2-D vs. 3-D Fixes Article by Ed Weston.
GPS Receiver Accuracy Article by C.J. Hoare, GlobeStar Positioning Services Inc.
Obtaining and Using Vector Maps with OziExplorer/OziMC Article by John Galvin (includes converting the data into maps uploadable to Lowrance receivers).
Need to really accurately convert position to various Datums ? See: Horizontal Time Dependent Positioning