Integrating artificial neural network and classical methods for unsupervised classification of optical remote sensing data

A novel system named unsupervised multiple classifier system (UMCS) for unsupervised classification of optical remote sensing data is presented. The system is based on integrating two or more individual classifiers. A new dynamic selection-based method is developed for integrating the decisions of the individual classifiers. It is based on competition distance arranged in a table named class-distance map (CDM) associated to each individual classifier. These maps are derived from the class-to-class-distance measures which represent the distances between each class and the remaining classes for each individual classifier. Three individual classifiers are used for the development of the system, K-means and K-medians clustering of the classical approach and Kohonen network of the artificial neural network approach. The system is applied to ETM + images of an area North to Mosul dam in northern part of Iraq. To show the significance of increasing the number of individual classifiers, the application covered three modes, UMCS@, UMCS#, and UMCS*. In UMCS@, K-means and Kohonen are used as individual classifiers. In UMCS#, K-medians and Kohonen are used as individual classifiers. In UMCS*, K-means, K-medians and Kohonen are used as individual classifiers. The performance of the system for the three modes is evaluated by comparing the outputs of individual classifiers to the outputs of UMCSs using test data extracted by visual interpretation of color composite images. The evaluation has shown that the performance of the system with all three modes outrages the performance of the individual classifiers. However, the improvement in the class and average accuracy for UMCS* was significant compared to the improvements made by UMCS@, and UMCS#. For UMCS*, the accuracy of all classes were improved over the accuracy achieved by each of the individual classifiers and the average improvements reached (4.27, 3.70, and 6.41%) over the average accuracy achieved by K-means, K-medians and Kohonen respectively. These improvements correspond to areas of 3.37, 2.92 and 5.1 km respectively. While the average improvements achieved by UMCS@ and UMCS#, respectively, compared to their individual classifiers were (0.77 and 2.79%) and (0.829 and 2.92%) which correspond to (0.61 and 2.2 km) and (0.65 and 2.3 km) respectively. Introduction Unsupervised classification of remotely sensed data is a technique of classifying image pixels into classes based on statistics without pre-defined training data. This means that the technique is of potential importance when training data representing the available classes is not available. Unsupervised classification is also important for providing a preliminary overview of image classes and more often it is used in the hybrid approach of image classification [1,2]. Correspondence: [email protected] Department of Computer Science, Faculty of Science, Duhok University, Duhok, Kurdistan Region, Iraq © 2012 Tahir; licensee Springer. This is an Open License (http://creativecommons.org/licenses/b medium, provided the original work is properly Several methods of unsupervised classification using classical or neural network approaches have been developed and used consistently in the field of remote sensing. The most commonly used of the classical approach is K-means clustering algorithm [3] while Kohonen network is the most commonly used one of the artificial neural network approach [4]. So far many research works have conducted to improve the accuracy of the unsupervised classifiers. Examples of these works are the use of Kohonen classifier as a pre-stage to improve the results of clustering algorithms such as agglomerative hierarchical clustering, K-means and threshold-based clustering algorithms [5-7]. Access article distributed under the terms of the Creative Commons Attribution y/2.0), which permits unrestricted use, distribution, and reproduction in any cited. Figure 1 The Scheme of the proposed UMCS. Tahir EURASIP Journal on Advances in Signal Processing 2012, 2012:165 Page 2 of 12 http://asp.eurasipjournals.com/content/2012/1/165 In those works one algorithm was used as a pre-stage to improve the classification results of another algorithm. That is, the final decision is made according to only one classifier’s decision. Methods involving a simultaneous use of more than one classifier in the so-called multiple classifier system (MCS) which is very common in the approach of supervised classification have not been conducted in the unsupervised classification of optical remote sensing data. See for example [8-10] for some of the MCS schemes form supervised classification of remote sensing data. The idea of MCS is based on performing more than two classifiers and integrating their decisions according to some prior or posterior knowledge concerning the output classes to reach the final decision. Prior knowledge is estimated from training data concerning the output classes while posterior knowledge, in general, represents the outputs of the individual classifiers. The operation of integration is done in one of two strategies, either by combining the outputs of the individual classifiers or by selecting one of the individual classifiers outputs. Many methods of integration have been developed for the implementation of MCS in the supervised approach of classification. Examples of combined-based methods of integration are the majority voting rule, which assigns the label scored by majority of the classifiers to the test sample [9] and Belief function, which is knowledge-based method. It is based on the probability estimation provided by the confusion matrix derived from training data set [11]. Examples of the dynamic classifier selection-based method of integration are classifier rank (CR) approach, which takes the decision of the classifier that correctly classifies most of the training samples neighboring the test sample [12] and the local ranking (LR) which is based on ranking the individual classifiers for each class according to the mapping accuracy (MA) of the classes [8]. In this article, an integrated system of unsupervised classification named unsupervised multiple classifier system (UMCS) is developed using individual classifiers from two different approaches, traditional (classical) and artificial neural network. The system is based on new integration method of the dynamic classifier selectionbased type. This method is based on class-distance maps (CDM) for the individual classifiers as the measure upon which the final decision is selected. The CDM of each individual classifier is generated from the measure of Euclidean distances between each class and the remaining classes of that individual classifier, named here as the class-to-class distance measurement (CCDM). The remaining parts of the article are organized as follows: In the following section, the proposed system is described and detailed explanations of its major modules are given. In section “Results”, the results of applying the system to ETM + images are shown and discussed. In section “Posterior interpretation of output classes”, posterior interpretation of the classification outputs is done. In section “Individuals and multiple classifiers comparison”, comparisons between the output results are made. In section “Evaluation of system performance”, the performance of the system is evaluated and finally some concluding remarks are given in the last section. (UMCS); the proposed system In this article, the proposed system of classification is called UMCS to be differentiated from the multiple classifier system (MCS) which is common in supervised classification. It is designed to host three individual unsupervised classifiers and can be adapted to any number on individual classifiers. The scheme of the system for three individual classifiers is shown in Figure 1. Each of the three classifiers, K-means, K-medians and Kohonen is implemented using multi-spectral images yielding three output images. These three output images are then entered to a color unification algorithm (CUA) in order to achieve class-to-class correspondence in the three output images. Finally, the three output images of the (CUA) are integrated using CDM generated from the Euclidean distance measurements between each class and the remaining classes within the classifier, named as (CCDM). The algorithms of color unification and classifier integration method are given in the following sections. CUA In most cases the order of classes resulting from different approaches of unsupervised classification are affected by the way of performing the operation of clustering and the order of data presented to the process of clustering. For instance, in the Kohonen network, the training phase usually starts by giving the initial weights Tahir EURASIP Journal on Advances in Signal Processing 2012, 2012:165 Page 3 of 12 http://asp.eurasipjournals.com/content/2012/1/165 which control the order of the outcome classes. Therefore in order to implement the proposed system, the corresponding classes in the individual classifiers must have the same order. To achieve this step, an algorithm named CUA is developed. The aim of this algorithm is to reorder the classes an all classifiers in order to assign same color to the three nearest classes of the three classifiers. This is done by fixing the order of classes in one classifier as a reference and reordering the classes of the other two classifiers. This algorithm requires the determination of the Euclidean distance between the center of each class in the referenced classifier and the centers of all classes in each of the other two classifiers. The nearest two classes each from one classifier are given the same order (color) of the current class in the reference classifier. Then the operation is repeated until the ordering of all classes in the three classifiers is reached. The algorithm does not require re-calculation of the class centers since these centers are calculated during the implementation of the classifiers. In K-means and K-medians classifiers, the last mean vectors and median vectors upon which the classifier have reached the convergence state represent the centers of the classes. In Kohonen classifier, the weight vectors to the output neurons are taken to be the centers of the classes. The procedures of the algorithm are: 1Read the centers of the classes for the three classifiers and set the class number i = 0. 2Increase class number i = i + 1. 3Calculate the Euclidean distance between the mean vector of Ci from the reference classifier and the mean vectors of all output classes in the other two classifiers. Dim 1⁄4 jjCi Pm jj for all m 1⁄4 i; ; ; k Din 1⁄4 jjCi Qn jj for all n 1⁄4 i; ; ; k Where; Dim is the Euclidean distance between class Ci from the reference classifier and class Pm from the second classifier. Din is the Euclidean distance between class Ci from the reference classifier and class Qn from the third classifier. ||.|| represents the norm operator. 4Exchange class order. Exchange the class order of the second classifier: if Dij < Dim for all m 1⁄4 i; ; ; k and j≠m Temp = Pj Pj = Pi Pi = Temp Exchange the class order of the third classifier: if Dil < Din ð Þ for all n 1⁄4 i; ; ; k and l≠n Temp = Ql Ql = Qi Qi = Temp 5Check the convergence of the algorithm. if (i < k) Go to step 2 else Go to step 6 6Stop. Integration method by CCDM As it was mentioned in the introduction, several methods of integrating the outputs of different classifiers are available. These methods were designed for MCS of the supervised type and they require a priori knowledge which most often can be estimated from the training data. For UMCS, the training data are not available and therefore this a priori knowledge cannot be obtained. The method of majority voting may be the only one which can be used to integrate the outputs of unsupervised classifiers since it only requires the final decisions of the three classifiers. However, this rule is influenced by the degree of correlation among the errors made by individual classifiers. When these errors are correlated (all classifiers produce incorrect but similar outputs) it leads to incorrect decision and when these errors are uncorrelated (each classifier produces a unique output) it leads to failure, [9]. In this article, a new method of integration is introduced. It is categorized as a selection-based approach and does not need prior knowledge. It requires a posterior knowledge which can be obtained from the outputs of the three classifiers. This posterior knowledge is the within classifier CCDM which is the measure of Euclidean distance between each class and all of the remaining classes within each individual classifier. This CCDM is then used to generate a table having N columns and N-1 rows, where N is the number of classes. The elements under each column represent the distances, stored in ascending way, from the class of that column to all of the remaining classes. For each individual classifier one CDM is generated. The procedures of implementing the algorithm are given below for UMCS made from three classifiers. It consists of two parts. In the first part, the CDM is generated. In the second part, the process of selecting the final decision is performed. The algorithm can easily be adapted to any number of classifiers. The flowchart of the algorithm is given in Figure 2.