Obstructions can be one of the main impediments to GPS surveys. In navigation an obstruction may be gone before it becomes a problem but for high accuracy surveys needing a block of time in one place, some planning will have to be done so that no point is surveyed without a clear view of the minimum of 5 satellites. Some areas which frequently cause problems are river valleys, where terrain cuts off satellites near to the horizon, or in mountainous regions where the same effect occurs.
Forests can produce a problem but the magnitude of the problem is dependent upon the type of forest.
A rain forest tends to have larger, wetter leaves which completely block GPS signals. A dry pine forest on the other hand can be quite surveyable. The thickness of the pine needles does not present much of a block for signals.