Improving Urban Land Cover Classiﬁcation with Combined Use of Sentinel-2 and Sentinel-1 Imagery

: Accurate land cover mapping is important for urban planning and management. Remote sensing data have been widely applied for urban land cover mapping. However, obtaining land cover classiﬁcation via optical remote sensing data alone is difﬁcult due to spectral confusion. To reduce the confusion between dark impervious surface and water, the Sentinel-1A Synthetic Aperture Rader (SAR) data are synergistically combined with the Sentinel-2B Multispectral Instrument (MSI) data. The novel support vector machine with composite kernels (SVM-CK) approach, which can exploit the spatial information, is proposed to process the combination of Sentinel-2B MSI and Sentinel-1A SAR data. The classiﬁcation based on the fusion of Sentinel-2B and Sentinel-1A data yields an overall accuracy (OA) of 92.12% with a kappa coefﬁcient (KA) of 0.89, superior to the classiﬁcation results using Sentinel-2B MSI imagery and Sentinel-1A SAR imagery separately. The results indicate that the inclusion of Sentinel-1A SAR data to Sentinel-2B MSI data can improve the classiﬁcation performance by reducing the confusion between built-up area and water. This study shows that the land cover classiﬁcation can be improved by fusing Sentinel-2B and Sentinel-1A imagery.


Introduction
Rapid urbanization has several negative effects, such as heavy haze, the urban heat island effect, and the degradation of urban ecosystem services. Accurate land cover maps can contribute to land use management, urban management, and sustainable development. Remote sensing technology has become a useful tool for monitoring land cover and urban expansion. In particular, remote sensing images with a medium spatial resolution (10 m to 100 m) have been widely adopted to classify land cover types. For instance, Landsat 7 ETM+ and Landsat 8 OLI data have been widely applied for land cover mapping [1][2][3]. Sentinel-1 and Sentinel-2, as two new-generation platforms, also provide free satellite imagery [4][5][6]. The 30 m and 10 m global land cover maps are produced using Landsat TM, ETM+, and Sentinel-2 Multispectral Instrument (MSI) satellite images [7,8]. Given its fine resolution, Sentinel-2 MSI data are able to provide fine-scale land cover classification at global and regional scales. However, optical sensors are usually hampered by atmospheric conditions [9][10][11]. The geometric and dielectric properties of the Earth's surface can be observed by Synthetic Aperture Rader (SAR) data. Additionally, microwaves can penetrate clouds, haze, and smoke. As a result, SAR data are not affected by weather conditions. SAR data are found to discriminate land cover types in clouded regions. Studies have The Wuhan metropolis, which is the capital of Hubei Province, is a representative city in central China [39]. Over the past four decades, Wuhan has experienced rapid urbanization because of the opening up and reform policy. The city of Wuhan has a great influence on the Yangtze River Economic Belt. The land cover types of Wuhan mainly include built-up area, water, forest, cropland, and bare soil [40][41][42]. To avoid the uncertainty introduced by image mosaicking, only one scene of a Sentinel-2B MSI image and one scene of a Sentinel-1A SAR image are used [43]. The study area (2223 km 2 ) covers the core regions of Wuhan (Figure 1). ISPRS Int. J. Geo-Inf. 2021, 10, x FOR PEER REVIEW 3 of 16 one scene of a Sentinel-1A SAR image are used [43]. The study area (2,223 km 2 ) covers the core regions of Wuhan ( Figure 1).

Remote Sensing Data
The Landsat 8 satellite that carries the Operational Land Imager (OLI) sensor was launched on 11 February 2013. OLI has five bands in the visible and near-infrared (VNIR) region and three bands in the shortwave infrared (SWIR) region [44]. These eight bands have a 30 m spatial resolution over a 185 km swath. The Sentinel-2B satellite that carries an innovative MSI was launched on 7 March 2017. The MSI has ten bands in the VNIR region and three bands in the SWIR region. The four bands (B02, B03, B04, and B08) have a 10 m spatial resolution over a 100 km swath. In addition, the six bands (B05, B06, B07, B8a, B11, and B12) have a 20 m spatial resolution over a 100 km swath [45].
The Sentinel-1A images acquired in the Interferometric Wide Swath (IW) mode are available. The Sentinel-1A IW level-1 Ground Range Detected (GRD) image acquired on 16 October 2018 is analyzed in this research. This Sentinel-1A scene is captured at 5 m by 20 m spatial resolution, in dual-polarization (VV + VH) [46,47].

Preprocessing of Sentinel-2B MSI and Sentinel-1A SAR Data
The Sentinel-2B MSI image acquired on 12 October 2018, Landsat 8 OLI image acquired on 15 September 2018, and Sentinel-1A SAR image acquired on 16 October 2018 are analyzed in this study. The Sentinel-2B image (bands 2-7,8a, [11][12] and Landsat 8 OLI image (bands 1-7) are radiometrically corrected in ENVI 5.5.1 (the Environment for Visualizing Images). A combination of the Sentinel-1A image and corresponding orbit file is imported into SARscape 5.2.1 [21]. A Frost filter with a 9×9 window is applied to reduce the noise. The intensity values of the Sentinel-1A image are converted into sigma naught (dB) by performing radiometric calibration. The Sentinel-2B MSI image and Sen-

Remote Sensing Data
The Landsat 8 satellite that carries the Operational Land Imager (OLI) sensor was launched on 11 February 2013. OLI has five bands in the visible and near-infrared (VNIR) region and three bands in the shortwave infrared (SWIR) region [44]. These eight bands have a 30 m spatial resolution over a 185 km swath. The Sentinel-2B satellite that carries an innovative MSI was launched on 7 March 2017. The MSI has ten bands in the VNIR region and three bands in the SWIR region. The four bands (B02, B03, B04, and B08) have a 10 m spatial resolution over a 100 km swath. In addition, the six bands (B05, B06, B07, B8a, B11, and B12) have a 20 m spatial resolution over a 100 km swath [45].
The Sentinel-1A images acquired in the Interferometric Wide Swath (IW) mode are available. The Sentinel-1A IW level-1 Ground Range Detected (GRD) image acquired on 16 October 2018 is analyzed in this research. This Sentinel-1A scene is captured at 5 m by 20 m spatial resolution, in dual-polarization (VV + VH) [46,47].

Preprocessing of Sentinel-2B MSI and Sentinel-1A SAR Data
The Sentinel-2B MSI image acquired on 12 October 2018, Landsat 8 OLI image acquired on 15 September 2018, and Sentinel-1A SAR image acquired on 16 October 2018 are analyzed in this study. The Sentinel-2B image (bands 2-7,8a, [11][12] and Landsat 8 OLI image (bands 1-7) are radiometrically corrected in ENVI 5.5.1 (the Environment for Visualizing Images). A combination of the Sentinel-1A image and corresponding orbit file is imported into SARscape 5.2.1 [21]. A Frost filter with a 9×9 window is applied to reduce the noise. The intensity values of the Sentinel-1A image are converted into sigma naught (dB) by performing radiometric calibration. The Sentinel-2B MSI image and Sentinel-1A SAR image are resampled to a 20 m resolution and coregistered to the Landsat 8 OLI image. Both Sentinel-2B MSI and Sentinel-1A SAR data are spatially subset in ENVI 5.5.1 to the study area ( Figure 2). The procedure for land cover classification is illustrated in Figure 3. tinel-1A SAR image are resampled to a 20 m resolution and coregistered to the Landsat 8 OLI image. Both Sentinel-2B MSI and Sentinel-1A SAR data are spatially subset in ENVI 5.5.1 to the study area ( Figure 2). The procedure for land cover classification is illustrated in Figure 3.

Reference Data Acquisition and Accuracy Assessment
A total of 435 regions of interest (ROIs) are selected for training and validation (Table 1) [48]. Pixels representing water, forest, cropland, and bare soil are selected from the false-color composite Sentinel-2B MSI image (Figure 2a). The built-up area consists of  tinel-1A SAR image are resampled to a 20 m resolution and coregistered to the Landsat 8 OLI image. Both Sentinel-2B MSI and Sentinel-1A SAR data are spatially subset in ENVI 5.5.1 to the study area ( Figure 2). The procedure for land cover classification is illustrated in Figure 3.

Reference Data Acquisition and Accuracy Assessment
A total of 435 regions of interest (ROIs) are selected for training and validation (Table 1) [48]. Pixels representing water, forest, cropland, and bare soil are selected from the false-color composite Sentinel-2B MSI image (Figure 2a). The built-up area consists of

Reference Data Acquisition and Accuracy Assessment
A total of 435 regions of interest (ROIs) are selected for training and validation (Table 1) [48]. Pixels representing water, forest, cropland, and bare soil are selected from the false-color composite Sentinel-2B MSI image (Figure 2a). The built-up area consists of asphalt, concrete, red-tiled roofs, and metal. In urban areas, asphalt roads and old concrete roofs can be easily confused with water. It is also difficult to distinguish water from shaded areas caused by high buildings. Shaded areas caused by high buildings are common in urban areas due to the spatial resolution of the Sentinel-2B MSI image. Assisted by Google Earth imagery (available online: https://google-earth.gosur.com/ (accessed on 1 July 2021)), it is observed that shaded areas are predominantly covered by concrete. Therefore, the shaded area is not regarded as a land cover type. Google Earth is useful for selecting suitable ROIs. Therefore, pixels representing built-up area are collected from the Sentinel-2B MSI image and Google Earth images. The classifications are assessed by the overall accuracy (OA) and kappa coefficient (KA). Each land cover type is evaluated by the producer's accuracy (PA), user's accuracy (UA), and F-score.

SVM-CK Algorithm
The SVM-CK algorithm is employed to process the remote sensing data because of its great performance. The SVM-CK algorithm has shown good performance because this classifier can use spatial information. The method requires three adjustable parameters: the penalty parameter (C), the kernel parameter (σ 1 ) for the remote sensing data, and the kernel parameter (σ 2 ) for the mean texture of remote sensing data. The ranges of these three parameters correspond to: C = (10, 10 2 , 10 3 , 10 4 , 10 5 , 10 6 ), σ 1 = (2 −5 , 2 −4 , . . . , 2 5 ), and σ 2 = (2 −5 , 2 −4 , . . . , 2 5 ). A 5-fold cross-validation with a grid search method is used for tuning these parameters [49]. The training pixels are used to select the optimal parameters in MATLAB 2017a software. The validation pixels are used for accuracy assessments and statistical comparisons.

Classification with Sentinel-2B MSI Data
The SVM-CK algorithm is used to process the Sentinel-2B MSI data. The classification using the SVM-CK algorithm achieves an OA of 91.54% with a KA of 0.88 ( Table 2). The highest PA is acquired for the built-up area (97.29%), followed by water (93.42%), bare soil (90.91%), forest (82.53%), and cropland (81.91%). The producer's accuracies for the built-up area, water, and bare soil are considerably high. The PA for the built-up area is 97.29% (18 pixels of the built-up area are misclassified as water, and 2 pixels of the built-up area are misclassified as forest) ( Table 2). The UA is highest for bare soil (100%), followed by built-up area (95.22%), water (92.65%), forest (85.77%), and cropland (76.17%). The user's accuracies for bare soil, built-up area, and water are considerably high. The UA for water is 92.65% because 18 pixels of the built-up area are wrongly classified as water ( Table 2). The F-score is highest for built-up area (96.24%), followed by bare soil (95.24%), water (93.03%), forest (84.12%), and cropland (78.93%). The confusion between the built-up area and water is apparent when only the Sentinel-2B MSI data are used. A majority filter is applied to process the land cover classification ( Figure 4).

Classification with Sentinel-1A SAR Data
The SVM-CK algorithm is also used to process the Sentinel-1A SAR data. The classification using the Sentinel-1A SAR image achieves an OA of 73.09% with a KA of 0.64 (Table 3). Among the five land cover types, the largest area is covered by built-up area (29.19%), followed by forest (29.18%), cropland (22.17%), water (14.08%), and bare soil (5.38%). A noticeable underestimation of the built-up area, water, and bare soil is observed when using the Sentinel-1A SAR image alone. The above results indicate that Sentinel-1A SAR data distinguish land cover types with lower accuracy than Sentinel-2B MSI data with the same spatial resolution. The highest PA is acquired for water (91.77%), followed by forest (89.73%), bare soil (76.45%), built-up area (74.36%), and cropland (17.09%). The producer's accuracies for water, forest, and bare soil are relatively high. The PA for the built-up area is 74.36% because 4 pixels of the built-up area are classified as water, 92 pixels of the built-up area are classified as forest, 62 pixels of the built-up area are classified as cropland, and 31 pixels of the built-up area are classified as bare soil ( Table 3). The PA for cropland is less than 25%, suggesting a large omission error. The

Classification with Sentinel-1A SAR Data
The SVM-CK algorithm is also used to process the Sentinel-1A SAR data. The classification using the Sentinel-1A SAR image achieves an OA of 73.09% with a KA of 0.64 (Table 3). Among the five land cover types, the largest area is covered by built-up area (29.19%), followed by forest (29.18%), cropland (22.17%), water (14.08%), and bare soil (5.38%). A noticeable underestimation of the built-up area, water, and bare soil is observed when using the Sentinel-1A SAR image alone. The above results indicate that Sentinel-1A SAR data distinguish land cover types with lower accuracy than Sentinel-2B MSI data with the same spatial resolution. The highest PA is acquired for water (91.77%), followed by forest (89.73%), bare soil (76.45%), built-up area (74.36%), and cropland (17.09%). The producer's accuracies for water, forest, and bare soil are relatively high. The PA for the built-up area is 74.36% because 4 pixels of the built-up area are classified as water, 92 pixels of the built-up area are classified as forest, 62 pixels of the built-up area are classified as cropland, and 31 pixels of the built-up area are classified as bare soil ( Table 3). The PA for cropland is less than 25%, suggesting a large omission error. The UA is highest for water (94.49%), followed by the built-up area (90.58%), bare soil (76.45%), forest (53.91%), and cropland (23.61%). The UA for water is 94.49% because 4 pixels of built-up area and 9 pixels of cropland are wrongly classified as water ( Table 3). The user's accuracies for forest and cropland are less than 60%, showing a large commission error for these two land cover categories. The producer's and user's accuracies for the built-up area, water, and bare soil are higher than 70% (Table 3). The F-score is highest for water (93.11%), followed by built-up area (81.67%), bare soil (76.45%), forest (67.35%), and cropland (19.83%). A majority filter is applied to process the land cover classification ( Figure 5). UA is highest for water (94.49%), followed by the built-up area (90.58%), bare soil (76.45%), forest (53.91%), and cropland (23.61%). The UA for water is 94.49% because 4 pixels of built-up area and 9 pixels of cropland are wrongly classified as water ( Table 3). The user's accuracies for forest and cropland are less than 60%, showing a large commission error for these two land cover categories. The producer's and user's accuracies for the built-up area, water, and bare soil are higher than 70% (Table 3). The F-score is highest for water (93.11%), followed by built-up area (81.67%), bare soil (76.45%), forest (67.35%), and cropland (19.83%). A majority filter is applied to process the land cover classification ( Figure 5).

Classification with the Combination of Sentinel-2B MSI and Sentinel-1A SAR Data
Sentinel-2B MSI and Sentinel-1A SAR data are fused to improve land cover classification at the pixel level. The fusion of Sentinel-2B MSI and Sentinel-1A SAR data is achieved via the layer-stacking method. The Sentinel-1A SAR data are combined as two

Classification with the Combination of Sentinel-2B MSI and Sentinel-1A SAR Data
Sentinel-2B MSI and Sentinel-1A SAR data are fused to improve land cover classification at the pixel level. The fusion of Sentinel-2B MSI and Sentinel-1A SAR data is achieved via the layer-stacking method. The Sentinel-1A SAR data are combined as two bands to the Sentinel-2B MSI data; an 11-dimensional dataset is obtained. The SVM-CK technique is also employed to process the combination of Sentinel-2B MSI and Sentinel-1A SAR data. The classification utilizing the SVM-CK algorithm yields an OA of 92.12% with a KA of 0.89 ( Table 4). The above results suggest that the inclusion of Sentinel-1A data to Sentinel-2B data can increase the OA of the classification. The highest PA is acquired for the built-up area (99.73%), followed by water (98.77%), bare soil (90.08%), cropland (79.90%), and forest (77.40%). The highest UA is obtained for water (100%) and bare soil (100%), followed by built-up area (96.33%), forest (84.33%), and cropland (70.98%). The F-score is highest for water (99.38%), followed by built-up area (98%), bare soil (94.78%), forest (80.71%), and cropland (75.18%). Among the five land cover types, the largest area is covered by built-up area (43.55%), followed by forest (21.98%), water (16.41%), cropland (11.34%), and bare soil (6.72%). A majority filter is applied to process the land cover classification (Figure 6). technique is also employed to process the combination of Sentinel-2B MSI and Sentinel-1A SAR data. The classification utilizing the SVM-CK algorithm yields an OA of 92.12% with a KA of 0.89 (Table 4). The above results suggest that the inclusion of Sentinel-1A data to Sentinel-2B data can increase the OA of the classification. The highest PA is acquired for the built-up area (99.73%), followed by water (98.77%), bare soil (90.08%), cropland (79.90%), and forest (77.40%). The highest UA is obtained for water (100%) and bare soil (100%), followed by built-up area (96.33%), forest (84.33%), and cropland (70.98%). The F-score is highest for water (99.38%), followed by built-up area (98%), bare soil (94.78%), forest (80.71%), and cropland (75.18%). Among the five land cover types, the largest area is covered by built-up area (43.55%), followed by forest (21.98%), water (16.41%), cropland (11.34%), and bare soil (6.72%). A majority filter is applied to process the land cover classification ( Figure 6).

Comparison of the Classifications Using Different Machine Learning Algorithms
The performance of the SVM-CK approach is evaluated by comparing it with the SVM and KELM-CK approaches (Figure 7) [50,51]. Three different algorithms, including SVM-CK, SVM, and KELM-CK, are applied to the Sentinel-2B MSI data. The classification based on the SVM-CK algorithm obtains the highest OA (91.54%), followed by SVM (90.43%) and KELM-CK (88.09%) algorithms (Table 5). Additionally, the McNemar test indicates that the difference between classifications using the SVM-CK and SVM algorithms is statistically significant (p < 0.05, Z = 2.22). Moreover, there is a statistically significant difference (p < 0.05, Z = 4) between the classifications using the SVM and KELM-CK algorithms. The above results indicate that the SVM-CK is more accurate than the SVM and KELM-CK in terms of land cover mapping. When using the KELM-CK algorithm, 78 pixels of the built-up area are classified as water, and 14 pixels of the built-up area are classified as forest. As a result, the PA for the built-up area is 87.52% when the KELM-CK algorithm is used (Table 5). In comparison, when using the SVM-CK algorithm, 18 pixels of the built-up area are classified as water, and 2 pixels of the built-up area are classified as forest. Therefore, the PA for the built-up area is 97.29% when the SVM-CK algorithm is used ( Table 5). The results prove that it is challenging to distinguish the built-up area from water when using the Sentinel-2B MSI image alone. The influence of the three different algorithms is evaluated by a comparative experiment in Area 1 (see Figure 2). The MSI false-color composite image (band 12, 8a, 2 in RGB) of Area 1 is shown in Figure 8a. The classification results using the SVM-CK and SVM algorithms present good differentiation between the built-up area and water (Figure 8b,c). However, some pixels of dark impervious surface are incorrectly classified as water when using the KELM-CK algorithm (Figure 8d). This confusion is caused by the spectral similarity between dark impervious surface and water. The classification utilizing the KELM-CK algorithm shows more errors of commission when classifying water. The mean texture of the Sentinel-2B bands can improve the OA of classification by reducing the heterogeneity of the same land cover type. Compared with the SVM-CK classification, the classification based on the SVM algorithm contains more speckles (Figure 8b,c).

Comparison of the Classifications with Different Remote Sensing Data
The classification implemented on the Sentinel-1A SAR image shows that the Sentinel-1A SAR data can distinguish the built-up area, water, and bare soil. The backscatter coefficient (σ 0 ) values of different land cover categories are affected by many factors. Microwave is sensitive to surface roughness, which contributes to identifying land cover types [52,53]. For both VV and VH polarizations, the built-up area has the highest mean σ 0 value while the water body has the lowest mean σ 0 value. Compared with the VV polarization, weaker backscatter is observed for the five land cover categories at VH polarization ( Figure 9). The above results suggest that SAR data provide complementary information to better distinguish the built-up area, water, and bare soil. The F-scores for forest and cropland are 67.35% and 19.83%, indicating that Sentinel-1A SAR data are inefficient for discriminating forest and cropland. The results show that the backscattering information of forest and cropland is similar. When using only Sentinel-1A SAR data, forest is easily confused with cropland ( Table 3). The value of transformed divergence is 0.77 (less than 1), suggesting that these two categories can be combined. The classification achieves an OA of 81.38% with a KA of 0.74 when forest and cropland are combined as vegetation ( Figure 10). The PA and UA for vegetation are 89.21% and 69.52%, respectively. The F-score for vegetation is 78.14%, showing that Sentinel-1A SAR data have efficient performance in discriminating the vegetation category. The above results reveal the potential of Sentinel-1A SAR data in land cover classification. To further improve the classification accuracy, multi-temporal Sentinel-1A SAR data can be used [54,55].
The classification implemented on the Sentinel-1A SAR image shows that the Sentinel-1A SAR data can distinguish the built-up area, water, and bare soil. The backscatter coefficient (σ 0 ) values of different land cover categories are affected by many factors. Microwave is sensitive to surface roughness, which contributes to identifying land cover types [52,53]. For both VV and VH polarizations, the built-up area has the highest mean σ 0 value while the water body has the lowest mean σ 0 value. Compared with the VV polarization, weaker backscatter is observed for the five land cover categories at VH polarization ( Figure 9). The above results suggest that SAR data provide complementary information to better distinguish the built-up area, water, and bare soil. The F-scores for forest and cropland are 67.35% and 19.83%, indicating that Sentinel-1A SAR data are inefficient for discriminating forest and cropland. The results show that the backscattering information of forest and cropland is similar. When using only Sentinel-1A SAR data, forest is easily confused with cropland ( Table 3). The value of transformed divergence is 0.77 (less than 1), suggesting that these two categories can be combined. The classification achieves an OA of 81.38% with a KA of 0.74 when forest and cropland are combined as vegetation (Figure 10). The PA and UA for vegetation are 89.21% and 69.52%, respectively. The F-score for vegetation is 78.14%, showing that Sentinel-1A SAR data have efficient performance in discriminating the vegetation category. The above results reveal the potential of Sentinel-1A SAR data in land cover classification. To further improve the classification accuracy, multi-temporal Sentinel-1A SAR data can be used [54,55].  To reduce the confusion between the built-up area and water, Sentinel-1A SAR data are synergistically combined with Sentinel-2B MSI data to improve the land cover classification. Using the Sentinel-2B MSI image alone, 18 pixels of the built-up area are misclassified as water, and 2 pixels of the built-up area are wrongly classified as forest. With the additional use of Sentinel-1A SAR data, only 2 pixels of the built-up area are misclassified as forest. Therefore, the confusion between the built-up area and water is greatly reduced. The PA for the built-up area is 97.29% in the classification based on Sentinel-2B MSI data. The classification implemented on the combination of Sentinel-2B MSI and Sentinel-1A SAR data achieves a PA of 99.73% for the built-up area, which corresponds to the highest Z value (Z = 3.84). Compared with the classification using Sentinel-2B MSI data via the SVM-CK algorithm, the classification using the combination of Sentinel-2B MSI and Sentinel-1A SAR data does not provide a statistically significant improvement in OA (p > 0.05, Z = 1.23). Another comparative study is performed to evaluate the effectiveness of the Sentinel-1A SAR data. When using Sentinel-2B MSI alone, it is noticeable that some areas of built-up area are misclassified as water in Area 2 (Figure 11a,b). The classification result obtained from the Sentinel-2B MSI image is spatially consistent with that obtained from the combination of Sentinel-2B MSI and Sentinel-1A SAR data (Figure 11b,d). The classification derived from the Sentinel-1A SAR data does not present a reasonable depiction of urban land cover types (Figure 11c). It is obvious that some areas of water are misclassified as bare soil (Figure 11c). When using Sentinel-2B MSI data alone, water is not easily confused with bare soil (Figure 11b). The misclassification of forest and cropland is also observed (Figure 11c). In comparison, the Sentinel-2B MSI image can distinguish the forest from the cropland (Figure 11b). Compared with the classification based on the Sentinel-2B MSI image, the classification based on the Sentinel-1A SAR image contains more speckles (Figure 11b,c). To reduce the confusion between the built-up area and water, Sentinel-1A SAR data are synergistically combined with Sentinel-2B MSI data to improve the land cover classification. Using the Sentinel-2B MSI image alone, 18 pixels of the built-up area are misclassified as water, and 2 pixels of the built-up area are wrongly classified as forest. With the additional use of Sentinel-1A SAR data, only 2 pixels of the built-up area are misclassified as forest. Therefore, the confusion between the built-up area and water is greatly reduced. The PA for the built-up area is 97.29% in the classification based on Sentinel-2B MSI data. The classification implemented on the combination of Sentinel-2B MSI and Sentinel-1A SAR data achieves a PA of 99.73% for the built-up area, which corresponds to the highest Z value (Z = 3.84). Compared with the classification using Sentinel-2B MSI data via the SVM-CK algorithm, the classification using the combination of Sentinel-2B MSI and Sentinel-1A SAR data does not provide a statistically significant improvement in OA (p > 0.05, Z = 1.23). Another comparative study is performed to evaluate the effectiveness of the Sentinel-1A SAR data. When using Sentinel-2B MSI alone, it is noticeable that some areas of built-up area are misclassified as water in Area 2 (Figure 11a,b). The classification result obtained from the Sentinel-2B MSI image is spatially consistent with that obtained from the combination of Sentinel-2B MSI and Sentinel-1A SAR data (Figure 11b,d). The classification derived from the Sentinel-1A SAR data does not present a reasonable depiction of urban land cover types (Figure 11c). It is obvious that some areas of water are misclassified as bare soil (Figure 11c). When using Sentinel-2B MSI data alone, water is not easily confused with bare soil (Figure 11b). The misclassification of forest and cropland is also observed (Figure 11c). In comparison, the Sentinel-2B MSI image can distinguish the forest from the cropland (Figure 11b). Compared with the classification based on the Sentinel-2B MSI image, the classification based on the Sentinel-1A SAR image contains more speckles (Figure 11b,c).

Conclusions
Classifying land cover types in an accurate manner is very important for monitoring urban environments. Taking the city of Wuhan as a case study, this paper has shown that the combined use of Sentinel-2B MSI and Sentinel-1A SAR data can provide an accurate land cover classification. When using the Sentinel-2B MSI image alone, the land cover classification using the SVM-CK algorithm achieves an OA of 91.54% with a KA of 0.88. We further evaluate the efficiency of SVM-CK by comparing it with SVM and KELM-CK. The results suggest that the classification utilizing the SVM-CK algorithm obtains the highest OA (91.54%), followed by the SVM (90.43%) and KELM-CK (88.09%) algorithms. The SVM-CK algorithm has the ability to further improve classification performance. On certain occasions, dark impervious surface can be easily confused with water. The SVM-CK algorithm has the ability to improve classification accuracy by reducing the confusion between dark impervious surface and water. Sentinel-1A SAR data are also applied for the discrimination of the five land cover types. The classification utilizing Sentinel-1A SAR data yields an OA of 73.09% with a KA of 0.64. Compared with the classification using Sentinel-2B MSI data, the classification based on Sentinel-1A SAR data yields a lower OA. When using Sentinel-1A SAR data alone, forest can be easily confused with cropland. The classification achieves an OA of 81.38% with a KA of 0.74 when forest and cropland are merged. The results show that Sentinel-1A SAR data can identify built-up area, water, vegetation, and bare soil. This research demonstrates that Sentinel-1A SAR data have the potential to classify different land cover types. Thus, Sentinel-1A SAR data can be used as an independent source when optical data are affected by weather conditions. Sentinel-1A SAR data have great advantages in monitoring

Conclusions
Classifying land cover types in an accurate manner is very important for monitoring urban environments. Taking the city of Wuhan as a case study, this paper has shown that the combined use of Sentinel-2B MSI and Sentinel-1A SAR data can provide an accurate land cover classification. When using the Sentinel-2B MSI image alone, the land cover classification using the SVM-CK algorithm achieves an OA of 91.54% with a KA of 0.88. We further evaluate the efficiency of SVM-CK by comparing it with SVM and KELM-CK. The results suggest that the classification utilizing the SVM-CK algorithm obtains the highest OA (91.54%), followed by the SVM (90.43%) and KELM-CK (88.09%) algorithms. The SVM-CK algorithm has the ability to further improve classification performance. On certain occasions, dark impervious surface can be easily confused with water. The SVM-CK algorithm has the ability to improve classification accuracy by reducing the confusion between dark impervious surface and water. Sentinel-1A SAR data are also applied for the discrimination of the five land cover types. The classification utilizing Sentinel-1A SAR data yields an OA of 73.09% with a KA of 0.64. Compared with the classification using Sentinel-2B MSI data, the classification based on Sentinel-1A SAR data yields a lower OA. When using Sentinel-1A SAR data alone, forest can be easily confused with cropland. The classification achieves an OA of 81.38% with a KA of 0.74 when forest and cropland are merged. The results show that Sentinel-1A SAR data can identify built-up area, water, vegetation, and bare soil. This research demonstrates that Sentinel-1A SAR data have the potential to classify different land cover types. Thus, Sentinel-1A SAR data can be used as an independent source when optical data are affected by weather conditions. Sentinel-1A SAR data have great advantages in monitoring urban flooding when the city of Wuhan is hit by a flood. Sentinel-1A SAR data can provide timely information for land cover classification. The built-up area can be confused with water when using Sentinel-2B MSI data alone. To reduce the confusion between the built-up area and water, Sentinel-1A SAR data are synergistically combined with Sentinel-2B MSI data for land cover mapping. The classification implemented on the combination of Sentinel-2B MSI and Sentinel-1A SAR data achieves an OA of 92.12% with a KA of 0.89. The methodology proposed in this study can be generalized to other areas worldwide. In addition, the SVM-CK algorithm can be applied to other applications, such as crop-type classification, vegetation type classification, and flood monitoring. The above results confirm that the fusion of Sentinel-2B MSI and Sentinel-1A SAR data efficiently improves land cover classification in cloud-prone regions.