Originally Posted by wphamilton

It's not a problem there, because the position errors will tend to average out over the longer term. It's only the instantaneous speed readings that will really have a problem.

Trees and even atmospheric conditions will throw a given reading off. Even then, even with everything perfect, without augmentation the civilian gps-only device is accurate to at best 3 meters, worst case 8 meters. Which means we simply can't expect a phone GPS to provide an accurate instantaneous speed reading but it can be can closer if the software is sophisticated enough.