I use a Garmin Edge 205 to record tracks from the various surveys that I have done. From trial and error I have learned that even small amounts of interference can reduce the ability of the gps to pinpoint its location. For instance, if its sitting by itself on the sidewalk it can generally zoom in to about +-4m. If its in my hand or I am looming over it, the accuracy drops to +-7m.

However, when the unit is sitting in my car on the passenger seat it can really struggle to accurately pinpoint itself.

Recently I tried mounting gps to the roof of my vehicle and have been rewarded with increased accuracy in my tracks. Tests out in the prairies (no large buildings, trees or stuff like it for miles and miles) have shown highly similar results for intentionally plotted routes. Bringing the GPS inside the car reduces the accuracy markedly.

That said, for most people without windshields that reflect satellite signals, the gps could probably be best placed on the dash.

That is all.