Missing band in mapping function using Google Earth Engine

Question

I built a function to add a a new band (b1_1) to each image of a image collection and I mapped it. It does seem to work, at least when printing the image collection and inspecting it in the console.

But when I trying to map on the "new collection" another function for charting the new band (b1_1) against some other pre-existing band, the new one doesn't exist anymore.

//mask and add delta DEM band
var add_band = function(image){
var glacier_img = image.select(['b1']);
var glacier_clip = glacier_img.clip(shp);
var dem_glacier = DEM_delta.clip(shp);

var vel_range = glacier_clip.lte(ee.Number(700)).and(glacier_clip.gte(ee.Number(-700)));
var dem_glacier_mask = dem_glacier.updateMask(vel_range);

var final_img = image.updateMask(vel_range).addBands(dem_glacier_mask);
return final_img;

}; 

//add a DEM band  to each image of the images collection
var imgCollection = imgCollection.map(add_band); 
print(imgCollection);


//chart 
var graph = function(image){
var id = image.id;
var image = ee.Image(image.id);
//print(id);

var result = image.reduceRegion(ee.Reducer.toList(),shp, 200);
var y = ee.Array(result.get('b1'));
var x = ee.Array(result.get('b1_1'));
var chart = ui.Chart.array.values(y, 0, x)
      .setSeriesNames(['b1'])
      .setOptions({
        title: 'Velocity vs. Delta_DEM ',
        hAxis: {'title': 'Delta_DEM'},
        vAxis: {'title': 'Velocity'},
        pointSize: 3,
        });

  // Print the chart.
  return print(chart);
  };


 imgCollection.evaluate(function(imgCollection) {  // use map on client-side
 imgCollection.features.map(graph);
 });

Answer 1

The problem is that you're attempting to use evaluate() on an entire imagecollection. The result from evaluate is just some metadata; the results are no longer images; they're just small dictionaries containing the image metadata.

If you're hoping to generate a chart per image for every image in a collection, you'll have to use a traditional for loop and get each image out of the collection 1 at a time (collection.toList(1, index). But there is no evaluate in that process.