Spatial vs. spectral resolution: which is important for lychee and longan classification from hyperspectral remote sensing image

Lychee and longan are important economic crops in South China. Fast identifying and acquiring their planting areas are important tasks. However, lychee trees and longan trees have similar appearances. So, they have similar spectral features. Therefore, obtaining the planting area of these trees from a remote sensing image becomes a challenge. Furthermore, unlike lychee, the planting area of longan in Guangdong province is too small to be identified. Therefore, spatial and spectral resolution may become important factors for the classification. To evaluate the influence of spatial and spectral resolution on lychee and longan classification, this paper compares different kinds of representative remote sensing data sources, including Sentinel-2 (high spatial resolution, multispectral image), Zhuhai-1 (high spatial resolution, hyperspectral image), Gaofen-5 (low spatial resolution, hyperspectral image), Ziyuan-1 02D (low spatial resolution, hyperspectral image), as well as the fused image of GF-5 with Sentinel-2. We aim at finding which spatial and spectral resolution can be suitable for classification. We combine a hierarchical classifier and Convolutional Neural Networks (CNNs) to classify the classes. The hierarchical classifier classifies the land use classes and the combined class of lychee and longan at the first level. And then it classifies lychee and longan from the combined class at the second level. The CNN is used as a base classifier in the hierarchical classifier. Three branches, i.e. spatial feature, spectral feature, and the joint of spatial and spectral feature branches, are the backbone of the network. 1D, 2D and 3D convolutional layers are designed to extract the spectral features, the spatial features and the joint features in the above branches, respectively. The experimental results show that spectral resolution is the most important factor for the lychee and longan classification, while spatial resolution cannot be ignored since longan occupies small planting areas. The hyperspectral images with 10m spatial resolution obtain the best performance. The image with 30m spatial resolution can hardly distinguish longan from lychee unless the spectral resolution reaches at least 10nm (e.g. GF-5). The multispectral image cannot distinguish these trees.