Export of reprojected layer from GEE fails

Question

I am trying to export a resampled image from Google Earth Engine, and keep getting errors such as -

Error: Reprojection output too large (23734x8520 pixels).

As shown in the following screenshot -

The error can be reproduced using the sample script from https://developers.google.com/earth-engine/resample#reduce-resolution as follows -

// Load a MODIS EVI image.
var modis = ee.Image(ee.ImageCollection('MODIS/006/MOD13A1').first())
    .select('EVI');

// Display the EVI image near La Honda, California.
Map.setCenter(-122.3616, 37.5331, 12);
Map.addLayer(modis, {min: 2000, max: 5000}, 'MODIS EVI');

// Get information about the MODIS projection.
var modisProjection = modis.projection();
print('MODIS projection:', modisProjection);

// Load and display forest cover data at 30 meters resolution.
var forest = ee.Image('UMD/hansen/global_forest_change_2015')
    .select('treecover2000');
Map.addLayer(forest, {max: 80}, 'forest cover 30 m');

// Get the forest cover data at MODIS scale and projection.
var forestMean = forest
    // Force the next reprojection to aggregate instead of resampling.
    .reduceResolution({
      reducer: ee.Reducer.mean(),
      maxPixels: 1024
    })
    // Request the data at the scale and projection of the MODIS image.
    .reproject({
      crs: modisProjection
    });

// Display the aggregated, reprojected forest cover data.
Map.addLayer(forestMean, {max: 80}, 'forest cover at MODIS scale');

The script works fine and the resampled image forestMean is shown on the map.

However, trying to export the image with the following expressions leads to the above-mentioned error -

// Continental USA polygon
var pol = ee.Geometry.Polygon([[[-126.21093749999999,23.241346102386135],[-67.8515625,23.241346102386135],[-67.8515625,48.922499263758255],[-126.21093749999999,48.922499263758255],[-126.21093749999999,23.241346102386135]]]);

// Export
Export.image.toDrive({
  image: forestMean,
  description: "forest_example",
  scale: 926.6254330555, 
  region: pol
});

It is important to note that the same error appears even when using a very small export region.

Update:

As Philipp Gärtner discovered, this seems to be an issue of export area size. Using a very small area indeed works.

For example, the following very small export area works -

While this one doesn't work -

Answer 1

It seems that the problem you are running into is that when Earth Engine is trying to reduce the resolution there are too many pixels going into the aggregation calculation at the scale you are requesting at. A way to circumvent the problem is to set the bestEffort argument to true and lower the maxPixel values in reduceResolution(). Here is a working example:

// Load a MODIS EVI image.
var modis = ee.Image(ee.ImageCollection('MODIS/006/MOD13A1').first())
    .select('EVI');

// Display the EVI image near La Honda, California.
// Map.setCenter(-122.3616, 37.5331, 12);
Map.addLayer(modis, {min: 2000, max: 5000}, 'MODIS EVI');

// Get information about the MODIS projection.
var modisProjection = modis.projection();
print('MODIS projection:', modisProjection);

// Load and display forest cover data at 30 meters resolution.
var forest = ee.Image('UMD/hansen/global_forest_change_2015')
    .select('treecover2000');
Map.addLayer(forest, {max: 80}, 'forest cover 30 m');

// Get the forest cover data at MODIS scale and projection.
var forestMean = forest
    // Force the next reprojection to aggregate instead of resampling.
    .reduceResolution({
      reducer: ee.Reducer.mean(),
      maxPixels: 64,
      bestEffort:true
    })
    // Request the data at the scale and projection of the MODIS image.
    .reproject({
      crs: modisProjection
    });

// Display the aggregated, reprojected forest cover data.
Map.addLayer(forestMean, {max: 80}, 'forest cover at MODIS scale');

// Continental USA polygon
var pol = ee.Geometry.Rectangle([-125,25,-67,49],'EPSG:4326',false);

Map.addLayer(pol,{color:'red'},'Export Region')

// Export
Export.image.toDrive({
  image: forestMean,
  description: "forest_example",
  scale: 926.6254330555, 
  region: pol,
  maxPixels:1e13
});

I tried the export over the CONUS and it took 8 minutes (resulting image below in QGIS). Be careful of the resulting values though...since you are lowing the number of pixels going into the resolution reduction, the MODIS scale values may not be absolutely accurate.

I hope this helps!