Coordintates seem to ALWAYS be off by 20-50 feet
How are you determining your error? If you are comparing your position to a map and you are getting error consistently, it could be that your map datum does not match the datum set in the GPS. Without WAAS turned on, your error should be @15'. With WAAS on it should be @ 6 to 9'. If you are using the Sattelite screen to determine your error, remember that is just estimated error. Higher estimated errors can be caused by satellites that are blocked or not visible to the GPS, or poor satellite geometry. It is normal for example, to have more error in high northern or southern latitudes due to less satellites being visible. With good reception you should be able to see 8 to 12 satellites on your satellite page. Signal strength is also important, You can have a lot of visible satellites but if the signal is weak your error wil increase. The signal can be weakened by heavy tree cover or high objects such as buildings or mountains. Such interference is called signal path error. Another form of signal path error is when the signal gets reflected off something before it gets to your GPS. For example; you're in a canyon and satellite 1 is to the east of you. The eastern rim of the canyon blocks the signal from satellite 1, however the signal bounces off the western rim and your GPS then picks it up. Because it took longer to get to your GPS, the GPS thinks that the satellite is farther away than it really is, causing a postion error. (This would not show up on your estimated position error shown on the satellite page because the GPS does not know this is happening.)
When WAAS is turned on it takes a channel that would normally be dedicated to a GPS satellite. It's possible that turning off WAAS helped you because having an extra GPS satellite improved your geometry which improved your accuracy more than the corrections WAAS is designed to make. The next time your error seems high, check how many satellites are being tracked and how strong the signal is.
Feb 13, 2008 |
Garmin eTrex Legend Cx GPS Receiver