On Nov. 29, 1864, U.S. soldiers attacked a village of about 500 Cheyenne and Arapaho people camped along the banks of Big Sandy Creek in southeastern Colorado. The troops massacred some 150 people, mostly women, children, and the elderly, who believed the U.S. Army would protect them. The government eventually condemned the killings. In November 2000, federal law authorized Sand Creek Massacre National Historic Site to recognize its significance in American history and importance to Cheyenne and Arapaho peoples, including descendents of the massacred. The National Park Service has acquired 920 of 12,583 acres within the authorized boundary. The site will not open to the public until the park service and its partners acquire enough private and state land to ensure its preservation. GIS technology will help make that a reality. GIS data helped pinpoint where the tragedy occurred. GIS mapping also shows the geographic relationship between the artifact sites, Big Sandy Creek, and views of the area from six different points. GIS specialists have updated the boundaries and status of land ownership, and the tribes’ oral history of the event identified significant areas. The park service added these new information layers, or themes, to the GIS model. A computer analysis of the model rated the various lands based on their historic value. This has helped the park service and its partners decide which lands to pursue, ultimately to establish a national historic site for our benefit and education.