Export all pixel NDVI values from a region using Earth Engine

Question

I need to export a CSV file with all pixel NDVI values from a clipped region but with my code below I just got part of pixel values, not all pixel values.

var sent2 = ee.ImageCollection("COPERNICUS/S2"),
    trueColour = {"bands":["B4","B3","B2"],"min":0,"max":3000},
    local = 
    /* color: #98ff00 */
    /* locked: true */
    ee.Geometry.Point([-38.67714056850234, -11.07113127584459]),
    geometry = 
    /* color: #98ff00 */
    /* locked: true */
    /* displayProperties: [
    {
      "type": "rectangle"
    }
    ] */
    ee.Geometry.Polygon(
     [[[-38.67902798420933, -11.068997041766817],
      [-38.67902798420933, -11.07215579800511],
      [-38.67490811116245, -11.07215579800511],
      [-38.67490811116245, -11.068997041766817]]], null, false);


var start_date = '2022-04-01';
var finish_date = '2022-04-10';

var image = ee.Image(sent2
 .filterDate(start_date, finish_date)
 .filterBounds(local)
 .sort('CLOUD_COVER',true)
 .first());

var true_image = sent2.median().clip(geometry);
Map.addLayer(true_image, trueColour,'Clipped True Image');

var palette = ['f4e1e1', 'ff0000', 'fa0000', 'F1B555', 'FCD163', '99B718',
           '74A901', '66A000', '529400', '3E8601', '207401', '056201',
           '004C00', '023B01', '012E01', '011D01', '011301'];

var ndvi = image.normalizedDifference(['B8', 'B4']);

var clippedNDVI = ndvi.clip(geometry);

var clippedImage = image.clip(geometry); //ClipGeometry can any geometry or feature or shapfile
var clippedImagendvi = clippedImage.normalizedDifference(['B8', 'B4']);

Map.addLayer(clippedImagendvi, {min: 0, max: 1, palette: palette},'Clipped Image ndvi');

Map.centerObject(local,15);

// Changing image to FeatureCollection
var coll = clippedNDVI.sample({
  region: geometry,
  geometries: true,  // This specifies that you want the lat-long, rather
  // than image samples without any position information.
});

// Export a .csv table of date, mean NDVI for watershed
Export.table.toDrive({
  collection: coll,
  folder: 'folder',
  description: 'ndviMandassaia',
  fileFormat: 'CSV',
  selectors: ['nd']
});

To export .csv from region I did need to change the clipped image to FeatureCollection by .sample() line, setting maximum number of picture (lines x columns) but it didn't work. Is there other manner to solve this problem?

Answer 1

If I understand correctly, you got less pixels in the FeatureCollection resulting from ee.Image().sample than you expected based on viewing the image in the Map. You are looking at the NDVI image at a Map Scale of 15 (approx 5m/px). The Sentinel2 image bands you are computing NDVI on have a native scale of 10m/px see here. It is important to specify the scale parameter in your ee.Image().sample() method, as this affects the resolution of the image you are interacting with upon run-time, and consequently in your case, how many available pixels there are in the image to sample from. See here on how Earth Engine handles scale and pyramiding.

Here is a code editor link: https://code.earthengine.google.com/8abc705104b2e9ce842bc85d97e07e54.

Change the number in the scale parameter on line 54 to see how this is working.