One of the most important steps to make use of any renewable energy is to perform an accurate estimation of solar energy. Incoming shortwave (300-4000 nm) solar radiation is an important component in evaluating environmental factors. In addition to local solar radiation, a detailed spatial and temporal information of Global Solar Radiation (GSR) is required in urban design and regional plans. Further, beside the atmospheric parameters, the variability of the elevation, the surface orientation and the obstructions due to elevations are a source of great localdifferences affecting the incoming solar radiation. For this reason, several models based on GIS techniques have been recently proposed and developed, which consider the local topography for retrieving global solar radiation on the Earth’s surface. Actual irradiation which is received at any time and location depends on many factors such as solar zenith angle, solar declination angle, atmospheric transmissivity (i.e. cloud amount, aerosol optical depth), diffused radiation, slope orientation and local topography. The commonly used radiation models (e.g. Angstrom-Prescott model) do not consider diffused radiation, atmospheric transmissivity (turbidity) and topography of the study sites; as they are not usualy available for model estimation. Using the Arc GIS tool, Batless et al. (2008) estimated the global solar radiation on a monthly basis. They showed that summer month estimations are more accurate than winter times. For a region with a different topography, Martinez et al. (2009) employed satellite images (Meteosat) and Digital Terrain Model for estimation of GSR. Their results showed reasonable performance for the model estimates. Boschet al. (2010) addressed the same problem.  To evaluate the potential of solar energy for Oman, Gastli and Charabi (2010) used Arc GIS method in their work. They found that Oman had a high potential of solar energy which could be used as a sustainable source of renewable energy.Iran is located in an unique part of the world, which experiences significant variation in altitude and latitude. The altitude of the country, which has a complex totography, varies from -40 to 5670 m above sea level. These geographical factors cause a wide range of climate types and different amounts of solar irradiance at the surface. The majority of the radiation modeling in Iran has relied on straightforward and simple radiation models such as Angstrom and Angstrom-Prescott. The main constraint of these models was the regional estimates of GSR which were not easily possible from the local estimates. As the first attempt in Iran, this work aimed to consider the effect of complex topography and altitude on surface GSR. 
    In this study, global solar radiation was produced for four central provinces of Iran using the Solar Analyst model implemented on ArcGIS 9.3. The Solar Analyst uses input parameters such as the diffuse fraction (k) and the atmospheric transmissivity (). These parameters are not usually available in radiation networks. In this work, the aforesaid parameters were obtained with use of global solar radiation data (as measured in radiation sites) and extraterrestrial radiation  (determined by astronomical models). Monthly local solar irradiation in the year 2007 was calculated from a digital terrain model (DTM) with spatial resolution of 300 m, diffuse fraction (k) and atmospheric transmissivity () for the study sites. Results were accordingly compared against the measured GSR values. Evaluation of the annual regional global solar radiation showed that the southern and northern slopes received the maximum and minimum incoming solar radiation, respectively. It was found that the elevation differences have less effect on the incoming global solar radiation than other topographic variables such as slope, surface orientation and the obstructions.