GEE基础学习——Hillshade山阴的计算和显示

很多时候我们需要计算山阴主要通过ee.Terrain.hillshade()来实现，具体代码如下：

// Hillshade example.  This is a demonstration of computing
// a hillshade from terrain data and displaying multiple
// layers based on multiple view geometries.  Hillshade
// creation is also provided by ee.Terrain.hillshade().
 
// Define a function to convert from degrees to radians.
function radians(img) {
  return img.toFloat().multiply(Math.PI).divide(180);
}
 
// Define a function to compute a hillshade from terrain data
// for the given sun azimuth and elevation.
function hillshade(az, ze, slope, aspect) {
  // Convert angles to radians.
  var azimuth = radians(ee.Image(az));
  var zenith = radians(ee.Image(ze));
  // Note that methods on images are needed to do the computation.
  // i.e. JavaScript operators (e.g. +, -, /, *) do not work on images.
  // The following implements:
  // Hillshade = cos(Azimuth - Aspect) * sin(Slope) * sin(Zenith) +
  //     cos(Zenith) * cos(Slope)
  return azimuth.subtract(aspect).cos()
    .multiply(slope.sin())
    .multiply(zenith.sin())
    .add(
      zenith.cos().multiply(slope.cos()));
}
 
// Compute terrain meaasures from the SRTM DEM.
var terrain = ee.Algorithms.Terrain(ee.Image('CGIAR/SRTM90_V4'));
var slope = radians(terrain.select('slope'));
var aspect = radians(terrain.select('aspect'));
 
// For loops are needed for control-flow operations on client-side
// operations.  Here Map.addLayer() is a client operation that needs
// to be performed in a for loop.  In general, avoid for loops
// for any server-side operation.
Map.setCenter(-121.767, 46.852, 11);
for (var i = 0; i < 360; i += 60) {
  Map.addLayer(hillshade(i, 60, slope, aspect), {}, i + ' deg');
}

这个代码的具体思路是首先，及建立一个函数，完成有角度向弧度的转换！

其次就是定义一个新的函数，来计算太阳方位角和高程并通过返回值计算 Hillshade = cos(Azimuth - Aspect) * sin(Slope) * sin(Zenith) +cos(Zenith) * cos(Slope)，另外需要注意，我们算数的(+, -, /, *)是不能执行的，因为这里没有用到image.expression进行，也没有合适的波段去进行选择，所以要用英文字母的加减乘除。

剩余的地方基本上就是通过影像分别计算slope和aspect然后进行函数的hillshade的函数计算。

最后进行图像的中心位置显示。

此外本程序还设置了一个循环，也就是每隔60度进行一个方向的展示：