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Urban sprawl is one of the significant issues faced by cities in Sri Lanka today. Urban sprawl is unplanned and uneven pattern of growth, driven by a multitude of processes and leading to inefficient resource utilisation. This study evaluates urban sprawl and its impact on rural land masses of Colombo district, Sri Lanka in 1997, 2009 and 2018 using Shannon’s entropy values. Maximum likelihood supervised classification was applied for Landsat 8 OLI/TIRS (Operational Land Imager/Thermal Infrared Sensor 2018) and Landsat 5 TM (Thematic Mapper 1997, 2009) satellite images to get the build- up areas and then the Shannon’s entropy values using GIS were assessed. Calculated built-up areas were 98.97 (14.6 %), 178.76 (26.38%), 276.80 (40.85%) km 2  and Shannon’s entropy values were 0.928, 1.009 and 1.059 for the years 1997, 2009 and 2018, respectively. Since Shannon’s entropy is an efficient indicator to measure urban sprawl, it is observed that Colombo district continues to sprawl from 1997 to 2018. The analysis of the results further showed that rapid increase of built-up area resulted in decrease in vegetation and agricultural areas. Calculated relative entropy values were 0.83, 0.91 and 0.95 for consecutive years, which has an increasing trend and exceeded the threshold value of 0.5 indicating a higher level of urban sprawl in Colombo district. Developed maps show that since 1997, the city has experienced pronounced urban sprawl along the main roads of Colombo district consuming rural lands at a faster pace as population shifts from urban areas. Prioritised policies in Government’s public investment programme 2017–2020 for sustainable urban development may help concentrate growth within targeted areas and restrain sprawling development in rural settings of Colombo district. Further, similar studies are encouraged to check the effectiveness of applying policies.

Assessment of Urban Sprawl and Its Impacts on Rural Landmasses of Colombo District: A Study Based on Remote Sensing and GIS Techniques