## 1 Introduction  
Urban areas have expanded at an accelerated speed during the last five decades, and rates of urban population growth are higher than the overall growth in most countries [1; 2; 3]. The accelerated development of urban areas and the anthropic pressure on the natural environment is becoming more and more prevalent recently due to population growth, resource depletion, and a modern, service-oriented economy. This has brought about many negative environmental repercussions to the world; for example, less precipitation, more dryness, higher temperatures [4; 5], and increasing the probability of man-made landslides [6; 7; 8; 9]. The conversion of the natural lands into urban areas, dominated by built-up and other associated lands, may have significant impacts on the ecosystem, which can result in negative aspects such as the urban heat island phenomenon and, on a larger scale, as global warming [10; 11]. Land use changes in urban areas usually occur because of high urbanization and residential development rates, causing the suburbanization process and consequently the urban sprawl (generally known as uncontrolled urban development), which implies negative effects on human communities, such as: overcrowding, traffic congestion, open spaces loss, increasing pollution, poor housing quality, etc. [12]. Built-up area delineation is a common challenge in terms of monitoring urban development and measuring urban sprawl. This action is focused on accurately drawing the border between the built-up area of urban and rural settlements and the natural landscape. Urban environment and built-up area delimitation are subjects frequently approached in the European Union documents: EEA Report No 11/2016 regarding Urban sprawl in Europe, Urban Agenda for the EU, SOER 2015--The European environment, Europe 2020 strategy and the Environment Action Programme to 2020 [12]. Most scientific studies that analyze different aspects of urban sprawl highlights, more or less, the causal relationship between the built-up areas and the urban sprawl phenomenon [13; 14; 15; 16]. In this sense, one of the best-documented and well-known pieces of research was elaborated by [PERSON] and [PERSON] [17], and according to their publication, the urban sprawl degree for any given territory, results and can be quantified by combining three essential components: (1) the size of the built-up areas; (2) the spatial configuration (dispersion) of the built-up areas in the landscape; (3) the uptake of built-up area per inhabitant or job. All these measures are reunited into the Weighted Urban Proliferation (WUP) index for urban sprawl quantification; this method also used by the European Environment Agency (EEA). Under these circumstances, the study of urban spatial expansion is very important for both urban and regional development planning, and always needs accurate and up-to-date data on built-up areas, such as: size, shape (geometry), pattern, spatial context, growth trend, and development directions. Mapping urban land in a timely and accurate manner is indispensable for quality spatial planning and rational territorial management. Identification of the patterns of sprawl and analyses of spatial and temporal changes would help immensely in the planning for proper infrastructure facilities [18; 19]. Over the past two decades, researchers have become increasingly interested in using remotely sensed imagers to address urban and suburban problems and various planning applications, such as: land use changes analysis and predictions [20; 21; 22], natural risks assessment and management [23; 24; 25; 26], urban heat island effect [27; 28; 29; 30; 31], transportation system management [32], recognizing zones susceptible to pollution, and for the purpose of crop mapping in agricultural areas [33].