1.The reception of GPS signals can be affected by objects overhead (e.g., buildings, tunnels, bridges, or trees) and by the weather. Also note that GPS signals cannot pass through metalized window films.
2.Professional Navigation uses high class GPS chips and a larger antenna to improve the navigation performance.Smartphones, on the other hand, improve their positioning accuracy with other wireless communication (4G LTE, Wi-Fi assistive)
They are able to refer to data from its built-in map for more accurate positioning.
3.The elevation information displayed on road signs is measured with professional equipments and/or techniques, such as Triangulation or Pressure instruments. Hence the results will be slightly different from those measured by GPS.