Urban Ecological Security Assessment Based on Sentinel-2A and Landsat 8 Image Data: A Case Study in Surakarta City and its Surroundings

A healthy ecosystem is the foundation of social and economic development. It is crucial to conduct ecological security assessments in rapidly urbanizing areas. Surakarta City is one of the cities in Central Java undergoing significant growth, driven by an increasing population year by year. This has led to changes in land use accompanied by shifts in environmental quality. This research aims to assess the urban ecological security of Surakarta City and its surroundings using remote sensing technology from satellite systems. The research methodology employs Sentinel-2A and Landsat 8 TIRS imagery to extract indicators of urban ecological security. The modeling method utilizes Principal Component Analysis (PCA). The findings of this study provide the distribution of urban ecological security levels within the research area.


Introduction
The ecological security of a nation serves as a crucial foundation of its national security system [1].Ecological security pertains to the extent to which humans are protected from ecological damage and environmental pollution [2].Rapid global economic development has led to various ecological issues, such as resource scarcity, declining habitat quality, and climate change [3].Environmental degradation due to conflicts related to regional resource issues, accompanied by global ecological problems, can ultimately impact the economic and political security of the region [4].Consequently, ecological security has become integral to national strategies in various countries, equivalent to national defense, economic, and food security fields [2].Ecological security can connect natural, social, and even economic systems, making it an intriguing research field within geography and ecology [5].
Surakarta City is one of the rapidly developing cities in Central Java, leading to an increasing demand for land for construction [31,32].Non-built land is transforming into built-up areas while available land is diminishing.Limited urban land has caused the city to expand outward, and if this growth is uncontrolled, it can lead to urban sprawl.Rampant urban sprawl phenomena can result in a decline in ecological and environmental quality [33].Another impact of urban growth is the reduction of green open spaces, leading to decreased environmental quality and causing the city to become increasingly heat-prone [34].Urban Heat Islands (UHI), urbanization, and climate change are interrelated, resulting in various environmental consequences (such as heat stress, loss of biodiversity, and air quality deterioration) at both local and regional levels [35].The high density of buildings, concrete surfaces, and limited green spaces in urban areas can contribute to the urban heat island effect.This phenomenon causes higher temperatures in cities than in surrounding rural areas, leading to increased energy consumption and discomfort for residents.
In addition to rapid land use changes, Surakarta City's location in low-lying areas, basins and traversed by the Bengawan Solo River also makes it prone to flooding [36,37].These floods affect 26 neighborhoods in 5 sub-districts within Surakarta City, with varying levels of vulnerability from low to high [36].
The effects of Urban Heat Islands, urbanization, and disasters can all significantly impact ecological security, referring to an ecosystem's sustainable ability to support diverse species and maintain its ecological functions and services.An urban ecological security assessment can comprehensively analyze threats occurring in Surakarta City using urbanization, ecological status, thermal comfort, and vegetation proportion indicators.Addressing these issues is crucial to maintain ecological security and ensure the long-term sustainability of the ecosystem.This research aims to determine urban ecological security distribution in Surakarta and its surroundings.

Study Area and Data Processing
The research area encompasses Surakarta City and its vicinity (Figure 1).Surakarta City, also known as "Solo," is a relatively large city on Java Island that supports neighboring significant cities such as Yogyakarta and Semarang.Geographically, Surakarta City is situated between 110° 45' 15" to 110° 45' 35" East Longitude and 7° 36' to 7° 56' South Latitude.Topographically, Surakarta City is characterized by lowland areas with elevations ranging from 80 to 130 meters above sea level and slope gradients between 0% to 15%.The city is also surrounded by several volcanoes and mountain ranges: Mount Merapi and Merbabu to the west, Mount Lawu to the east, and the Sewu Mountain Range to the south.The climate in Surakarta City is tropical, with temperatures ranging from 25.7°C to 28.7°C.The highest rainfall occurs in February, while the highest humidity is recorded in January and March [38].
This study utilized Sentinel-2A image data acquired on May 4th, 2018, and May 28th, 2023, along with Landsat 8 TIRS image data acquired on May 28th, 2023.The multitemporal Sentinel imagery was employed to derive urban expansion intensity index parameters, while Landsat 8 TIRS data were used to obtain urban thermal field variation index (UTFVI) parameters.The Sentinel 2A and Landsat 8 TIRS images underwent initial processing involving radiometric calibration, radiometric correction, and geometric correction.Radiometric correction employed the Top of Atmosphere (TOA) reflectance method and atmospheric correction using the Dark Object Subtraction (DOS1) method.Subsequently, the geometric correction was performed using an image-to-image with quadratic polynomial transformation and a nearest-neighbor method to achieve an error level of less than 1 pixel.All images use a pixel size of 20 meters.
The research area covers the city of Surakarta and its surroundings in a rectangular shape.The road network data is sourced from the 1:25,000 scale Rupa Bumi Indonesia (RBI) Map the Geospatial Information Agency issued.The rectangular research area covers Surakarta City and its surroundings 3 because Surakarta City has been saturated with built-up land and has grown beyond its administrative boundaries, so it is necessary to expand the observation area to obtain a more comprehensive analysis.

Methods
The urban ecological security assessment considers socioeconomic, urbanization, and environmental parameters [39].The selected parameters are based on previous studies.This research chooses seven factors from three components: urbanization, environmental and ecological, and socioeconomic.The dynamic land use component encompasses the urban expansion intensity index and development intensity index.The environmental and ecological component consists of the Normalized Difference Built-Up Index (NDBSI), Normalized Difference Moisture Index (NDMI), vegetation proportion, and Urban Thermal Field Variation Index (UTFVI).Meanwhile, the socioeconomic component involves road density.Each of these parameters will be elucidated as follows:

Classification of LULC
The satellite images rectified for both years (2018, 2023), with the 2023 image, were classified using a Support Vector Machine (SVM) classifier.The SVM method is a hybrid classification method capable of extracting land cover patterns by separating data points in feature space through a hyperplane [40].The primary classification output used in this study is the built-up land class, which achieves an overall classification accuracy (>90%) based on a reference dataset collected from field surveys (July 2023) and the Google Earth database.The interpretation of the built-up land in 2018 was derived by subtracting the buildings in 2023 while referencing the 2018 image.

Urban Expansion Intensity Index
This approach offers tangible proof of the impact of population dynamics and management consequences on the land system, enabling an assessment of the immediate effects of urbanization [41].This stage requires land use maps for the years 2018 and 2023.The algorithm utilized is as follows [42]:

City Development Intensity Index
The Urban Development Intensity Index assesses the current anthropogenic disturbance intensity status resulting from urbanization [42].This index requires the calculation of building density within the research area.This parameter is constructed from the estimated building density using the Normalized Difference Built-up Index (NDBI), which is then proportionally adjusted based on the total area of a specific spatial unit.
where, CDIIi = City Development Intensity Index Di = The proportion of urban built-up area within sampling block i during a designated monitoring timeframe TAᵢ = stands for the total area of the unit under consideration.

Dryness index
The Normalized Difference Build-up and Soil Index (NDBSI) can assess the disruption of natural landscapes due to rapid urban expansion and exposure of vacant land [43][44] [45].

Moisture index
The Normalized Difference Moisture Index (NDMI) monitors drought stress and vegetation moisture content.Investigating vegetation as a component of the urban system enables the control and modeling of biophysical processes within the urban environment [46] [47].

Urban Thermal Field Variation Index (UTFVI)
This index is derived from the Land Surface Temperature (LST) values.It is one of the most common methods for detecting urban heat islands (UHI) [48] and revealing UHI's ecological impacts.The urban thermal zones are determined using UTFVI, the following equation [49]: where Tlst is land surface temperature, and Tmean is the mean LST of the area.

Proportion of vegetation (Pv)
Pv represents the vegetation proportion and can be derived from the Normalized Difference Vegetation Index (NDVI) as Equation  = ( −    −  ) 2  (8)

Density of road
The country's infrastructure and amenities, like its road networks, can be employed to assess human impact on ecosystems [50].Elevated population density implies a greater concentration of road networks, accompanied by heightened transportation frequency; furthermore, notable alterations signify more recurrent human engagements.Road data are sourced from the Rupa Bumi Indonesia (RBI) Map of 1: 25,000 scale.The map indicating density was created using ArcGIS 10.8 software and the line density tool.

Urban Ecological Security Index
study develops the Urban Ecological Security Index (UESI) model using the Principal Component Analysis (PCA) method with seven parameters.The PCA approach can reduce the effect of interdependence between evaluation indexes/indicators and reduce the burden of index selection.However, there are occasional weighting variations among indicators, which may not necessarily reflect the true importance of each parameter.The PCA method is often used to model remote sensing data.UESI values range from 0 to 1, where values close to 1 indicate ecological security, while values close to 0 indicate ecological insecurity.
Where ri represents the variance attributed to the principal component (PC), and i indicates the number of retained PCs.The computation of the contribution ratio ri is conducted subsequently: =   ∑    =1 (10) where ei represents the eigenvalue of PCi [50].

Ecological Parameters
The spatial distribution of seven ecological parameters in Surakarta City and its surrounding areas (Figure 2) reveals that the highest level of urban development intensity is located in the southern part of Surakarta City, particularly around the Pasar Kliwon Subdistrict.Meanwhile, urban expansion intensity is predominantly directed outward from Surakarta City, especially towards east-southeast.Agricultural land exhibits high moisture, while urban areas have low moisture content.The analysis of the NDBSI indicates that relatively dry conditions characterize a significant portion of the study areas.The timing of image acquisition heavily influences the conditions.In this specific study, the imagery was obtained during the dry season, leading to the prevalence of dry paddy fields across agricultural land.Surakarta City's vegetation proportion is notably low, particularly in the city center.This is attributed to the natural land being extensively replaced by built-up areas.As inferred from the UTFVI values, thermal comfort conditions depict that urban areas experience poor thermal comfort.In contrast, areas outside the urban core exhibit increasing comfort.More vegetation can enhance thermal environmental quality, promoting greater comfort.The highest road density is observed in the Banjarsari

Characteristics of Urban Ecological Security
Remote sensing can provide real-time data and effective large-scale monitoring for assessing regional eco-environmental conditions.In this study, urban ecological security is assessed using the PCA method, which can extract significant information from multi-factor environmental datasets and evaluate urban ecological security on a medium scale.The results of the PCA analysis that formulate the UESI values are presented in Table 1.The values of UESI in the study area are obtained from Table 1 as follows: The higher UESI value indicates that the ecological conditions are improving and vice versa (Figure 3).The spatial distribution of the urban ecological security index indicates that the conditions are highly insecure in the central part of Surakarta City, and the farther one moves away from the city center, the better the ecological security conditions become.Moderately secure conditions are often found on the city outskirts, which serve as buffer zones between settlements and agricultural land.On the other hand, the highly secure level pertains to vegetated areas such as mixed gardens and plantations.The moderately secure level requires greater attention because it is the most susceptible to change; the high urban expansion intensity index indicates this.This index signals the rapid changes taking place in the region.
Concerning the parameters constituting this UESI, the very insecure conditions are primarily influenced by the City Development Intensity Index, road density, and the Urban Thermal Field Variation Index (UTFVI).The City Development Intensity Index is an indicator used to assess urban ecological security.This index reflects the density of buildings within urban areas.A higher building density indicates the loss of other land cover or usage elements, mainly green open spaces and water bodies.High building density also affects potential temperature, average radiant temperature, and outdoor thermal comfort [52].The urban surroundings and nearby ecosystems are progressively declining due to the speedy pace of urban development [53].
Meanwhile, UTFVI is an index that portrays the quality of urban health and ecology in terms of the level of thermal comfort present in the urban environment.It evaluates environmental health related to ecological thermal comfort, considering the Urban Heat Island (SUHI) effect.A low Urban Thermal Factor of Vegetation Index (UTFVI) value indicates poor thermal comfort levels, and this is due to the high proportion of built-up land combined with the lack of green open space.Conversely, outside urban residential areas, the UTFVI value increases, signifying an improvement in thermal comfort levels.In a spatiotemporal context, UTFVI can demonstrate how the increase in building density in metropolitan areas contributes to the deterioration and warming of the ecosystem [54].The dense road network in this area indicates significant activity.The thickness of roads is closely linked to increased population density, heightened human engagement, and elevated socioeconomic endeavors.This heightened road density offers frequent transportation options and convenient facilities for human activities.However, these amplified transportation and socioeconomic concentrations adversely influence the ecological stability [50].

Conclusion
The findings indicate that the central region of Surakarta City requires enhanced ecological protection.Ecological soundness becomes more evident as one moves farther from the urban core.Moderate security levels are observed at the city's periphery, functioning as a protective buffer.In contrast, the most robust ecological security is characterized by abundant vegetation.It is essential to highlight that the moderately secure zone represents a dynamic area susceptible to alterations, evident through its high urban expansion index.
The Urban Ecological Security Index (UESI) is mainly influenced by the City Development Intensity Index, road density, and the Urban Thermal Field Variation Index (UTFVI) parameters.This study underscores the intricate interplay among urban development, ecological dynamics, and thermal comfort.Attaining sustainable urban planning necessitates a delicate equilibrium between urban growth and environmental well-being.

Acknowledgments
This research was supported by the UGM Vocational College Community Fund research activity in 2023, for which we thank you.

UESI = 1 -Figure 3 .
Figure 3. Spatial distribution of Urban Ecological Security in Surakarta City and its surroundings