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How a map works, Mercator, tiles, and your GPS pin
从经纬度到屏幕像素：一张地图如何在后台被切割成瓦片，并通过墨卡托投影定位你的GPS点。
You drag a map, it loads more map. You drop a pin, it lands exactly where you tapped. This feels like one seamless thing, but underneath, every "slipp…