Turn your iPhone into an Altimeter. Key Features: - Accuracy Display - Current Position on Map - Easy to Use Please note: Walls, vehicle roofs, tall buildings and other obstructions can block line of sight to GPS satellites. To improve the accuracy, make sure that you maintain a clear view of the horizon in several directions. GPS accuracy varies depending on the number of visible GPS satellites. Locating all visible satellites can take several minutes, with accuracy gradually increasing over time.