The present invention is related to location positioning systems, and more particularly, to a method and apparatus for making accuracy improvements to a GPS receiver's navigation calculations. According to a first aspect, the invention includes maintaining a table of height attributes for major cities and urban areas around the world (contour table) in the GNSS chipset. The information in the table preferably includes latitude, longitude of the city along with height attributes of those cities, such as the average, min, max height and region boundary etc. Additional information such as average gradient could also be saved in the table. According to further aspects, when GPS signals are degraded in environments such as urban canyons, the height information can be obtained from the table and used to improve the navigation solution.