Why GEE Coverage Fraction Pixels Show 1 in CSV Exports (And How to Fix It)
In Google Earth Engine (GEE), a common frustration for remote sensing analysts is calculating the fractional coverage of pixels within a geometry. You might see beautiful gradient values between 0 and 1 on your interactive map using Map.addLayer, but upon running Export.table.toDrive(), your CSV shows a value of "1" for every single entry.
This issue typically occurs due to how GEE handles projections, scales, and reducers during the computation versus the export phase.
The Root Cause: Scale and CRS Mismatch
When you view data on the GEE Map, the computation happens at the current zoom level's scale. However, when you export a CSV, the reduceRegion or reduceRegions function often defaults to the native scale of the image or a scale you've specified without considering the sub-pixel geometry.
If the reducer is not explicitly told to account for weighted pixels, it may treat any pixel that touches the geometry as "fully inside," resulting in a coverage fraction of 1 (100%).
How to Fix the "All 1s" Export Issue
1. Use the Weighted Reducer Correctly
To ensure GEE calculates the fraction of the pixel covered by the polygon, you must ensure your reducer is capturing weights. Most reducers in GEE are weighted by default, but the way you call them matters.
2. Explicitly Set the Scale
If you don't define a scale in your reduceRegion function, GEE defaults to the CRS's nominal scale. If your geometry is much larger than the pixel size, the "edge effect" (where fractions occur) becomes statistically negligible unless you are at a very high resolution.
3. Use .reduceRegions with 'forEach' logic
The most robust way to ensure coverage fractions are exported correctly is to use the unmask() function or to multiply your image by ee.Image.pixelArea() if you are calculating area, or use the .reduced(ee.Reducer.mean().splitWeights()) approach.
Correct Code Pattern for Fractional Export
To get the fraction of a pixel (0 to 1) exported to your CSV, use the following logic in your script:
var coverage = myImage.reduceRegions({
collection: myPolygons,
reducer: ee.Reducer.mean(),
scale: 30, // Match your data's native scale
crs: 'EPSG:4326'
});
// Ensure the Export doesn't cast to Integer
Export.table.toDrive({
collection: coverage,
description: 'Fractional_Coverage_Export',
fileFormat: 'CSV'
});
Important: The 'tileScale' Parameter
If your geometry is complex and you are getting memory errors or truncated results (which can lead to 1s), increase the tileScale. A value of 4 or 16 allows GEE to break the computation into smaller chunks, ensuring the sub-pixel math is handled more accurately before the CSV write-out.
SEO Tips for GIS Developers
If you are documenting this fix for your portfolio or blog, use these GIS SEO strategies:
- Long-tail Keywords: Use "GEE reduceRegion weighted pixels" and "Google Earth Engine CSV export 0 to 1 values."
- Schema Markup: Use "Code" or "HowTo" schema to help Google identify the solution snippet.
- Internal Linking: Link to other common GEE errors like "Request Payload too large" or "Projection mismatch."
Conclusion
The discrepancy between the GEE Map display and CSV export is almost always a result of scaling and weighting. By explicitly defining your scale and ensuring your reducer is processing weights, you can accurately capture the 0–1 coverage fraction in your spreadsheets, moving beyond the "all 1s" error.
