Perform Accuracy Assessment for two FeatureCollections

Question

I want to perform an binary Accuracy Assessment (ErrorMatrix) for two FeatureCollections containing lakes as polygons. One collection contains the results of a classification (needed to convert raster to vectors to calculate area size and to remove smaller objects) and the other one with validation data representing reality. A polygon represents a lake with landcover 1 as property. How can I merge both collections to have the specific columns needed to perform ErrorMatrix()? I tried using the method descriped here https://developers.google.com/earth-engine/guides/joins_spatial but didn't seem to work for an ErrorMatrix(). Might there be any other way to create a Confusion Matrix for two FeatureCollections?

Answer 1

I've found the following solution to my problem: I can use the methods of the ROC-Analysis to calculate the true and false classified areas. Here's my function for that:

var validateLakes = function(classified, truth){
  //clip classified lakes to sampleareas
  classified = classified.filterBounds(sampleareas);
  
  //convert required FeatureCollections to single multipolygon Feature
  var classified_union = ee.Feature(classified.union().first());
  truth = ee.Feature(truth.union().first());
  var area = ee.Feature(sampleareas.union().first());
  
  //calculate tp, tf, fp & fn areas
  var tp = classified_union.intersection(truth);
  var tn = area.difference(classified_union.union(truth));
  var fp = classified_union.difference(truth);
  var fn = truth.difference(classified_union);
  
  /*
  //check results
  Map.addLayer(tp, {palette: 'FF0000'}, 'tp');
  Map.addLayer(tn, {palette: 'FF0000'}, 'tn');
  Map.addLayer(fp, {palette: 'FF0000'}, 'fp');
  Map.addLayer(fn, {palette: 'FF0000'}, 'fn');
  */
  
  //calculate size of areas
  var tp_area = ee.Number(tp.area());
  var tn_area = ee.Number(tn.area());
  var fp_area = ee.Number(fp.area());
  var fn_area = ee.Number(fn.area());
  
  //calculate tpr, fdr and accuracy
  //tpr = tp / (tp + fn)
  var tpr = tp_area.divide(tp_area.add(fn_area));
  //fdr = tp / (fp + tp)
  var fdr = fp_area.divide(fp_area.add(tp_area));
  //accuracy = (tp + tn) / (tp + tn + fp + fn)
  var numerator = tp_area.add(tn_area);
  var denominator = tp_area.add(tn_area.add(fp_area.add(fn_area)));
  var accuracy = numerator.divide(denominator);

  //function for adding values to classfied FeatureCollection
  var addProps = function(feature){
    feature = feature.set('TPR', tpr);
    feature = feature.set('FDR', fdr);
    feature = feature.set('ACCURACY', accuracy);
    return feature;
  };

  //return classified Collection with new properties
  return classified.map(addProps);
};