Reprojection issue in Image collection after Filtering using Google Earth Engine

Question

I am trying to reproject an image collection, but it fails after I apply filtering by (space, time, and cloudy cover percentage)!?

Here is the code:

Map.centerObject(geometry);

var S2_f = S2.filterDate('2018-07-03', '2018-07-24')
             .filterBounds(geometry)
             .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 5));


Map.addLayer(S2_f, {min: 0,  max:3000, bands: ['B4', 'B3', 'B8A']},'Before Reprojection');

print('projection original', S2_f.projection());

var proj = ee.Projection('EPSG:2062');

print('nominal scale', proj.nominalScale()); 

var S2_r = S2_f.reproject(proj, null, 10);

Map.addLayer(S2_r, {min: 0,  max:3000, bands: ['B4', 'B3', 'B8A']}, 'After Reprojection');

The error message is:

S2_f.projection is not a function

https://code.earthengine.google.com/20877d4ab79887bbbb663ffe89367b2e

Answer 1

You can't reproject an ImageCollection. You can only reproject images. So your code would work like this:

var S2_r = S2_f.first().reproject(proj, null, 10);

If you want to reproject all images in the collection you need to map over the collection and reproject every single Image.

var S2_r = S2_f.map(function(image){
  return image.reproject(proj, null, 10);
})

However be aware that unless you absolutely need to reproject an Image, it is not advised to do so, since it can add a lot of unnecessary processing overhead. See https://developers.google.com/earth-engine/guides/projections