Homogeneous Terrain Unit Extraction by Integrating Superpixel Segmentation and Multiscale Region Merging: A Case Study in the Deeply Incised Valleys of Southeastern Tibet

Mapping mountain surfaces requires spatial units that represent both hillslope-scale structure and local within-slope terrain heterogeneity. Hydrological slope units provide limited representation of within-slope objects, whereas general object-based segmentation is sensitive to fragmentation and scale selection. We developed a homogeneous terrain unit extraction framework based on superpixel segmentation and multiscale region merging (SSM-HTU), in which initial slope units serve as local statistical references and within-slope terrain objects are generated through slope-unit-conditioned morphometric representation, superpixel initialization, distribution-sensitive region merging, and a nested partition hierarchy. The framework was applied to the 5136 km2 Yuqu River Basin in southeastern Tibet. Of 971 expert-interpreted reference HTUs, 680 were reserved for independent geometric evaluation; 1329 historical landslides were additionally used for supplementary spatial association analysis across mapping-unit schemes. Relative to the eCognition Multiresolution Segmentation (MSS) baseline, SSM-HTU showed a slight decrease in Precision from 0.8432 to 0.8340, while Recall (directional reference-object coverage) increased from 0.7615 to 0.8011, area-weighted IoU from 0.6710 to 0.6945, and Boundary F1 at a 12.5 m tolerance from 0.5980 to 0.6810, indicating greater reference-object coverage, spatial overlap, and boundary correspondence without uniform improvement across all geometric metrics. Across four geomorphological zones, area-weighted IoU ranged from 0.671 to 0.724 and Boundary F1 from 0.651 to 0.709, with non-monotonic regional variation. Mapping-unit schemes also yielded factor-dependent spatially stratified associations, underscoring the importance of spatial support in downstream statistical analysis. SSM-HTU therefore provides an object-based mapping framework for representing local within-slope terrain heterogeneity within a hillslope-scale statistical context in deeply incised valleys.