I have some GPS points that are close to where they should be, but not nearly close enough (horizontal precision is around 10 meters). Looking at the positions which comprise these points (there are 10-15 positions per point), the positions are in a nearly straight line. Normally, I would expect to see a more or less round grouping of positions with the point somewhere in the middle, which I do see for most points in this dataset.
The points were collected on a 10-20 foot wide road through a forest, with handheld Trimble units (the antenna was in the handheld, not on a pole), and I was told that in most cases the points were taken by someone who had their back against a tree. The area is in the middle of a bayou, there are no nearby overhead obstructions or buildings nearby, overhead or otherwise. I've also differentially corrected the locations with base data from a nearby CORS base station.
One obvious explanation is that the person was actually walking and not standing still when taking these points, but I've been told that wasn't the case. My only other guess as to what's causing this pattern of positions is multipath distortion. I don't think these trees are big or dense enough for that, and the fact that the positions are in a nearly straight line seems too convenient.
So does this look like multipath distortion, or something else? The red point in this screenshot is the worst of the bunch, but there are others with the same problem. In this spot, I would have expected the red point to be pretty close to the southernmost (bottom-most in this screenshot) GPS position (the small black dots). As you can see, points 740, 736, and 737 have pretty tight clusters of positions, the blue points are ones with much higher precision. 738 and to a lesser extent 739 have a bit of the straight line problem but not as bad as the red point.