From what I've read, and experienced in the field, GPS signals can be blocked by both physical objects (buildings, hilltops, really dense canopy) and certain types of magnetic fields (think transformers on a telephone pole). This may cause the receiver to place points that appear to "jump" or otherwise not be where they're supposed to be. I've had this happen while collecting GPS data both in a vehicle and on foot.
Usually, there are not that many, and I just edit them out using Sketch Properties to select the points I don't want (shift-click in the list to select a range of points).
If the track or collection of points is much larger, and significant numbers of errors are recorded, I have been known to go back outside and recollect the GPS data. Even doing that, there are still little "blips" or "goofs" that may require manual cleanup in an edit session.
This is for your garden-variety, consumer Garmin or Magellan type GPS receivers. If you have target areas that require more reliable acquisition of GPS data, you may wish to look in to the more capable (and expensive) Trimble or Leica, or other, brands of GPS equipment. Some have software where you can set a sensitivity or tolerance level that helps keep the wayward points down to a minimum. Also, there may be post-processing software that can further reduce the amount of unwanted points. Maybe.
I've also had to resort to guesstimating a portion of a GPS track by editing it while referencing an aerial photo. Some spots, deep canyons, downtown in a big city, way down inside a heavily vegetated drainage, simply won't get an acceptable GPS signal no matter who's hardware your using. Some times one simply has to draw in the blank spots, as much of a pain as that may be.
Hopefully, some of this is helpful.
Gregory
