By Rick Mueller

The USDA/National Agricultural Statistics Service provides meaningful, accurate, and objective geospatial information and services utilizing satellite imagery and ground truth data to produce a supervised land use classification called the Cropland Data Layer (CDL).  More specifically, this NASS remote sensing area program produces a public domain crop specific geospatial land cover product over the major crop producing states in the continental US.  Indications of major cropland planted and harvested acres are provided from the CDL for internal NASS use.   On January 28, 2010 NASS issued an official press release and Tweet announcing the release of the National 2009 CDL.  This was the first time that NASS completed a national geospatial data product and released the CDL within just a few months after the growing season concluded.  Currently all continental U.S. States with the exception of Florida are complete, as Florida will be released sometime in March.  The '09 data are freely available for download from the NASS website or the NRCS Geospatial Data Gateway.



The '09 National CDL is the result of multiple long term synergistic partnerships with multiple federal agencies that have expanded NASS' geospatial efficiencies and capacity to produce this end-of-season land cover product.  The CDL method leverages the Satellite Image Archive from the USDA/Foreign Agricultural Service that provides satellite data collected from the Indian Resourcesat-1 AWiFS sensor, and the U.S. Geological Survey (USGS) provides the Landsat V Thematic Mapper [TM] and Landsat VII Enhanced Thematic Mapper [ETM]).  The USDA Farm Service Agency (FSA) provides confidential ground truth data called the Common Land Unit (CLU) and associated Administrative data to build a supervised land cover classification with major emphasis on crop identification.  The NASS June Agricultural Survey (JAS) also confidential, is leveraged to build a regression model of crop area during the growing season, and provides NASS with indications of crop area at the state, district and county levels.  Additionally, ancillary inputs include;  the USGS National Elevation Dataset (NED), the USGS National Land Cover Dataset 2001 (NLCD 2001), the USGS Percent Imperviousness 2001 and USGS Percent Forest Canopy 2001 and the National Aeronautics and Space Administration (NASA) Moderate Resolution Imaging Spectroradiometer (MODIS) 250 meter 16 day Normalized Difference Vegetation Index (NDVI) composites.



A variety of commercial software applications are utilized to build this data product.  ESRI's ArcGIS is used to edit ground truth from the CLU and JAS datasets.  ERDAS Imagine is used in conjunction with the public domain NLCD Toolkit and See5 for image preparation, processing, sampling, and decision tree classification functions.  SAS is utilized for regression based area estimation. 

Each state's classification is evaluated using a rigorous formal accuracy assessment; the accuracy assessment is published in the metadata with each CDL.  The accuracy of the land cover classifications are evaluated using independent validation data sets generated from the FSA CLU data (agricultural categories) and the NLCD 2001 (non-agricultural categories).  The Producer's Accuracy is generally 85% to 95% correct for the major crop-specific land cover categories.  The official DVD contains additional accuracy assessment information that is not available through the Geospatial Data Gateway in the form of an associated confidence layer.  It is important to note that the internal acreage estimates produced using the CDL are not simple pixel counting as it is more of an 'Adjusted Census by Satellite'.

The USDA's National Agricultural Statistics Service (NASS) conducts hundreds of surveys every year and prepares reports covering virtually every aspect of U.S. agriculture. Production and supplies of food and fiber, prices paid and received by farmers, farm labor and wages, farm finances, chemical use, and changes in the demographics of U.S. producers are only a few examples. NASS is committed to providing timely, accurate, and useful statistics in service to U.S. agriculture.

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