Google Earth Engine – Resolve ‘Parameter Element is Required’ Error

demgoogle-earth-enginejavascriptmodis

In the below code I want to put elevation average values in to the list but returns error:

enter image description here

code link: https://code.earthengine.google.com/6337849ec707ae2e229ba4bdb02e5fff

var startDate = '2000-01-01';
var endDate = '2020-01-01';

 
var maxBound = ee.Image("JRC/GSW1_2/GlobalSurfaceWater")
.select('max_extent').clip(geometry)
.gt(0);
 

var dem = ee.ImageCollection("JAXA/ALOS/AW3D30/V3_2")
.median()
.clip(geometry)
.select('DSM')
.rename('DEM');
 
 
 
var NDWI = function(img){
  var bands = img.select('sur_refl_b04','sur_refl_b02').multiply(0.0001).clip(geometry);
  var index = bands.normalizedDifference(['sur_refl_b04','sur_refl_b02']).rename('NDWI')
  return index.addBands(dem)
  .copyProperties(img,['system:time_start','system:time_end']);
};

var modisNDWI = ee.ImageCollection("MODIS/006/MOD09A1")
.filterDate(startDate,endDate)
.filterBounds(geometry)
.map(NDWI);


print(modisNDWI)


var calcArea = function(img,list){
  var thr = img.select('NDWI').gt(0.5).and(maxBound.eq(1));
  var mask = thr.updateMask(thr);
  var area = mask.multiply(ee.Image.pixelArea());
  var totalarea = area.reduceRegion(ee.Reducer.sum(), geometry);
  
  var boundary = mask.reduceToVectors({
    geometry: geometry,
    scale: 250,
    geometryType: 'polygon',
    eightConnected: false,
    bestEffort: true
  });
  
  var demClip = img.select('DEM').clip(boundary);
  var eleAve = demClip.reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: boundary,
    scale: 250
  })
    
    
  return ee.List(list).add(eleAve.get('DEM'));};

var first = ee.List([]);
var AREA = ee.List(modisNDWI.iterate(calcArea, first));
print('elevation average:',AREA);

Best Answer

It's probably a better idea to map the function, rather than using iterate. The former runs parallel and is much faster. Also, as your 'geometry' is a geometry object, there's no need to use .reduceToVectors, which can be computationally expensive.

To calculate the elevation of each image in the collection:

var calcMeanElev = function(img){
  var eleAve = img.select("DEM").reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: geometry,
    scale: 250
  })
    
  return img.set('mean_elev', eleAve.get('DEM'))};

var area = modisNDWI.map(calcMeanElev)

In the same function, you can add additional statistics to the image using .set().

In case you want to calculate the elevation of just unmasked pixels (in your script you're not, but maybe you want to?), you can use:

var calcMeanElev = function(img){
  var eleAve = img.updateMask(thr).select("DEM").reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: geometry,
    scale: 250
  })
    
  return img.set('mean_elev', eleAve.get('DEM'))};

var area = modisNDWI.map(calcMeanElev)

To return a list of all calculated mean elevation values use .aggregate_array():

var listOfElevationMeans = area.aggregate_array('mean_elev')
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