One idea for the observation above is that, having one’s iPhone in a pocket, some GPS satellite signals need to travel through the body which has a different speed-of-light than space or air. This body hypothesis is easily tested: when going back on a trail, switch the side the iPhone is on to keep the GPS satellite locations the same with respect to the body and the phone. The gap should be less when using the opposite pocket comparing to keeping the phone in the same pocket. Using data from a short (415 m) out-an-back on a narrow trail gave the result show in Figure 5.
The tracks were recorded using a Fitbit app, and the gaps were calculated using geodetic distances (WGS 84 model). The effect is clearly visible in the graphs; and statistically highly significant (p < 10-20). Also useful to note is that the single-frequency GPS is often said to have an accuracy of 30 meters, which is shown here is not random noise: gap distances indicate that noise levels are much, much less.
My curiosity pertained to the traces recorded while hiking, but to fully understand the behavior of the iPhone GPS, repeated experiments that are controlled, for example, for time of day because the GPS satellites orbit every 12 hours, exact locations (such as USGS markers or survey data), and iPhone position and orientation. One can also think about variations between iPhones, tracing apps, and even GPS hardware. The tools I could develop quickly in IDL, such as the file conversions, geodetic distance calculation, and trace matching can easily be applied to the analysis of any such experiment. The results could be used to improve the tracing apps and real-time locations, for example to give more timely warnings that one has strayed from an intended path.