house, roof, trees, mountains, buildings, etc) and the signal from the satellite is REFLECTED by some object. The reflecting surface may be: buildings, mountains, the ground, or any object that happens to be a radio reflector at 1.6Ghz.
3) Multipath are radio signals which have traveled FURTHER to get to your receiver than they should have. This can result in your GPS miscalculating its position because the signals may have traveled from feet to miles further to get to you than a direct line of sight signal path would have been.
4) Multipath can cause longer term "stable" errors or it can cause your position to wander at varying rates (even thousands of miles per hour if your GPS could follow such speeds). Sometimes GPS wanderings caused by multipath can cause your GPS to "jump" from one position to another as the multipath signal "comes and goes" and causes your GPS to jump from using one group of erroneous signals to another. These "jumps" can add substantial distances to the tracklog measurements in some GPS receivers.
A related problem with the tracklog occurs when you are in a difficult terrain (tree cover, mountains, etc.). There, you my momentarily lose GPS lock but most GPS receivers do not notify you of the lost lock for perhaps 30 seconds. But during this interval, most hiking GPS receivers stop recording movement until the GPS lock returns. Thus, you may end up with a shorter GPS measured tracklog distance than you know the path or trail to be.
Knowing your exact physical situation wherein you had the "large spikes" in position or speed is of course impossible. This because the satellites are constantly in motion and are in different configurations in the sky from moment to moment.
However, this can be said in general:
If your GPS does not have a clear sky view** then multipath errors are possible, even likely. These multipath errors can often cause both position and speed "spikes" on any consumer GPS receiver. EVEN IF you have a clear sky view, multipath is possible, but the possibility of multipath errors are at least minimized with a clear sky view since it is probable that the GPS will have more than the minimum 4 satellites in view and can perhaps throw out the "bad" measurements. NOTHING here should be construed to suggest that a "clear sky view" will always eliminate multipath or position or speed excursions under all circumstances. These multipath errors are just the nature of the technology at this point.
** A clear sky view means that the GPS can see in roughly a hemisphere of clear sky. That is: Your GPS antenna is unobstructed in ALL directions. For these reasons, an amplified external GPS antenna accessory may be desirable in a particular situation. Putting your GPS (or your GPS antenna) on your hat is a proven method to IMPROVE performance in difficult areas. But! Difficult areas are by definition difficult. You will not be disappointed if you expect to have large excursions in speed/distance/tracklog measurements in situations where multipath exists.