TIL: Exporting a GEE Image Using Rasterio Transforms

I recently needed to export an image from Earth Engine to overlay with a local GeoTIFF. Translating the CRS and transforms between the local rasterio metadata and the format expected by GEE to get identical grids turned out to be surprisingly frustrating, so I thought I’d do a quick write up to hopefully save myself a future headache.

Reference Metadata

Assuming we have a local GeoTIFF reference.tif, we can grab the relevant metadata with:

import rasterio

with rasterio.open("reference.tif") as ref:
    ref_profile = ref.profile

print(ref_profile)

which gives us something similar to:

{
'driver': 'GTiff', 
'dtype': 'float32', 
'nodata': -3.3999999521443642e+38, 
'width': 2160, 
'height': 1080, 
'count': 19, 
'crs': CRS.from_wkt('GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AXIS["Latitude",NORTH],AXIS["Longitude",EAST],AUTHORITY["EPSG","4326"]]'), 
'transform': Affine(0.16666666666666666, 0.0, -180.0, 0.0, -0.16666666666666666, 90.0), 
'blockxsize': 2160, 
'blockysize': 1, 
'tiled': False, 
'compress': 'lzw', 
'interleave': 'band'
}

Translating for Earth Engine

The ref_profile from rasterio includes all the information that Earth Engine needs to recreate the grid, but not in the format that it expects.

When you export an image from GEE, you can provide any combination of region, scale, dimensions, crs, and crsTransform, but some of those provide redundant information and override each other. To fully define a grid, you just need the last three¹: dimensions, crs, and crsTransform.

dimensions is easy. Just format the width and height into a "WIDTHxHEIGHT" string.

dimensions = f"{ref_profile['width']}X{ref_profile['height']}"

crs is similarly easy. You could just use ref_profile["crs"].to_string() to get the EPSG code, but ref_profile["crs"].to_wkt() provides the full coordinate reference system specification, so it’ll be more reliable for less common projections.

crs = ref_profile["crs"].to_wkt()

crsTransform is the tricky one. rasterio offers two output styles (GDAL and Shapely) for its affine transformation, neither of which match the mystery² format expected by Earth Engine. After some trial and error, the expected format seems to be (following the naming format of affine):

a b c d e f

which differs from the GDAL format of

c a b f d e

or the Shapely format of

a b d e c f

The Converter

Putting all that together, you export an Earth Engine image to match a local GeoTIFF’s grid with:

def get_export_args(ref_tif):
    with rasterio.open(ref_tif) as src:
        ref_profile = src.profile

    transform = ref_profile["transform"]

    return dict(
        dimensions=f"{ref_profile['width']}X{ref_profile['height']}",
        crs=ref_profile["crs"].to_wkt(),
        crsTransform=[
            transform.a, transform.b, transform.c,
            transform.d, transform.e, transform.f
        ]
    )

ee.batch.Export.image.toDrive(
    image=my_image,
    description="overlay_image",
    **get_export_args("reference.tif")
)