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In Google Earth Engine I am calculating Sentinel-2 NDVI and then want to rescale my ndvi imagery from 10m to 30m to match Landsat. Finally, I want to combine the 30m Sentinel-2 with the Landsat imagery to give a two band image and export them in a specific projection (UTM). I know that with composites (such as ndvi) the projection information is lost. I have tried to correct for this but my code below doesn't seem to work - plotting the data shows the Sentinel-2 is still at 10m. And if I try and export the data I get the error "Error: Can't transform (224518.0,-661010.5)".

How can I reassign the projection correctly and create a 30m resolution ndvi image for Sentinel-2?

All the other examples I can find are for simple images rather than composites.

// Cloud mask Landsat 8
function cloudMaskL8sr(image) {
  var cloudShadowBitMask = (1 << 3);
  var cloudsBitMask = (1 << 5);
  var qa = image.select('QA_PIXEL');
  var mask = qa.bitwiseAnd(cloudShadowBitMask).eq(0)
                 .and(qa.bitwiseAnd(cloudsBitMask).eq(0));
  return image.multiply(0.0000275).add(-0.2).updateMask(mask);
}

// Cloud mask using the Sentinel-2 QA band.
function maskS2clouds(image) {
  var qa = image.select('QA60');
  var cloudBitMask = Math.pow(2, 10);
  var cirrusBitMask = Math.pow(2, 11);
  var mask = qa.bitwiseAnd(cloudBitMask).eq(0).and(
             qa.bitwiseAnd(cirrusBitMask).eq(0));
  return image.divide(10000).updateMask(mask).copyProperties(image, ['system:time_start']);
}

// Get landsat 8 collection
var ls8_collection = ee.ImageCollection("LANDSAT/LC08/C02/T1_L2") 
                      .filterBounds(geometry) 
                      .filterDate(ee.Date.fromYMD(2020,1,1),ee.Date.fromYMD(2020,12,31))
                      .map(cloudMaskL8sr)
                      .select(['SR_B4','SR_B5']);

// Calculate landsat ndvi                      
var ls8_ndvi = ls8_collection
                .median()
                .normalizedDifference(['SR_B5','SR_B4'])
                .clip(geometry)
                .rename('LS8_NDVI');

// reset scale and projection                
ls8_ndvi =  ls8_ndvi.setDefaultProjection({
    crs: 'EPSG:32360',
    scale: 30
    }) 
                      
// Get Sentinel-2 data
var s2_collection = ee.ImageCollection('COPERNICUS/S2_SR')
                      .filterBounds(geometry)
                      .filterDate(ee.Date.fromYMD(2020,1,1),ee.Date.fromYMD(2020,12,31))
                      .filterMetadata('CLOUDY_PIXEL_PERCENTAGE','less_than',50) // Need to include this to reduce teh amount of data
                      .map(maskS2clouds)
                      .select(['B4','B8'])      
                      
// calculate ndvi
var s2_ndvi = s2_collection
                .median()
                .normalizedDifference(['B8','B4'])
                .clip(geometry)
                .rename('S2_NDVI');                     
                
// Change resolution and assign projection
s2_ndvi =   s2_ndvi.setDefaultProjection({
    crs: 'EPSG:32360',
    scale: 30
    })                

// Combine Landsat and Sentinel-2 into a single two-band image
var combined = ee.Image.cat([s2_ndvi,ls8_ndvi])
print(combined)

// Plot (as a test)
var ndviParams = {min:-0.25, max:1, palette:'CE7E45,DF923D,F1B555,FCD163,99B718,74A901,66A000,529400,3E8601,207401,056201,004C00,023B01,012E01,011D01,011301'}; 
Map.addLayer(s2_ndvi, ndviParams, 'S2');
Map.addLayer(ls8_ndvi, ndviParams, 'LS8');

// Export image to drive
Export.image.toDrive({
  image: combined,
  description : 'combined',
  scale : 30,
  crs: 'EPSG:32360',
  region : geometry
})

1 Answer 1

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Your geometry isn't listed, but most likely it exceeds the bounds of the UTM projection you're using. That projection is only valid for 6 degrees of longitude.

2
  • Good thought, thanks for the pointer. It does explain the error message on the export. BUT no, that's not the issue. I've now set the AOI to be a box around the city of Sheffield, UK; an area ~15km by 11km. That's pretty much in the middle of that UTM zone. But it still won't export "Export too large: specified 1390226400 pixels (max: 100000000)." I realize I can change max pixels but ultimately I want to export a whole Landsat scene. Furthermore, the map display is showing that the S-2 imagery is still at 10m resolution not 30m resolution so I'm still not getting this right somewhere.
    – Ddee
    Commented Aug 20, 2021 at 12:34
  • You're expected in increase maxPixels as necessary. That's only 1G, so it shouldn't be a problem. Setting the default projection doesn't do anything, it just sets a guide. If you really want to reproject pixels to 30m, use reproject(), not defaultProjection. But the export, running at 30m, is going to do that automatically. Commented Aug 21, 2021 at 13:13

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