*Article* **Hyperspectral Image Classification Based on Semi-Supervised Rotation Forest**

#### **Xiaochen Lu \* , Junping Zhang \*, Tong Li and Ye Zhang**

School of Electronics and Information Engineering, Harbin Institute of Technology, Harbin 150001, China; ltong@hit.edu.cn (T.L.); zhye@hit.edu.cn (Y.Z.)

**\*** Correspondence: lxchen09@163.com (X.L.); zhangjp@hit.edu.cn (J.Z.); Tel.: +86-0451-8640-3020 (J.Z.)

Received: 20 July 2017; Accepted: 1 September 2017; Published: 6 September 2017

**Abstract:** Ensemble learning is widely used to combine varieties of weak learners in order to generate a relatively stronger learner by reducing either the bias or the variance of the individual learners. Rotation forest (RoF), combining feature extraction and classifier ensembles, has been successfully applied to hyperspectral (HS) image classification by promoting the diversity of base classifiers since last decade. Generally, RoF uses principal component analysis (PCA) as the rotation tool, which is commonly acknowledged as an unsupervised feature extraction method, and does not consider the discriminative information about classes. Sometimes, however, it turns out to be sub-optimal for classification tasks. Therefore, in this paper, we propose an improved RoF algorithm, in which semi-supervised local discriminant analysis is used as the feature rotation tool. The proposed algorithm, named semi-supervised rotation forest (SSRoF), aims to take advantage of both the discriminative information and local structural information provided by the limited labeled and massive unlabeled samples, thus providing better class separability for subsequent classifications. In order to promote the diversity of features, we also adjust the semi-supervised local discriminant analysis into a weighted form, which can balance the contributions of labeled and unlabeled samples. Experiments on several hyperspectral images demonstrate the effectiveness of our proposed algorithm compared with several state-of-the-art ensemble learning approaches.

**Keywords:** ensemble learning; hyperspectral; rotation forest; semi-supervised local discriminant analysis
