Think this may clear some things up. Courtesy of another forum member here, Boyd.
From the National Park Services website:http://mms.nps.gov/gis/gps/gps4gis/how2 ... S-WASO.doc
If you are using a WAAS (Wide Area Augmentation System) enabled receiver in an environment where you are getting intermittent signal from the WAAS satellite then the WAAS option should be disabled. Examples would be a forested, urban or canyon environment. If the WAAS option is left enabled in this type of operating environments then you very likely will get incomplete differential information from the satellite resulting in a position that has a much worse accuracy than an autonomous GPS position (no differential corrections). This is because the Garmin receivers will produce a differential position first even if it is a 2D position before it will give you a lower DOP (Dilution of Precision) position.
Quote:The estimated positional error (EPE) is really HPE (horizontal positional error). On the new Garmins (GPSMap 76, 76s’s and V’s, etc.) the EPE is approximately at a 95% (or 2 sigma)
confidence level when you are not using differential corrections (WAAS or NDGPS) this drops to a 68% (or 1 sigma) confidence level when you are using differential corrections.