GEE计算山体阴影

使用高程数据来计算山体阴影

主要功能

使用高程数据来计算不同角度的山体阴影，并且在map中添加为多个图层展示。

代码

// 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');
}

步骤分析

1. 定义函数radians(img)实现角度弧度转换
2. 定义函数hillshade(az, ze, slope, aspect)来计算山体阴影
3. 创建ee对象，通过ee.Alogrithms.Terrain来计算坡度坡向等地形度量指标
4. ee对象特定图层提取
5. 地图对象设置显示中心，缩放等级
6. 按照角度循环计算山体阴影，添加计算的山体阴影图层，设置显示参数

主要方法

1. img.toFloat()
   Casts the input value to a 32-bit float.
   Arguments:
   this:value (Image):
   The image to which the operation is applied.
   Returns: Image

将输入影像投影为32bit浮点型。
本例中，转换为浮点型之后，使用了Math.PI和除法divide()实现角度弧度转换函数radians()

2. 山体阴影计算
   hillshade(az, ze, slope, aspect)

• 方位角计算az
  将输入的方位角由角度转化为弧度值

• 高度角计算ze
  将输入的高度角由角度转化为弧度值

• 山体阴影计算
  Hillshade = cos(Azimuth - Aspect) * sin(Slope) * sin(Zenith) + cos(Zenith) * cos(Slope)

3. 按照60°为步长，计算从0到360度的不同方位角的山体阴影

• for 循环实现对i的计算山体阴影时输入方位角的控制
• addLayer添加图层，使用i+'deg'来对图层进行命名