Sky Surveys

Sky surveys represent a fundamental data basis for astronomy. We usethem to map in a systematic way the universe and its constituents andto discover new types of objects or phenomena. We review the subject,with an emphasis on the wide-field, imaging surveys, placing them ina broader scientific and historical context. Surveys are now the largestdata generators in astronomy, propelled by the advances in informationand computation technology, and have transformed the ways in whichastronomy is done. This trend is bound to continue, especially with thenew generation of synoptic sky surveys that cover wide areas of the skyrepeatedly and open a new time domain of discovery. We describe thevariety and the general properties of surveys, illustrated by a number ofexamples, the ways in which they may be quantified and compared, andoffer some figures of merit that can be used to compare their scientificdiscovery potential. Surveys enable a very wide range of science, and that isperhaps their key unifying characteristic. As new domains of the observableparameter space open up thanks to the advances in technology, surveys areoften the initial step in their exploration. Some science can be done withthe survey data alone (or a combination of data from different surveys),and some require a targeted follow-up of potentially interesting sourcesselected from surveys. Surveys can be used to generate large, statisticalsamples of objects that can be studied as populations or as tracers of largerstructures to which they belong. They can be also used to discover orgenerate samples of rare or unusual objects and may lead to discoveriesof some previously unknown types. We discuss a general framework ofparameter spaces that can be used for an assessment and comparison ofdifferent surveys and the strategies for their scientific exploration. As we aremoving into the Petascale regime and beyond, an effective processing andscientific exploitation of such large data sets and data streams pose manychallenges, some of which are specific to any given survey and some ofwhich may be addressed in the framework of Virtual Observatory andAstroinformatics. The exponential growth of data volumes and complexity makesa broader application of data mining and knowledge discovery technologiescritical in order to take a full advantage of this wealth of information.Finally, we discuss some outstanding challenges and prospects for thefuture.