U.S corn output increased from 1.8 billion bushels in 1879 to 12.7 billion bushels in 2007. Concurrently, the footprint of production changed substantially. Failure to take proper account of movements means that productivity assessments likely misattribute sources of growth and climate change studies likely overestimate impacts. Our new spatial output indexes show that 16 to 21 percent of the increase in U.S. corn output over the 128 years beginning in 1879 was attributable to spatial movement in production. This long-run perspective provides historical precedent for how much agriculture might adjust to future changes in climate and technology.