ImageCollection contains Images with different CRS

Question

I'm using Google Earth Engine Python API in Jupyter. I'm working with an ImageCollection from Sentinel 2 Surface Reflectance dataset for a given area on a single day and filtering by a region of interest (roi) polygon.

The ImageCollection only contains 2 images and one of these appears to be in a different CRS (EPSG: 32631) the other being in the expected EPSG: 4326. This is causing problems for further processing. The code for creating the image collection is:

roi = ee.Geometry.Polygon(
[[[3.94, 51.6],
    [5.3, 51.6],
    [5.3, 52.15],
    [3.94, 52.15],
    [3.94, 51.6]]], None, False)


dataset = ee.ImageCollection("COPERNICUS/S2_SR")\
    .filterDate('2020-04-20', '2020-04-21')\
    .filterBounds(roi)

Here is an screenshot of the collection in Jupyter:

I have two questions:
1. How can I ensure all images in a collection are in the same CRS (and what is the most robust method for achieving this)?
2. Why are images from one dataset in different CRSs?

Answer 1

How are you checking which CRS the two images have? To me, both seems to be EPSG: 32631:

roi = ee.Geometry.Polygon(
[[[3.94, 51.6],
    [5.3, 51.6],
    [5.3, 52.15],
    [3.94, 52.15],
    [3.94, 51.6]]], None, False)


dataset = ee.ImageCollection("COPERNICUS/S2_SR")\
    .filterDate('2020-04-20', '2020-04-21')\
    .filterBounds(roi)


list = dataset.toList(2)    
i1 = ee.Image(list.get(0))
i2 = ee.Image(list.get(1))

print(i1.select(0).projection().crs().getInfo())
print(i2.select(0).projection().crs().getInfo())