给你一张半世纪前的卫星图，看看那时我们的城市
将日历翻回半个世纪前，在那个剑拔弩张的时代，美国曾使用侦察卫星探查、记录过中国的每一寸土地。时过境迁，战争的威胁减退，最初的一批卫星图像也逐渐「过时」，于是被分批解密作为科学资料公之于众。
如今我们可以在美国地质勘探局（USGS）的官网上下载到这几批卫星图像，这些拍摄于上世纪六、七十年代的照片意外地发挥了历史记录的功能，忠实地重现出我们父辈生活的土地。通过比对，我们能发现河流如何改道、城镇如何取代农田，数十年来的城市格局变迁，在卫星照片的对比之下一览无余。
想要下载包含某地的卫星图像，我们首先要注册美国地质勘探局官网账号并登录地球探索（Earth Explorer）页面。
在页面右侧的地图上，通过鼠标点击锚点划定想要搜索的区域，然后点击左下方的 Data Sets 进入下一步设置。
需要寻找的卫星图像一般在 Declassified Data 数据库里，勾选以后点击 Additional Criteria 进一步筛选，比如说限定高分辨率及可下载的图像。完成以后点击 Result 即可得出包含框选区域的高清卫星照片列表。
点击每张照片下方的「脚印」按钮，照片所包含的区域便会在右侧地图以半透明色块显示，尽量选择那些完全包含所选区域的照片作为自己的目标，然后点击第五个按钮就能下载到自己的电脑里了。
因为服务器限制，下载速度一般会很慢，动辄超过 1 GB 的图像可能要好几个小时。
下载完成以后解压会得到几张 .tif 格式的图片，因为照片中没有任何定位，城市面貌又大幅改变，因此建议以河流网的形态作为辅助定位自己想要找的区域。
比较往往会带来一些意外发现。比如说我比对了自己家乡五十年前的卫星图，才发现那条以河流命名的狭窄街道原来曾是隔壁小河的河道，随着河流的截弯取直才改为街道——也就不奇怪为什么只有双车道并导致今天的长期交通堵塞了。
你的家乡半个世纪来发生了什么变化？欢迎来评论区分享你的发现。
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