HOTA: Hierarchical Overlap-Tiling Aggregation for Large-Area 3D Flood Mapping

TL;DR

HOTA introduces a multi-scale inference strategy that enhances large-area 3D flood mapping by combining hierarchical overlap-tiling with deep learning and DEM differencing, enabling accurate, rapid flood extent and depth estimation.

Contribution

This work presents HOTA, a novel plug-and-play multi-scale inference method that improves 3D flood mapping accuracy without retraining models, integrating segmentation and depth estimation.

Findings

Improved IoU from 73% to 84% over baseline.

Achieved mean boundary error less than 0.5 meters.

Demonstrated effectiveness on real flood case study.

Abstract

Floods are among the most frequent natural hazards and cause significant social and economic damage. Timely, large-scale information on flood extent and depth is essential for disaster response; however, existing products often trade spatial detail for coverage or ignore flood depth altogether. To bridge this gap, this work presents HOTA: Hierarchical Overlap-Tiling Aggregation, a plug-and-play, multi-scale inference strategy. When combined with SegFormer and a dual-constraint depth estimation module, this approach forms a complete 3D flood-mapping pipeline. HOTA applies overlapping tiles of different sizes to multispectral Sentinel-2 images only during inference, enabling the SegFormer model to capture both local features and kilometre-scale inundation without changing the network weights or retraining. The subsequent depth module is based on a digital elevation model (DEM) differencing method, which refines the 2D mask and estimates flood depth by enforcing (i) zero depth along the flood boundary and (ii) near-constant flood volume with respect to the DEM. A case study on the March 2021 Kempsey (Australia) flood shows that HOTA, when coupled with SegFormer, improves IoU from 73\% (U-Net baseline) to 84\%. The resulting 3D surface achieves a mean absolute boundary error of less than 0.5 m. These results demonstrate that HOTA can produce accurate, large-area 3D flood maps suitable for rapid disaster response.