A 1:200,000 lithological geological map with 14 categories after standardization

A <b>unified lithological classification system</b> is the fundamental prerequisite for large-scale remote sensing lithological mapping. The study area spans <b>eight 1:200,000 geological map sheets</b> produced in different years, which poses a major challenge to achieving classification consistency. Specifically, to obtain a complete and unified lithological reference map for the study area, two key issues must be addressed: (1) <b>polygonal inconsistencies at map boundaries</b>, and (2) <b>the unification of lithological classification systems</b>. These challenges are among the principal reasons why <b>remote sensing-based lithological mapping</b> has rarely been implemented at national or provincial scales.To address the first issue, two principles were applied:<br>(1) Only <b>one of the two adjacent polygons</b> was modified at each boundary. When auxiliary data (e.g., 1:500,000 geological maps) clearly indicated the lithology of one polygon, the category of the neighboring polygon was adjusted accordingly. Otherwise, lithological types were reassigned to <b>minimize boundary discontinuities and category inconsistencies</b>.<br>(2) When reference geological maps failed to provide clear lithological classifications, <b>regional and local geological maps</b>, relevant scientific literature (Yang et al., 2022; Zhu et al., 2025; Zhao et al., 2006), <b>expert judgment</b>, <b>PCA visualization</b>, and <b>high-resolution remote sensing imagery</b> (e.g., Google Earth, 0.5–2 m resolution) were jointly used for auxiliary interpretation. Lithological inferences were made logically based on <b>surface texture, tone, and geomorphological patterns</b> combined with geological reasoning. For instance, a linear dike should not terminate abruptly or sharply change direction at a map boundary.After resolving these boundary discrepancies, the initial lithological map contained <b>87 lithological categories</b> and <b>2,656 polygons</b>. The second challenge involved <b>reconstructing a unified classification system</b> for the eight 1:200,000 geological maps to eliminate internal <i>semantic noise</i> and establish a more stable and consistent foundation for large-scale remote sensing lithological mapping. This process represents the <b>most practical approach</b> to achieving true large-scale, cross-sheet lithological mapping (Supplementary Text S1). However, it is inherently complex and requires <b>expert judgment</b> combined with analyses at <b>regional and supra-regional stratigraphic scales</b>.Three core principles were established for this reconstruction:<b>Dominant lithology rule.</b> We assume that the first-mentioned lithology typically occupies a broader spatial extent than those mentioned subsequently, thereby emphasizing the primary lithology. This principle aligns with many geological mapping guidelines (Cohen et al., 2013; Hartmann et al., 2012) and was adapted from Dürr et al. (2005). For example, “metaconglomerate” and “metaconglomerate and sandstone” were merged into “metaconglomerate–sandstone mixture.”<b>Lithology-first principle, stratigraphy as auxiliary</b> (Supplementary Text S2). Stratigraphic information was retained only when it remained largely consistent within standardized lithological categories. Units exhibiting major stratigraphic inconsistencies or conflicts were temporarily treated as lithology-only classes without age attribution.<b>Spectral separability principle.</b> Lithological units with distinct names but highly similar spectral characteristics—such as “quartzite” and “quartzite and quartz schist”—were merged into a single category (“quartzite”) in consideration of the practical spectral resolution limits of remote sensing classification.Finally, three graduate students specializing in geology independently conducted lithological reinterpretation following the above principles. Their standardized results were <b>cross-validated</b>, and <b>a majority-vote approach</b> was adopted, with final expert adjudication applied to any remaining disagreements. This process produced a <b>single, unified, and standardized lithological classification system</b>.Through this systematic standardization procedure, we effectively <b>eliminated subjective inconsistencies</b> related to obsolete, ambiguous, or regionally specific geological terminology. The lithological descriptions were consolidated into <b>14 standardized lithological categories</b>, and the number of polygons was reduced from <b>2,656 to 1,407</b>.