前言：主要介绍利用GEE获取Modis下的GPP数据。

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1.GPP

• 总初级生产力（Gross primary productivity，GPP）是指单位时间内生物（主要是绿色植物）通过光合作用所固定的有机碳量，又称总第一性生产。
• 本博客获取的Modis_GPP数据主要是通过遥感的方法来计算的GPP。

2.MOD系列和MYD系列区别

• Terra卫星的观测数据为MOD数据，Terra是于当地的上午10:30过境。
• aqua卫星观测数据为MYD数据，auqa是于当地下午1:30过境。
• A3是NPP数据，每一年进行合成。

3.MOD17A2H(500m/8天)

var geometry = ee.FeatureCollection('users/www1573979951/luyixian');
Map.centerObject(geometry,6);
var dataset = ee.ImageCollection('MODIS/006/MOD17A2H').filter(ee.Filter.date('2014-01-01', '2014-12-31'));
print(dataset)
var npp = dataset.select('Gpp');
var nppVis = {
  min: 0.0,
  max: 19000.0,
  palette: ['bbe029', '0a9501', '074b03'],
};
Map.addLayer(npp.mean().clip(geometry), nppVis, 'NPP');

4.MYD17A2H(500m/8天)

4.1 MYD17A2H下的指数

主要有Gpp、PsnNet、Psn_QC指数

var geometry = ee.FeatureCollection('users/www1573979951/luyixian');
Map.centerObject(geometry,6);
var dataset = ee.ImageCollection("MODIS/006/MYD17A2H")
.filter(ee.Filter.date('2010-01-01', '2010-12-31'))
.filterBounds(geometry);
print(dataset);

4.2 遥感影像查看

var geometry = ee.FeatureCollection('users/www1573979951/luyixian');
Map.centerObject(geometry,6);
var dataset = ee.ImageCollection("MODIS/006/MYD17A2H")
.filter(ee.Filter.date('2010-01-01', '2010-12-31'))
.filterBounds(geometry).select('Gpp');
print(dataset);
var gppVis = {min: 0.0, max: 600.0, palette: ['bbe029', '0a9501', '074b03']};
Map.addLayer(dataset.mean().clip(geometry), gppVis, 'Gpp');//2010年的均值

5.GPP日数据下载(以MYD17A2H为例)

var geometry = ee.FeatureCollection('users/www1573979951/luyixian');
Map.centerObject(geometry,6);
var dataset = ee.ImageCollection("MODIS/006/MYD17A2H")
.filter(ee.Filter.date('2010-01-01', '2010-12-31'))
.filterBounds(geometry).select('Gpp');
print(dataset);
function exportImageCollection(imgCol) {
  var indexList = imgCol.reduceColumns(ee.Reducer.toList(), ["system:index"]).get("list");
  indexList.evaluate(function(indexs) {
    for (var i=0; i<indexs.length; i++) {
      var image = imgCol.filter(ee.Filter.eq("system:index", indexs[i])).first();
      image = image.clip(geometry);
      //tif数据下载
      Export.image.toDrive({
        image: image,
        description: 'modis_gpp_'+indexs[i],
        fileNamePrefix: 'modis_gpp_'+indexs[i],
        folder: 'Modis',
        region: geometry,
        scale: 500,
        crs: "EPSG:4326",
        maxPixels: 1e13
      });
    }
  });
}
exportImageCollection(dataset);

6.GPP月数据下载(以MYD17A2H为例)

var geometry = ee.FeatureCollection('users/www1573979951/luyixian');
var dataset = ee.ImageCollection("MODIS/006/MYD17A2H");
for(var i=2018;i<=2020;i++){
  for(var j=1;j<=12;j++){
    var data_collection = null;
    switch(j){
      case 1:
      case 3:
      case 5:
      case 7:
      case 8:
      case 10:
      case 12:
        data_collection = dataset.filterDate(i+'-'+j+'-01',i+'-'+j+'-31').select('Gpp');
        break;
      case 4:
      case 6:
      case 9:
      case 11:
        data_collection = dataset.filterDate(i+'-'+j+'-01',i+'-'+j+'-30').select('Gpp');
        break;
      case 2:
        data_collection = dataset.filterDate(i+'-'+j+'-01',i+'-'+j+'-28').select('Gpp');
        break;
    }
    var YR_collection = data_collection.mean().clip(geometry);
    Export.image.toDrive({
      image: YR_collection,
      description: i+'-'+j,
      fileNamePrefix: i+'-'+j,
      scale: 500,
      region: geometry,
      maxPixels: 1e13,
      folder: 'Modis'
    })
  }
}

7.GPP年数据下载(以MYD17A2H为例)

var geometry = ee.FeatureCollection('users/www1573979951/luyixian');
var dataset = ee.ImageCollection("MODIS/006/MYD17A2H");
for(var i=2018;i<=2020;i++){
  var data_collection = dataset.filterDate(i+'-01-01',i+'-12-31').select('Gpp');
  // print(data_collection)
  var YR_collection = data_collection.mean().clip(geometry);
  Export.image.toDrive({
    image: YR_collection,
    description: i,
    fileNamePrefix: i,
    scale: 500,
    region: geometry,
    maxPixels: 1e13,
    folder: 'Modis'
  });
}