There are two types of coordinate representations: 2D and 3D. When GPS can receive signals from four or more satellites, it can calculate the local 3D coordinates: longitude, latitude, and altitude. If it can only receive signals from three satellites, it can only calculate the 2D coordinates: accuracy and latitude. At this time, it may also display altitude data, but this data is invalid. Most GPS systems can not only display coordinates in Lat/Long format, but also use coordinate systems such as UTM (Universal Transverse Mercator) to display coordinates. However, we generally use the LAT/LONG system, which is mainly determined by the coordinate system of the map you are using. When Selective Availability is turned on, the horizontal accuracy of GPS is between 100-50 meters, depending on the amount and strength of satellite signals received. If you have arrived according to GPS instructions, take a look around and you should find your target within an area about the size of a football field.

　　When SA is turned off, the accuracy can reach about 15 meters. The high accuracy is worse due to the system structure. The display method of longitude and latitude can generally be chosen according to one's own preferences, including "hddd. dddd", "hddd * mm. mm", "hddd * mm" ss. s "(where" * "represents" degree ", the same below). The length of the Earth's meridian is 39940.67 kilometers, with a latitude change of 110.94 kilometers, a minute of 1.849 kilometers, and a second of 30.8 meters. The equatorial circle is 40075.36 kilometers, and the latitude in Beijing is around 40 degrees north latitude, with a latitude circle length of 40075 * sin (90-40), At one point, the longitude of this place was 276 kilometers, with one minute being 1.42 kilometers and one second being 23.69 meters. You can choose a certain display method and change the numbers to correspond to how many meters the ground has moved. This way, you can establish an approximate correspondence between the longitude and latitude and the actual mileage. Most GPS systems have the function of calculating the distance between two points, which can provide accurate distance between two coordinates. There are two ways to display height: English and metric. Go to the GPS SETUP page and set it to metric, so that other displays such as speed and distance will also be in metric.

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