In 2013 Hansen et al. used Landsat satellite imagery to map global forest loss and gain from 2000 to 2012 at a spatial resolution of 30 meters. This was the first study to quantify a globally consistent record of forest change. Using these techniques the University of Maryland prepares a weekly set of alerts, called GLAD alerts, which identify 30-meter pixels (from a total of about a billion pixels) that have recently been cleared. Although the data are updated on a weekly basis, the amount of time between detections depends on cloud cover. Persistent cloud coverage found in many tropical countries limits the monitoring frequency, in some cases for months at a time. Alerts become confirmed when more than one Landsat image flags the pixel as an alert. GLAD alerts are currently available for 16 tropical countries; Brazil, Burundi, Brunei, Peru, Cameroon, East Timor, Central African Republic, Indonesia, Democratic Republic of Congo, Malaysia, Equatorial Guinea, Papua New Guinea, Gabon, Republic of Congo, Rwanda, and Uganda.
Hansen, M.C., P.V. Potapov, R. Moore, M. Hancher, S.A. Turubanova, A. Tyukavina, D. Thau, S.V. Stehman, S.J. Goetz, T.R. Loveland, A. Kommareddy, A. Egorov, L. Chini, C.O. Justice, and J.R.G. Townshend. 2013. “High-Resolution Global Maps of 21st Century Forest Cover Change.” Science 342 (6160): 850–853.