GPS coordinate

　　There are 2D and 3D coordinates. When GPS can receive signals from 4 or more satellites, it can calculate 3 local micro coordinates: longitude, latitude and altitude. If it can only receive signals from 3 satellites, it can only calculate 2D coordinates: precision and latitude. At this time, it may also display altitude data, but this data is invalid. Most GPS can display coordinates not only in Lat/Long mode, but also in UTM (Universal Transverse Mercator) and other coordinate systems. But we generally use LAT/LONG system, which is mainly determined by the coordinate system of the map you use. When the Selective Availability (a measure implemented by the U.S. Department of Defense to reduce GPS accuracy) is turned on, the horizontal accuracy of GPS is 100-50 meters, depending on the number and strength of satellite signals received. If you have arrived according to the GPS instructions, look around and you should find your target in an area about the size of a football field.

　　When SA is turned off, the accuracy can reach about 15m. The high accuracy is worse due to the system structure. The display mode of longitude and latitude can generally be selected according to your own preferences. Generally, there are "hddd. ddddd", "hddd * mm. mm", "hddd * mm" ss. s "" (in which "*" represents "degree", the same below). The length of the earth's meridian is 39940.67 km. The latitude changes once to 110.94 km, one minute to 1.849 km, one second to 30.8 m, and the equator circle is 40075.36 km. The latitude of Beijing is about 40 degrees north latitude, The length of the latitude circle is 40075 * sin (90-40). The longitude here once closed 276 kilometers, and one opening closed 1.42 kilometers, one second closed 23.69 meters. You can select a display mode, and change each number to correspond to how many meters the ground moves, so that you can establish a approximate correspondence between longitude and latitude and the actual mileage. Most GPS have the function of calculating the distance between two points, which can give the accurate distance between two coordinates. The height will be displayed in both English and metric systems. Enter the SETUP page of GPS and set it to metric system, so that other displays such as speed and distance will also be metric system.

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