  A global site selection for astronomy was performed with 1 km spatial
resolution ($\sim$ 1 Giga pixel in size) using long term and up-to-date
datasets to classify the entire terrestrial surface on the Earth. Satellite
instruments are used to get the following datasets of Geographical Information
System (GIS) layers: Cloud Coverage, Digital Elevation Model, Artificial Light,
Precipitable Water Vapor, Aerosol Optical Depth, Wind Speed and Land Use --
Land Cover. A Multi Criteria Decision Analysis (MCDA) technique is applied to
these datasets creating four different series where each layer will have a
specific weight. We introduce for the first time a ``Suitability Index for
Astronomical Sites'' namely, SIAS. This index can be used to find suitable
locations and to compare different sites or observatories. Mid-western Andes in
South America and Tibetan Plateau in west China were found to be the best in
all SIAS Series. Considering all the series, less than 3 \% of all terrestrial
surfaces are found to be the best regions to establish an astronomical
observatory. In addition to this, only approximately 10 \% of all current
observatories are located in good locations in all SIAS series. Amateurs,
institutions or countries aiming to construct an observatory could create a
short-list of potential site locations using layout of SIAS values for each
country without spending time and budget.The outcomes and datasets of this
study has been made available through a web site, namely ``Astro GIS Database''
on \texttt{\url{www.astrogis.org}}.
