Hfrhyu

I am trying to calculate the area of watershed within each polygon for a set of square polygons (1km*1km) in my study area. I converted a water image to a featureCollection of water polygons, it works great. I get an error when I try to intersect the square polygon layer to the water featureCollection. I also would like to export the result to a csv.

//my study area

var studyArea = ee.FeatureCollection('users/oamram/ABM_Kenya/StudyArea');

//square polygons 1km in dimensions

var polygons = ee.FeatureCollection('users/oamram/ABM_Kenya/Polygons1km');


Map.centerObject(studyArea,13);




var start = '2014-01-01';

var finish = '2014-01-31';

//get water dataset

var dataset = ee.ImageCollection('JRC/GSW1_0/MonthlyHistory')
 .filter(ee.Filter.date(start, finish));

 //select water layer

 var water = dataset.select('water');

//identify water area

 var sumWater = water.reduce(ee.Reducer.sum());

//clip to my study area

 var clipped = sumWater.clip(studyArea);



var waterVis = 
 min: 0.0,
 max: 2.0,
 palette: ['ffffff', 'fffcb8', '0905ff'],
;

//Apply watermask- water values =2 in this layer//;

var maskedComposite = clipped.updateMask(clipped.eq(2));



// Convert the water areas to vectors.

var vectors = maskedComposite.reduceToVectors(
 geometry: studyArea,
 crs: maskedComposite.projection(),
 scale: 30,
 geometryType: 'polygon',
 eightConnected: false,
 labelProperty: 'water'
);


**//everything works up to here**


// Compute the intersection, for each feature in the feature collection.

var polyIntersect = polygons.map(function(feature)
 var intersection = vectors.intersection(feature, ee.ErrorMargin(1));

 return feature.set('Intersect': intersection.area().divide(1000 * 1000));
);

The error is get is this:

vectors.intersection is not a function

vectors is a FeatureCollection and hasn't got an intersection method. That method is for the class Feature. This is what I'd do:

// cast vectors
vectors = ee.FeatureCollection(vectors)

// make vectors a list
var vectorList = vectors.toList(vectors.size())

var polyIntersect = polygons.iterate(function(feature, list)
 list = ee.List(list)
 feature = ee.Feature(feature)

 var intersection = vectorList.map(function(feat) 
 feat = ee.Feature(feat)
 var intersection = feat.intersection(feature, ee.ErrorMargin(1));
 return ee.Feature(intersection).set('Intersect': intersection.area().divide(1000 * 1000))
 )

 return list.add(intersection)
, ee.List([]));

polyIntersect = ee.FeatureCollection(ee.List(polyIntersect).flatten())
Map.addLayer(polyIntersect)

link: https://code.earthengine.google.com/cef9d625cce4f0078c258e38ceff46f6