An overview of the parcels

In southwestern Kansas, most fields have a circular shape due to center-pivot irrigation. This ASTER image shows fields that are 800 and 1,600 meters in diameter.

Green areas in the image are healthy vegetation. Light colored cultivated fields are fallow or recently harvested. Common crops are corn, wheat and sorghum.

In Europe, satellite measurements of the fields' exact size are used to double-check farmer's land declaration for subsidies.

Question : How is it possible to identify crop types from space ?