� The use of GPS data and a high-resolution geoid model (Geoid99) to estimate accurate GPS-derived orthometric heights will be directly associated with the implementation of NAVD 88.  It is important that users initiate a program to convert their products to NAVD 88.  The conversion process is not a difficult one, but will require time and resources.  There will be several different conversion techniques available.  The technique used will depend on the accuracy requirement of the user, I.e., procedures developed for conversion of less accurate GIS/LIS products will be different than procedures developed for conversion of USGS NGVD 29 published height values.