Exploiting Convolutional Neural Networks With Deeply Local Description for Remote Sensing Image Classification

The extraction of features from the fully connected layer of a convolutional neural network (CNN) model is widely used for image representation. However, the features obtained by the convolutional layers are seldom investigated due to their high dimensionality and lack of global representation. In this study, we explore the uses of local description and feature encoding for deeply convolutional features. Given an input image, the image pyramid is constructed, and different pretrained CNNs are applied to each image scale to extract convolutional features. Deeply local descriptors can be obtained by concatenating the convolutional features in each spatial position. Hellinger kernel and principal component analysis (PCA) are introduced to improve the distinguishable capabilities of the deeply local descriptors. The Hellinger kernel causes the distance measure to be sensitive to small feature values, and the PCA helps reduce feature redundancy. In addition, two aggregate strategies are proposed to form global image representations from the deeply local descriptors. The first strategy aggregates the descriptors of different CNNs by Fisher encoding, and the second strategy concatenates the Fisher vectors of different CNNs. Experiments on two remote sensing image datasets illustrate that the Hellinger kernel, PCA, and two aggregate strategies improve classification performance. Moreover, the deeply local descriptors outperform the features extracted from fully connected layers.