Why is the Sentinel-2 image plot triangular in Google Earth Engine?

Question

I'm plotting the least cloudy Sentinel image in MGRS tile '11SPV', but it's rendered as a narrow triangle on the Western edge of the tile. I've changed the temporal range but the same thing occurs. Is this a clash of projections, or something else entirely?

The co-ordinates of tile 11SPV are:

EPSG    32611  
MGRS_REF      
36.13956045 -115.88852197   
36.124095832 -114.77760861  
35.223124941 -114.80245835  
35.238087535 -115.90095919 

UTM_WKT MULTIPOLYGON(((600000 4000020,600000 3890220,709800 3890220,709800 4000020,600000 4000020)))

LL_WKT  MULTIPOLYGON(((-115.888519424674 36.1397407305084,-115.902146161155 35.1499160380147,-114.697342097699 35.1330063316074,-114.668785361013 36.1222067277992,-115.888519424674 36.1397407305084)))

Screenshot of the output below.

// var START_DATE = '2017-02-15';
// var END_DATE = '2017-04-15';
var MGRS_TILE = '11SPV';

var s2_unsorted = ee.ImageCollection('COPERNICUS/S2')
                    .filterMetadata('MGRS_TILE', 'equals', MGRS_TILE)
                    .filterDate('2020-07-01', '2020-07-31');
var s2_sorted = s2_unsorted.sort('CLOUDY_PIXEL_PERCENTAGE');
var least_cloudy_image = ee.Image(s2_sorted.first());

Map.centerObject(least_cloudy_image);
Map.addLayer(
  least_cloudy_image,
  {bands:'B4,B3,B2', min:0, max:3000},
  'least cloudy image'
);    
print('least cloudy image', least_cloudy_image);

// It can be useful to print out the image collection, and the list of metadata properties to confirm that it is working the way you expect:

var MAX_IMAGES_IN_LIST = 100;
var getCloudMetadata = function (coll) {
  return coll.toList(MAX_IMAGES_IN_LIST).map(
    function (i) {
      return ee.Image(i).get('CLOUDY_PIXEL_PERCENTAGE');
    }
  );
};

// print(
//  'START_DATE', START_DATE,
//  'END_DATE', END_DATE,
//  'MGRS_TILE', MGRS_TILE
//);
print(
  's2 (unsorted)', s2_unsorted,
  'CLOUDY_PIXEL_PERCENTAGE metadata', getCloudMetadata(s2_unsorted)
);
print(
  's2 (sorted)', s2_sorted,
  'CLOUDY_PIXEL_PERCENTAGE metadata', getCloudMetadata(s2_sorted)
);

Answer 1

Sentinel 2 data is distributed as granules (tiles) that are 100x100 km2 ortho-images in UTM/WGS84 projection (source).

Because the satellite path is not aligned with axes of the UTM projections, the tiles along the satellite path edge appear as triangles or trapezoids.

The following script visualizes the boundaries of the S-2 granules imaged on a single day.

// Copyright 2021 Google LLC.
// SPDX-License-Identifier: Apache-2.0

var s2 = ee.ImageCollection('COPERNICUS/S2')
          .filterDate('2020-02-03', '2020-02-04');
Map.addLayer(ee.FeatureCollection(s2), {}, 'Granule boundaries'); 

Link to script: https://code.earthengine.google.com/a63bc097148f1b9dadc626597a270c29

Note the darker areas are region of overlap between the granules. And across the boundaries of UTM zones, the granules may be repeated (i.e. once using each UTM zone projection).

Answer 2

Quite normal. Lots of them are slivers. If you get the data from SciHub https://scihub.copernicus.eu/dhus/#/home, you can see the shape of the raster that will be downloaded. Sometimes they're square, sometimes they're triangles.