A novel dual skip connection mechanism in U-Nets for building footprint extraction

The importance of building footprints and their inventory has been recognised as an enabler for multiple societal problems. Extracting urban building footprint is complex and requires semantic segmentation of very high-resolution (VHR) earth observation (EO) images. U-Net is a common deep learning architecture for such segmentation. It has seen several re-incarnation including U-Net++ and U-Net3+ with a focus on multi-scale feature aggregation with re-designed skip connections. However, the exploitation of multi-scale information is still evolving. In this paper, we propose a dual skip connection mechanism (DSCM) for U-Net and a dual full-scale skip connection mechanism (DFSCM) for U-Net3+. The DSCM in U-Net doubles the features in the encoder and passes them to the decoder for precise localisation. Similarly, the DFSCM incorporates increased low-level context information with high-level semantics from feature maps in different scales. The DSCM is further tested in ResUnet and different scales of U-Net. The proposed mechanisms, therefore, produce several novel networks that are evaluated in a benchmark WHU building dataset and a multi-resolution dataset that we develop for the City of Melbourne. The results on the benchmark dataset demonstrate 17.7% and 18.4% gain in F1 score and Intersection over Union (IoU) compared to the state-of-the-art vanilla U-Net3+. In the same experimental setup, DSCM on U-Net and ResUnet provides a gain in five accuracy measures against the original networks. The codes will be available in a GitHub link after peer review.