http://www.d.dccam.org/Projects/Maps/Mapping_1975-79.htm

Killing Fields & Mass Grave Sites

The Documentation Center of Cambodia (DC-Cam) is an independent Cambodian non-governmental organization dedicated to collecting original source materials from the Khmer Rouge Regime. As part of its work, DC-Cam uses global satellite position mapping combined with fieldwork to document mass graves nationwide. To date, it has identified over 388 genocide sites containing 19,733 mass graves (these are defined as any pit containing 4 or more bodies, although some graves hold over 1,000) dating from the Khmer Rouge regime.

A History of Democratic Kampuchea (1975-1979), Khamboly Dy (2007), Phnom Penh: Documentation Center of Cambodia, p. 4.