In this article, horizontal global solar radiation and daylight illumination level measurements from Istanbul are analysed. The measurements were made at a central meteorological station near the Bosphorus, which is used as the reference location. Global solar radiation data collected at an additional seven stations near this site were also analysed and correlated with the reference point values. Due to their respective locations, observations at several of these stations exhibit characteristics typical of rural areas within the city, whereas two of them exhibit characteristics associated with urban areas. The illumination values for stations for which measurements were not made are estimated using the correlations between the illumination level at the reference station and the solar radiation received at the other locations. Representative linear regression analysis is performed. The measured and estimated values are used to evaluate the spatial properties of illumination levels in and around Istanbul using the kriging method. The daylight illumination level varies strongly with the seasons, especially in the southern parts of Istanbul, where the city centre is located. Using the strong correlation between daylight illumination and global solar radiation at these stations, the spatial variability of illumination in this region is evaluated and presented in the form of maps. Global solar radiation and daylight illumination levels tend to decrease from south to north in and around Istanbul. Due to the complex physical effects associated with urban surroundings, large deviations from solar radiation and daylight illumination values that are relatively consistent across the other stations are observed for the two stations located in the centre of Istanbul.