This proposal explains procedures of using regional and local geoid undulations to improve and convert the global positioning system (GPS) elevations (ellipsoidal heights) into orthometric heights. The Geoid Undulation Model (Geoid 2003) for the North America has reached the centimeter level accuracy. Although the GPS accuracy has reached to millimeter level, the elevation component (converted to orthometric height) has not been optimized to the same level of accuracy as X and Y. This research will select a test site in North Georgia covering a 2 x 2 of hilly as well as plain area. The test site will also include an urban area such as the City of Atlanta, and a large water body such as the Lake Lanier. The 2 x 2 area is divided into 1 x 1, 5' x 5', and 1' x 1' grid elements to compute global, regional, and local geoid undulations. Gravity data will be observed and compared against the gravity data obtained from the GRACE program. Also, the accuracy of existing geoid undulation in Georgia will be compared with the developed geoid undulation model, which will incorporate local and regional level gravity anomalies.