Google Earth Engine（GEE）——區域面積統計pixelArea()

ee.Image.pixelArea()

生成一個影像值，影像值是計算像素的平方米，傳回一個單波段影像"area."

Generate an image in which the value of each pixel is the area of that pixel in square meters. The returned image has a single band called "area."

No arguments.

Returns: Image

很多時候我們需要計算區域統計面積，内置函數ee.Image.pixelArea()十分簡單友善，通過将單個像素的面積乘以構成對象的像素數（由 确定

connectedPixelCount()

）來計算對象面積。像素區域由生成的圖像提供

ee.Image.pixelArea()

。

 代碼：

// Make an area of interest geometry centered on San Francisco.
var point = ee.Geometry.Point(-122.1899, 37.5010);
var aoi = point.buffer(10000);

// Import a Landsat 8 image, subset the thermal band, and clip to the
// area of interest.
var kelvin = ee.Image('LANDSAT/LC08/C01/T1_TOA/LC08_044034_20140318')
  .select(['B10'], ['kelvin'])
  .clip(aoi);

// Display the thermal band.
Map.centerObject(point, 13);
Map.addLayer(kelvin, {min: 288, max: 305}, 'Kelvin');

// Threshold the thermal band to set hot pixels as value 1, mask all else.
var hotspots = kelvin.gt(303)
  .selfMask()
  .rename('hotspots');

// Display the thermal hotspots on the Map.
Map.addLayer(hotspots, {palette: 'FF0000'}, 'Hotspots');
// Uniquely label the hotspot image objects.
var objectId = hotspots.connectedComponents({
  connectedness: ee.Kernel.plus(1),
  maxSize: 128
});

// Display the uniquely ID'ed objects to the Map.
Map.addLayer(objectId.randomVisualizer(), null, 'Objects');

// Compute the number of pixels in each object defined by the "labels" band.
var objectSize = objectId.select('labels')
  .connectedPixelCount({
    maxSize: 128, eightConnected: false
  });

// Display object pixel count to the Map.
Map.addLayer(objectSize, null, 'Object n pixels');
// Get a pixel area image.
var pixelArea = ee.Image.pixelArea();
print("pixelArea",pixelArea)
//.
var objectArea = objectSize.multiply(pixelArea);
print("objectArea",objectArea)
// Display object area to the Map.
Map.addLayer(objectArea,
             {min: 0, max: 30000, palette: ['0000FF', 'FF00FF']},
             'Object area m^2');