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