What is the difference between ambient pressure and barometric presure when using and plotting with a gps?
Your question is really puzzling me because, to my knowledge, no GPS shows anything about atmospheric pressure. I’ll just give you some notions common to pilots and let you pick what you want if you find it interesting. A GPS calculates your position by doing spherical trigonometry. Because the earth is not a perfect sphere, this is first done to an approximation ellipsoid. But even so, due to variations in the mass of the earth, even the oceans are not at the same level everywhere. Because of that, a difference between the ellipsoid and the actual sea level is calculated as a table and used in the GPS. It is called the geoidal height. Since the mean sea level (MSL) is our reference to draw maps and calculate elevations, a barometer can be used but then, it must be calibrated. Aircraft find their altitude from an altimeter that is simply a barometer. But, prior to departure, it must be adjusted to show the correct pressure at that moment and place. This is called the QNH, i.e. the pres
