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How big is the error?
Two things come to mind:
1.) Make sure your GPS is set to use the WGS84 geoid.
This is what MapPoint uses, as well as the satellites in the GPS network. However, different geoids have their uses, and some GPS devices will report their coordinates using different geoids by default. Most have options so that you can change the geoid.
Geoid-related errors of 100m or so are easily possible. They will appear to be systematic (similar amount and direction of error, when operating in the same regional area).
2.) Consumer GPS is generally less accurate than most people assume. This might be your problem if the error varies, the reported position over-shoots when you stop or turn, that type of thing.
Errors are typically 10m or so.
They are worse if the receiver does not have a clear view of the sky - eg. hills, buildings, etc. or the position of the receiver/antennae in the car.
Richard |