[GIS] Image Export fails in Google Earth Engine because “export region contains no valid (un-masked) pixels

exportgoogle-earth-enginemosaicsentinel-2

I want to export Sentinel-2 mosaics for rectangular ROIs. For this, I intend to use a script following this example: https://code.earthengine.google.com/ce1a151ce06497b20cf1793715cb0120

However, whenever I want to run the export task in the "Tasks" manager, I receive the following error after some time: "Error: Export region contains no valid (un-masked) pixels."

This leaves two questions:

1) Why are there no images for the right half of the ROI? The ImageCollection actually contains 9 images. Is that a Sentinel-2 inherent problem?

2) How can I deal with this situation when automated exporting is the goal? I can imagine two general options:
a) Export the image anyway (e.g. with the non-covered pixels set to NaN) – but how will GEE let me do this? b) Apply a check if such "empty" pixels are contained in my mosaic image. (Unfortunately, I have no idea how to achieve this.)

Best Answer

1st: The exact code you posted (https://code.earthengine.google.com/ce1a151ce06497b20cf1793715cb0120) did export the image correctly. So the error cannot be reproduced. May be, you changed the 'ROI' to a place where the filtered collection had no images.

2nd: There are no images because you filtered by cloud percentage, and so, it found images that suited the condition only in the left part of the ROI. If you comment like:

//.filter(ee.Filter.lte('CLOUDY_PIXEL_PERCENTAGE', 5))

you'll see that there are more images and you get the whole ROI (of course you get a part of the image plenty of clouds).

3rd: if automated exporting is your goal, maybe you should migrate to Python, but if you don't mind clicking the run button over and over, this would be one approach to download images only if the filtered collection has images (don't blame me on the getInfo =):

//Define date range
var startDate = ee.Date.fromYMD(2017,7,1);
var endDate = ee.Date.fromYMD(2017,7,31);

//Load Sentinel-2 image collections
var coll_s2 = ee.ImageCollection("COPERNICUS/S2");

//Filter Sentinel-2 collection for ROI and cloud-coverage.
//Keep only images with less than 5% clouds
var coll_s2_filtered = coll_s2.filterDate(startDate, endDate)
                              .filter(ee.Filter.lte('CLOUDY_PIXEL_PERCENTAGE', 5))
                              .sort('system:time_start',false);

// Define the FeatureCollection!
var fc = ee.FeatureCollection(LIST_OF_POINT_FEATURES)

// Iterate over the FeatureCollection to create a List
// with images where there is data avialable
var listn = ee.List(fc.iterate(function(elem, ini){
  var rectangle = ee.Feature(elem.buffer(20000).bounds());
  var roi = rectangle.geometry();
  var col = coll_s2_filtered.filterBounds(roi);
  return ee.Algorithms.If(col.size(), 
            ee.List(ini).add(coll_s2_filtered.select(['B4','B3','B2']).mosaic().clip(roi)), 
            ee.List(ini))
}, ee.List([])))

listn = ee.List(listn)

print(listn)

// As Export is a Client-side function, you have to iterate the list
// in a client-side way

for (var n = 0; n<listn.size().getInfo(); n++) {
  var i = ee.Image(listn.get(n))
  Export.image.toDrive({
    image: i,
    description: 'image_'+n.toString(),
    scale: 10,
    folder: "tests",
    region: i.geometry()
    });
}

But if you want to change the filter regarding on whether it finds or not images, that would be different.

Related Solutions

[GIS] How to find Sentinel-2 dataset ID on Google Earth Engine

Two issues with your script:

1) A single image works best when you use ee.Image instead of ee.ImageCollection.

2) You switched up the granule name. The first part is the sensing time and the second the ingestion time. An acquisition taken in 2016 cannot be ingested in 2015. It should be 20150828T110656_20160412T015159_T30SVG not the other way round.

As a complete script (on EarthEngine):

// not in Collection - switched sensing and acquisition time
// var s2 = ee.Image('COPERNICUS/S2/20160412T015159_20150828T110656_T30SVG');
// in Collection - correct granule
var s2 = ee.Image('COPERNICUS/S2/20150828T110656_20160412T015159_T30SVG');
Map.addLayer(s2, {bands: ['B8', 'B4', 'B3'], max: 4000}, 'S2')
Map.centerObject(s2, 8);

[GIS] “Batch processing” data of multiple ROIs in Google Earth Engine

Here's an example that hopefully does something like what you want, but you'll need to modify it to meet your requirements for "manually selected marker points" and Sentinel 2.

In general, do NOT use for-loops or getInfo(). To be clear, DO NOT USE FOR LOOPS unless you have a very good reason to be doing so. If you don't know, use map() for all the reasons described here, here, here and here. (And check the same guides for why to NOT use getInfo() or convert to a list as I've done here.) The reason it's OK to do this here (necessary, in fact) is because Export is a client side function and you can't use a client function in map().

// Get some imagery to play with.
var landsat = ee.ImageCollection("LANDSAT/LC08/C01/T1")
    .filterDate('2016-01-01', '2017-01-01');

var composite = ee.Algorithms.Landsat.simpleComposite({
  collection: landsat,
  asFloat: true
});

var rgbVis = {bands: ["B4", "B3", "B2"], min:0, max: 0.3};
Map.addLayer(composite, rgbVis, "RGB");

// This is a hacky way to get a pixel grid at arbitrary resolution.
var pixels = ee.Image.random().multiply(10000000).toInt32()
    .reduceToVectors({
      reducer: ee.Reducer.countEvery(),
      geometry: Map.getBounds(true),
      geometryType: 'bb' ,
      eightConnected: false,
      scale: 20000,
      crs: 'EPSG:4326'
    });
Map.addLayer(pixels);

// Only do this is you have a few regions.  Not suitable for 
// large feature collections.
var pixelsList = pixels.toList(pixels.size());

// This is one of the few places in the EE API where you need 
// a for-loop and a getInfo() call.  Export is a client function.
for (var i=0; i<pixels.size().getInfo(); i++) {
  Export.image.toDrive({
    image: composite, 
    description: 'foo_' + i, 
    fileNamePrefix: 'foo_' + i, 
    region: ee.Feature(pixelsList.get(i)).geometry(), 
    scale: 30, 
  });
}

If you really need precise control over the export regions, you can make lists of coordinates and turn those into a collection of the ROIs you want to export. All that aside, you still need to click 'Run' on the exports. If you want it completely automated, use the Python API and ee.batch.Export followed by task.start().

Best Answer

Related Solutions

[GIS] How to find Sentinel-2 dataset ID on Google Earth Engine

[GIS] “Batch processing” data of multiple ROIs in Google Earth Engine

Related Question