The GPS is accurate to at least 20 feet with a clear view of the sky. The problem is the underlying maps. The woman who lost her son in Death Valley was led astray by a poor choice of maps in her GPS. Maps of remote areas may be from government sources like the 7.5' USGS topos. These maps show century old mining trails as dirt roads. Naturally, her vehicle became stuck on one of these "roads". Also, there have been two cases of people being guided onto unmaintained, dirt, logging roads in winter in Oregon.
