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Question: error in the position in the map in a garmin 3206

I have a Garmin 3206 GPS Chartplotter sounder. At dock the position of the boat in the map is OK, but after sailing to a far location ( 30 Nm or more) the boat position appears to be wrong. In some cases the boat appears to be over ground. How can I correct this?
Nelson. nelsonviajes@gmail.com