New Ensemble Models for Shallow Landslide Susceptibility Modeling in a Semi-Arid Watershed

: We prepared a landslide susceptibility map for the Sarkhoon watershed, Chaharmahal-w-bakhtiari, Iran, using novel ensemble artiﬁcial intelligence approaches. A classiﬁer of support vector machine (SVM) was employed as a base classiﬁer, and four Meta / ensemble classiﬁers, including Adaboost (AB), bagging (BA), rotation forest (RF), and random subspace (RS), were used to construct new ensemble models. SVM has been used previously to spatially predict landslides, but not together with its ensembles. We selected 20 conditioning factors and randomly portioned 98 landslide locations into training (70%) and validating (30%) groups. Several statistical metrics, including sensitivity, speciﬁcity, accuracy, kappa, root mean square error (RMSE), and area under the receiver operatic characteristic curve (AUC), were used for model comparison and validation. Using the One-R Attribute Evaluation (ORAE) technique, we found that all 20 conditioning factors were signiﬁcant in identifying landslide locations,


Introduction
Landsliding is a complex natural phenomenon and is responsible for considerable loss of life and damage to engineered infrastructure worldwide [1]. Considerable efforts are being made to understand the causes and triggers of landslides to reduce these losses [2]. Considerable research has been carried out to identify and classify terrain that is susceptible to landslides. A common key strategy is to prepare a landslide susceptibility map that shows areas that are prone to landslides within a region of interest. Such maps can provide understandable and useful information that helps planners better manage susceptible areas [3][4][5]. As yet, there is no universal approach for preparing landslide susceptibility maps, thus landslide researchers are experimenting with new methods and techniques [6]. Novel quantitative approaches proposed by, researchers in recent years include logistic regression [7][8][9], multivariate regression [10][11][12], discriminant analysis [13,14], certainty factor [15,16], index or entropy (IOE) [17,18], spatial multi-criteria evaluation (SMCE) [19], statistical index [20,21], and analytic hierarchy processes [22,23].
Machine learning models are dissimilar to hydrological models that are based on hydro-geomorphological conditions within a single watershed and must be calibrated for use in other watersheds. Machine learning algorithms are not subject to this limitation; rather they are applicable in all situations. The algorithms have different distribution functions, which result in different performance. This characteristic of machine learning models has led to the development of several algorithms, with the best ones chosen to prepare susceptibility maps for specific regions. Differences in the performance of machine learning algorithms stem from differences in inputs (conditioning factors) to the model, because the distribution function of a particular algorithm is constant. Conditioning factors differ regionally; therefore, the performance of a particular algorithm is also different. Important differences relate to climate and land use, including earthworks (cuts and fills), urbanization (buildings and roads), and altered drainage [70,71]. Climate change can change the occurrence and intensity of extreme precipitation events, and hence may affect the occurrence of rainfall-triggered landslides [72][73][74][75][76]. A method for accurately predicting the spatial distribution of landslides is one of the controversial researched subjects among landslide researchers. The more the accurate method, the lower the damages from landslides will be. Indeed, with a reliable and timely landslide prediction, fatal events will be reduced and infrastructure better protected. Newly developed landslide prediction models have been proposed for use around the world. However, it is widely acknowledged that model uncertainties preclude the universal application of a single model. Therefore, for a given case study, powerful and flexible models should be tested and evaluated to generate the most accurate landslide susceptibility map possible. The primary objective of our research is to increase knowledge of landslide susceptibility mapping by introducing some novel machine learning ensemble approaches that combine the SVM algorithm as a base classifier with the AB, BA, RF, and RS ensemble models. The SVM is a powerful and functional algorithm that can be used as a benchmark model when a new model is developed and tested. Although it has been widely applied in landslide susceptibility mapping, SVM ensembles have not previously been used. We assess the performance of our proposed new ensemble models with a variety of statistical metrics and the AUC.

The Study Area
The study area is the Sarkhoon watershed in the Chahar Mahaal and Bakhtiari province, Iran ( (Figure 1). The watershed has an area of about 187 km 2 and ranges in elevation from 1370 m to 3376 m above sea level. The mean slope angle is 22.6 degrees, but some slopes are nearly vertical. The Sarkhoon watershed is one of several catchments that drain to the Persian Gulf via the Karoon River. Precipitation data provided by the regional water company of Chaharmahal-Bakhtiari province indicate that the average annual rainfall is about 523 mm in a semi-arid region at the mouth of the catchment and 1360 mm in its headwaters; the average for the entire watershed is 848 mm/year. Nearly 77% of the watershed is pasture land; the remainder includes dense forest (13.6%), semi-dense forest (3.4%), dry-farming land (2.4%), and barren land (0.7%). The area is dominated by sedimentary rocks of Cretaceous age, including marl of the Mishan Formation, sandstone and marl of the Aghajari Formation, conglomerate and sandstone of the Bakhtiari Formation, dolomite of the Jahrum Formation, limestone of the Tarbur Formation, limestone of the Sarvak-Illam Formation, and marl and limestone of the Gurpi Formation (Table 1). Quaternary alluvium is found on terraces and along floodplains and channels [58]. Our inventory of 98 landslide points included 55 transnational slides, 22 complex landslides and 21 rotational slides ranging in size from 100 to 60,000 m 2 ( Figure 2). includes dense forest (13.6%), semi-dense forest (3.4%), dry-farming land (2.4%), and barren land (0.7%). The area is dominated by sedimentary rocks of Cretaceous age, including marl of the Mishan Formation, sandstone and marl of the Aghajari Formation, conglomerate and sandstone of the Bakhtiari Formation, dolomite of the Jahrum Formation, limestone of the Tarbur Formation, limestone of the Sarvak-Illam Formation, and marl and limestone of the Gurpi Formation (Table 1). Quaternary alluvium is found on terraces and along floodplains and channels [58].

Materials and Methods
We schematically summarize the workflow of our study in Figure 2. The major steps in this procedure are 1) data collection and processing (input), 2) modeling, and 3) evaluation.

Figure 2.
Examples of landslides in the study area [58].

Data Collection and Processing
We prepared a landslide inventory map to determine relationships between the spatial pattern of landslides and different conditioning factors [52]. We identified 27 landslide polygons using the archived 1:20,000-scale aerial photograph of the study area from Natural Cartographic Center of Iran  [58].

Datasets
We prepared a landslide inventory map to determine relationships between the spatial pattern of landslides and different conditioning factors [52]. We identified 27 landslide polygons using the archived 1:20,000-scale aerial photograph of the study area from Natural Cartographic Center of Iran (NCC) (Figure 1). We then determined their exact locations in the field with a global positioning system (GPS). The polygons of landslides were converted to the points using "feature to point" in ArcGIS 10.2 environment, with a focus on the center point of landslide scarps. An additional 71 landslides have previously been identified by natural resource managers employed by Chaharmahal and Bakhtiary province. These features were located through field survey. We digitized all 98 landslide points to produce the landslide inventory map (Figure 1 In addition to landslide locations, a sufficient number of non-landslide locations are required to produce a landslide susceptibility map. We employed a random sampling strategy [77][78][79] to select a total of 98 points without landslides. These points include stable slopes, plateaus, and flood plains with different characteristics that are distributed uniformly in the Sarkhoon watershed. We next divided the landslide and non-landslide locations into a training dataset (70% of the sites) and a validation dataset (30% of the sites).

Landslide Conditioning Factors
We chose 20 conditioning factors for modeling based on our knowledge of the study area, a literature review, and expert opinion. They include land cover/use, lithology, rainfall, elevation, slope angle, aspect, sediment transport index (STI) or slope length and steepness factor of USLE, general curvature, profile curvature, plan curvature, longitudinal curvature, tangential curvature, solar radiation, stream power index (SPI), topographic position index (TPI), topographic wetness index (TWI), terrain roughness index (TRI), distance to rivers, distance to roads, and distance to faults. The land cover/use map has seven classes: dry farm land, semi-dense forest, dense forest, semi-dense pasture, dense pasture, and residential areas. This map was extracted from Landsat 8 (25 April 2016) with a supervised machine learning classification algorithm of support vector machine (SVM). We constructed the lithology and fault maps from 1:100,000-scale geology maps prepared by Geological Survey and Mineral Explorations of Iran (Ardales and Dehdez sheets). Ten geologic units are included on the lithology map (Table 1). A 50-m buffer was added to mapped faults to prepare the distance-to-fault map. The annual rainfall map was constructed from 42 years of rainfall data (1972-2014) from nine meteorological stations, which were supplied by the regional water company of Chaharmahal-Bakhtiari province. We extrapolated the data spatially based on the elevations of the meteorological stations. The geomorphological factor maps were derived from a 12.5-m Digital Terrain Model (DTM) extracted from ALOS PALSAR data that were provided by the Alaska Satellite Facility (https://vertex.daac.asf.alaska.edu/#). The elevation, slope angle, aspect, STI/LS, general, profile, plan, longitudinal and tangential curvatures, solar radiation, SPI, TPI, TWI, TRI, and distance to river maps were constructed from the DEM using ArcGIS 10.2 and SAGA 6.0.0 software. The distance-to-road map was constructed from the road network provided by the Iranian National Cartographic Center (INCC) in DGN format and at a scale of 1:25,000 ( Figure 3).

Factor Selection Using the One-R Attribute Evaluation (Orae) Technique
Factor or feature selection is an important part of machine learning modeling. Feature selection techniques affect model performance by increasing data quality, which results in improved comprehension of the machine learning algorithm results [80]. These techniques decrease the dimensionality of feature space, prevent redundancy, and decrease noise and over-fitting problems [81]. Wrapper and filter methods are available for selecting the most important features or factors for modeling. Machine learning algorithms can use wrapper methods to make factor accuracy estimates. Filter methods use statistical correlation between a set of factors and the target factor to select the most important factors. In general, filter methods are faster and more scalable than wrapper techniques, and also have lower computational complexity [81]. We used a filter technique, One-R Attribute Evaluation (ORAE), in this study. ORAE as a simple feature-selection algorithm first proposed by Holte [82]. For each factor, one rule (one-R) in the training dataset is separately constructed, and the rule with the smallest error metric is selected for modeling. We computed error metrics for each factor and each factor value. Based on the smallest calculated error metric, ORAE will independently sort all factors [83,84]. Figure 4 shows the code of the One-R algorithm for factor selection.

Factor Selection Using the One-R Attribute Evaluation (Orae) Technique
Factor or feature selection is an important part of machine learning modeling. Feature selection techniques affect model performance by increasing data quality, which results in improved comprehension of the machine learning algorithm results [80]. These techniques decrease the dimensionality of feature space, prevent redundancy, and decrease noise and over-fitting problems [81]. Wrapper and filter methods are available for selecting the most important features or factors for modeling. Machine learning algorithms can use wrapper methods to make factor accuracy estimates. Filter methods use statistical correlation between a set of factors and the target factor to select the most important factors. In general, filter methods are faster and more scalable than wrapper techniques, and also have lower computational complexity [81]. We used a filter technique, One-R Attribute Evaluation (ORAE), in this study. ORAE as a simple feature-selection algorithm first proposed by Holte [82]. For each factor, one rule (one-R) in the training dataset is separately constructed, and the rule with the smallest error metric is selected for modeling. We computed error metrics for each factor and each factor value. Based on the smallest calculated error metric, ORAE will independently sort all factors [83,84]. Figure 4 shows the code of the One-R algorithm for factor selection.

Support Vector Machine (Svm) Algorithm
Support Vector Machine (SVM), introduced by Vapnik and Guyon [85,86], is a machine learning classifier that has been used to solve many real world problems, including landslide prediction [49,87], flood prediction [88], and forest fire prediction [89]. It is based on the principle of structural risk minimization of statistical learning theory to reduce the error and the complexity of computation. In using the SVM, an optimal hyper-plane is constructed in order to separate two classes and one class, assigned as "1", is located above the hyper-plane and the other, assigned "0", is located below it.

Support Vector Machine (Svm) Algorithm
Support Vector Machine (SVM), introduced by Vapnik and Guyon [85,86], is a machine learning classifier that has been used to solve many real world problems, including landslide prediction [49,87], flood prediction [88], and forest fire prediction [89]. It is based on the principle of structural risk minimization of statistical learning theory to reduce the error and the complexity of computation. In using the SVM, an optimal hyper-plane is constructed in order to separate two classes and one class, assigned as "1", is located above the hyper-plane and the other, assigned "0", is located below it.

Ensemble/Meta Classifier Algorithms
We used the following meta-classifier algorithms ( Figure 5): AdaBoost (AB), Bagging (BA), Random Subspace [1], and Rotation Forest (RF). Each of these meta-classifiers was first processed with a classifier and then with an ensemble. Model development and evaluation were done with ArcGIS 10.2 and Weka 3.9 software. We schematically summarize the workflow of our study in Figure 5. The major steps in this procedure are (1) data collection and processing (input), (2) modeling, and (3) evaluation.  AdaBoost is a popular data-boosting algorithm involving iterative training of a series of individual classifiers in an ensemble [90]. It can provide solutions for two-class, multi-class singlelabel, multi-class multi-label, single label category, and regression problems [91]. We followed the approach of Hong et al. [19] and used AdaBoost to adaptively resample data in choosing training samples. We used successive iterations to assign weights to the data, such that the subsequent iteration incorporates reweighted data that were previously misclassified. This approach provided us with a weighted sum of ensemble predictions.
Bagging is a bootstrap sampling technique used with ensemble methods [92]. We used BA to randomly sample and replace data while generating multi-sample training subsets. Generated subsets were used to create a decision tree and later incorporate them into the final model. We used this technique to improve our classification accuracy by decreasing the variance of classification errors.
We used the random subspace algorithm as our third classifier. Boot [93] reported that averaging forecasts constructed from many randomly selected fixed-size subsets substantially lower the mean squared forecast error. Instead of selecting subsets of available predictors, random projection regression creates a low-dimensional subspace by averaging over-predictors using random weights drawn from a normal distribution. In this study, we followed the methodology of Pham et al. [94], who used the RS method to assess landslide hazards in Vietnam.
Rotation Forest is an ensemble technique developed by Rodriguez et al. [95] to improve the predictive capability of single classifiers. We trained the RF model by sequentially: 1) deriving subsets after dividing the attribute sets; 2) resampling the sample subsets and transforming features on the generated subsets; 3) realigning the rotation matrix according to the sequence of original attribute sets; 4) training base classifiers using the rotated sample subsets; and 5) integrating the results of AdaBoost is a popular data-boosting algorithm involving iterative training of a series of individual classifiers in an ensemble [90]. It can provide solutions for two-class, multi-class single-label, multi-class multi-label, single label category, and regression problems [91]. We followed the approach of Hong et al. [19] and used AdaBoost to adaptively resample data in choosing training samples. We used successive iterations to assign weights to the data, such that the subsequent iteration incorporates reweighted data that were previously misclassified. This approach provided us with a weighted sum of ensemble predictions.
Bagging is a bootstrap sampling technique used with ensemble methods [92]. We used BA to randomly sample and replace data while generating multi-sample training subsets. Generated subsets were used to create a decision tree and later incorporate them into the final model. We used this technique to improve our classification accuracy by decreasing the variance of classification errors.
We used the random subspace algorithm as our third classifier. Boot [93] reported that averaging forecasts constructed from many randomly selected fixed-size subsets substantially lower the mean squared forecast error. Instead of selecting subsets of available predictors, random projection regression creates a low-dimensional subspace by averaging over-predictors using random weights drawn from a normal distribution. In this study, we followed the methodology of Pham et al. [94], who used the RS method to assess landslide hazards in Vietnam.
Rotation Forest is an ensemble technique developed by Rodriguez et al. [95] to improve the predictive capability of single classifiers. We trained the RF model by sequentially: (1) deriving subsets after dividing the attribute sets; (2) resampling the sample subsets and transforming features on the generated subsets; (3) realigning the rotation matrix according to the sequence of original attribute sets; (4) training base classifiers using the rotated sample subsets; and (5) integrating the results of  (1) in Rodriguez et al. [95] for the matrix equation that we used). The flowchart for the landslide susceptibility modeling and analysis of spatial data of the watershed is shown in Figure 5.

Performance and Evaluation of the Landslide Models
Model validation is essential to confirm the validity of results. Indeed, both the training and validation datasets should be checked for, respectively, model goodness-of-fit and prediction accuracy [68]. In this study, statistical metrics and area under the receiver operating characteristic curve (AUC) were used to evaluate the performance of the SVM algorithm and its ensembles.

Statistical Metrics
We used sensitivity, specificity, accuracy, and kappa metrics to evaluate and test the performance of our machine learning algorithms. All these metrics can be formulated on the basis of four types of possible consequences (PC)-True Positive (TP), False Positive (FP), True Negative (TN), and False Negative (FN). If the landslide pixels are correctly or incorrectly classified as landslide, TP and FP are, respectively, dominant. In contrast, if the landslide pixels are correctly or incorrectly classified as non-landslide, they are, respectively, TN and FN [44]. The higher the values of sensitivity, specificity, and accuracy (Equation (1)), or the lower the RMSE, the better the models perform [45]. A kappa index value of 1 indicates a perfect model, whereas −1 represents a non-reliable model. Sensitivity, specificity, accuracy, kappa, and RMSE can be formulated as follows: where n is the total number of landsides, X P is the predicted probability values by training or validation datasets, X A is the actual (output) probability values obtained from landslide susceptibility model, and RMSE is root mean square error.

ROC Curve
The ROC curve is a two-dimensional plot used to evaluate the overall performance of models. It has been used in most landslide modeling studies [5,[44][45][46]. The x and y axes of the plot are, respectively, the sensitivity and 100-specificity [17,96,97]. The area under the ROC curve (AUC) provides a measure of the performance of the landslide model [45,98]. AUC for a perfect model is 1; for an inaccurate model it is 0.5 [26,34,46]. If P and N are total number of landslides and non-landslides, respectively, the AUC can be expressed as follows:

Factor Selection
The most important factors for landslide occurrence are shown in Figure 6.  (Figure 6). The low predictive capability of distance to the river is consistent with the fact that most landslides are far from the river channels.

Factor Selection
The most important factors for landslide occurrence are shown in Figure 6.  (Figure 6). The low predictive capability of distance to the river is consistent with the fact that most landslides are far from the river channels.

Shallow Landslide Modeling Process
The SVM and its ensemble models were built using training and validation datasets. The goodness-of-fit analysis indicates that all models predicted the spatial distribution of landslides well (Tables 2 and 3). The SVM model had the highest sensitivity (0.853) in the training process, whereas the RS model had the highest sensitivity (0.655) in the validation process. The RF and RS models had the highest specificity for the training and validation datasets-0.884 and 0.931, respectively, indicating that 88% and 93% of non-landslide locations were correctly classified as non-landslides. Additionally, the RF model performed best in the training process based on accuracy (0.855), kappa

Shallow Landslide Modeling Process
The SVM and its ensemble models were built using training and validation datasets. The goodness-of-fit analysis indicates that all models predicted the spatial distribution of landslides well (Tables 2 and 3). The SVM model had the highest sensitivity (0.853) in the training process, whereas the RS model had the highest sensitivity (0.655) in the validation process. The RF and RS models had the highest specificity for the training and validation datasets-0.884 and 0.931, respectively, indicating that 88% and 93% of non-landslide locations were correctly classified as non-landslides. Additionally, the RF model performed best in the training process based on accuracy (0.855), kappa (0.710), RMSE (0.318) and AUC (0.944) criteria, and the RS model performed best in the validation process based on accuracy (0.793), kappa (0.561), RMSE (0.386), and AUC (0.886).

Development of Landslide Susceptibility Maps
We constructed the landslide susceptibility maps using SVM and its ensemble models. We established five susceptibility classes-very low susceptibility (VLS), low susceptibility (LS), moderate susceptibility (MS), high susceptibility (HS), and very high susceptibility (VHS) -using different classification methods, including natural breaks (NB), quantile (Q), and geometrical intervals (GI). To select the best classification method, we calculated, for each model, all observed landslides in each susceptibility class. The results, shown as histograms of all models (Figure 7), indicate that the most landslides are located in the VHS class.
In the case of the susceptibility map generated by the AB model, the VHS class determined by the natural breaks, quantile, and geometrical interval methods covered, respectively, 23.76%, 19.74%, and 25.78% of the watershed pixels, and 72.45%, 65.31%, and 75.51% of the observed landslide pixels. Therefore, for the AB model, we selected the geometrical interval method as the most appropriate method for classification of landslide susceptibility. Accordingly, we chose this method for the BA, RS, and RF susceptibility maps. We used the natural break method to classify the SVM and prepare the susceptibility map. In the case of the AB model, the VLS, LS, MS, HS, and VHS classes covered, respectively, 18.77%, 19.74%, 15.66%, 20.04%, and 25.78% of the study area (Figure 7). For the BA model, the VHS class had the largest area (30.    (Figure 7). The maps show that the northeastern, middle, eastern, southeastern, and southern parts of the Sarkhoon watershed are more susceptible to landslides than northern and western parts. The VHS areas are mostly located along roads, and VLS areas are in the highlands, where human activity is reduced and there are no roads. respectively, 18.77%, 19.74%, 15.66%, 20.04%, and 25.78% of the study area (Figure 7). For the BA model, the VHS class had the largest area (30.39%), followed by the LS (21.36%), MS (18.88%), HS (15.30%), and VLS (14.07%) classes (Figure 7). The corresponding values for the RS model are 13.27%, 25.77%, 19.02%, 16.28% and 25.66%; for the RF model, they are 14.73%, 25.50%, 18.24%, 15.59%, and 25.94%. In the case of SVM model, the VHS class has the largest area (26.77%), followed by the VLS (20.35%), LS (19.00%), HS (17.88%), and MS (16.0%) classes. Figure 8 shows the landslide susceptibility maps derived with SVM and its ensemble models with selected classification methods. For example, the classes calculated using the natural break method are VLS (0.0004-0.2003), LS (0.2003-0.3963), MS (0.3963-0.6080), HS (0.6080-0.8157), and VHS (0.8157-1.000) (Figure 7). The maps show that the northeastern, middle, eastern, southeastern, and southern parts of the Sarkhoon watershed are more susceptible to landslides than northern and western parts. The VHS areas are mostly located along roads, and VLS areas are in the highlands, where human activity is reduced and there are no roads.

Validation and Comparison of Landslide Susceptibility Maps
The performance of the new ensemble SVM models was compared to that of the SVM model using the area-under-the-ROC curve (AUC) for both training and validation datasets. The ROC curves of the susceptibility maps of the AB, BA, RF, RS, and SVM models in the training step are shown in Figure 9a.
It can be seen that the SVM and all of its ensemble models perform well in landslide susceptibility mapping. The RS model is the best performer, with an AUC of 0.873, indicating an accuracy of 87.3%. It followed by the RF (0.871), SVM (0.869), BA (0.857), and AB (0.851) models.
shown in Figure 9a. It can be seen that the SVM and all of its ensemble models perform well in landslide susceptibility mapping. The RS model is the best performer, with an AUC of 0.873, indicating an accuracy of 87.3%. It followed by the RF (0.871), SVM (0.869), BA (0.857), and AB (0.851) models.
ROC curves for validation dataset are shown in Figure 9b. The RS model performed best, with an AUC of 0.837, followed by the RF (AUC = 0.34), SVM (0.810), AB (0.807), and BA (0.779) models. Although our model performance in the validation step is slightly higher than in the training step, the ROC curves for the training and validation steps yielded higher AUC values for the training dataset than those for the validation dataset. The reason is that the same landslides that were used to build the landslide models were used for performance analysis in the training step.

Discussion
In recent years, machine learning algorithms have outperformed expert knowledge and analytic methods in landslide susceptibility mapping [99,100]. A large number of machine learning methods and techniques have been developed, but there is, as yet, no standard method that is accepted by researchers around the world. Previous research has shown complex and non-linear relations between landslide locations and conditioning factors that differ from one region to another and through time [101]. Our objective in this study was to demonstrate the power of SVM and its ensembles adaBoost, bagging, random subspace, and rotation forest in landslide susceptibility mapping in the Sarkhoon watershed in Iran. The SVM algorithm has been widely employed to map landslide susceptibility [98,[102][103][104][105][106][107], but not with the ensembles used in this study.
Landslides are affected by a variety of geo-environmental factors, some of which are more important than others. Strategies for factor and feature selection are used in machine learning to improve the performance of models. The most important factors are objectively and preferentially selected, and factors that lead to noise and over-fitting problems are removed to enhance the predictive capability of the model [108]. Using the One-R Attribute Evaluation (ORAE) technique with 10-fold cross-validation, we show that all 20 of our conditioning factors were significant and thus were applied in the modeling and evaluation. However, distance to road is the most important factor for landslide occurrence. Roads disturb the natural environment of slopes, leading to a greater likelihood of landslides in hilly areas (e.g., [69,94,98,109,110]).
SVM is a powerful nonlinear machine learning algorithm in classification problems because it is solidly based on statistical learning theory and is able to find the optimal decision function based on a training dataset [111]. Several studies have concluded that the SVM method is more efficient than Although our model performance in the validation step is slightly higher than in the training step, the ROC curves for the training and validation steps yielded higher AUC values for the training dataset than those for the validation dataset. The reason is that the same landslides that were used to build the landslide models were used for performance analysis in the training step.

Discussion
In recent years, machine learning algorithms have outperformed expert knowledge and analytic methods in landslide susceptibility mapping [99,100]. A large number of machine learning methods and techniques have been developed, but there is, as yet, no standard method that is accepted by researchers around the world. Previous research has shown complex and non-linear relations between landslide locations and conditioning factors that differ from one region to another and through time [101]. Our objective in this study was to demonstrate the power of SVM and its ensembles adaBoost, bagging, random subspace, and rotation forest in landslide susceptibility mapping in the Sarkhoon watershed in Iran. The SVM algorithm has been widely employed to map landslide susceptibility [98,[102][103][104][105][106][107], but not with the ensembles used in this study.
Landslides are affected by a variety of geo-environmental factors, some of which are more important than others. Strategies for factor and feature selection are used in machine learning to improve the performance of models. The most important factors are objectively and preferentially selected, and factors that lead to noise and over-fitting problems are removed to enhance the predictive capability of the model [108]. Using the One-R Attribute Evaluation (ORAE) technique with 10-fold cross-validation, we show that all 20 of our conditioning factors were significant and thus were applied in the modeling and evaluation. However, distance to road is the most important factor for landslide occurrence. Roads disturb the natural environment of slopes, leading to a greater likelihood of landslides in hilly areas (e.g., [69,94,98,109,110]).
SVM is a powerful nonlinear machine learning algorithm in classification problems because it is solidly based on statistical learning theory and is able to find the optimal decision function based on a training dataset [111]. Several studies have concluded that the SVM method is more efficient than and outperforms other expert knowledge and machine learning techniques. Most recently, Tien Bui et al. [46] compared SVM with index of entropy, decision tree, and naïve bayes models, and concluded that SVM has higher prediction capability and is more powerful than the others. Likewise, Pham et al. [100] compared SVM with logistic regression, Fisher's linear discriminant analysis, Bayesian network, and naïve bayes models, and reached a similar conclusion. The SVM method is superior to the artificial neural network (ANN) method because training by ANN is difficult and tends to overly emphasize local optima. The SVM method does not require interpretation based on rules, as do, for example, the decision tree algorithms. In comparison to logistic regression, SVM requires greater machine memory and processing time, but these issues are not critical for spatial predictions of landslides [100].
The RS and RF classifiers combined with the SVM algorithm provided the best goodness-of-fit, performance, and prediction accuracy for, respectively, the training and validation datasets of this study. The BA and AB ensemble methods failed to improve the performance or power prediction of the SVM. The RS and RF classifiers did a better job of decreasing the variance, bias, and noise of the training dataset than the BA and AB methods, and prevented the problem of over-fitting in the modeling process.
Similar studies [5,44,52,69,91,98] have shown that the RS classifier can enhance the goodness-of-fit and prediction accuracy of the naive Bayes tree classifier for spatial predictions of landslides. Pham et al. [98] reported that the RF in combination with the Fuzzy Unordered Rules Induction Algorithm (FURIA) is more powerful than FURIA, AB-FURIA, and BA-FURIA ensemble models, or SVM. Pham et al. [69] concluded that the RF ensemble model outperforms the Radial Basis Function (RFRBF) neural network, logistic regression, multi-layer perceptron neural networks (MLP Neural Nets), and naïve bayes models. Shirzadi et al. [44] prepared landslide susceptibility maps based on different sample sizes and raster resolution and the use of an alternating decision tree (ADTree) algorithm as a base classifier. Their ensemble models included Multiboost, BA, RF, and RS algorithms. They concluded that the RSADT model with a sample size of 60%/40% and 10-m raster resolution of 70/30% had the highest prediction accuracy, whereas the MBADT was more capable with a sample size of 80/20% and a 20-m raster resolution of 90/10%. Some researchers have concluded that the BA classifier performs better than other ensemble and machine learning base classifiers. For example, Bui et al. [112] concluded that the BA classifier used in combination with a decision tree (DT) model has the highest prediction capability and outperforms the DT and ABDT models. Bui et al. [113] combined the functional tree (FT) algorithm with BA, AB, and Multiboost ensembles, and concluded that it has higher prediction capability compared to the other models mentioned above.
Based on the discussion above, we conclude that the quality of results depend largely on the type of base classifier used. On one hand, a weak base classifier, such as a decision tree algorithm, will likely produce a weak ensemble model [114]. On the other hand, a strong base classifier such as SVM will not necessarily produce a powerful ensemble model. If an ensemble improves the power prediction of a base classifier, it will effectively decrease the over-fitting, noise, and variance problems of the training dataset during the modeling process. We argue that base classifiers and their ensembles should be tested and evaluated before they are accepted and used for landslide susceptibility mapping. The RSSVM model is a promising technique for spatial predictions of landslides and can be used in our study area for land-use planning. We recognize, however, that our method must be tested in areas with different terrain and geology before it can be more widely used.

Conclusions
In this study, we developed machine learning ensemble models to prepare landslide susceptibility maps in an effort to better manage landslide-prone areas in the Sarkhoon watershed in the Chahar Mahaal and Bakhtiari Province, Iran. We developed the models using 98 landslide locations (scarp or rupture zone pixels) and 20 conditioning factors tested by the ORAE technique. This tool allowed us to select the most important factors, decrease the over-fitting and noise problems inherent in the training dataset, and increase the reliability and prediction accuracy of the models. Most of observed landslides in the study area are within the high and very high susceptibility classes. The output of the modeling process using the ORAE technique confirmed that the SVM and its ensembles performed well in recognizing landslide-prone locations in the study area. The results indicate that, among the four Meta classifiers, rotation forest and random subspace are more powerful and flexible than adaboost and bagging for modeling landslides in the study area. They significantly enhanced the power prediction of SVM as a base classifier. Additionally, rotation forest model had the highest prediction accuracy in generating landslide susceptibility maps. Overall, the findings can be summarized as follows: 1.
All 20 conditioning factors are significantly associated with landslide occurrence, while the most important factor is distance to road. It is followed by rainfall and land-use factors, which implies that human activities, including interference with runoff, are the main causes of landslides in the study area.

2.
The proposed RF-SVM model is a promising technique for generating accurate and useful landslide susceptibility maps in other areas with similar geo-environmental characteristics. 3.
We recommend the combination and integration of rotation forest and SVM for building the landslide susceptibility model.

4.
The RF-SVM ensemble model outperformed the RS-SVM model, the BA-SVM model, and the AB-SVM model, and therefore is an appropriate and reasonable tool for landslide susceptibility mapping. 5.
The ensemble model described in this paper is recommended as a robust model for landslide susceptibility assessment and hazard and risk management and reduction.

Conflicts of Interest:
The authors declare no conflicts of interest.