When Spatial Blocking Makes Distance Buffering Redundant: Structural Inertness versus Small Effects

Spatial blocking and distance buffering both limit dependence between training and test samples, but their marginal roles are rarely separated. We define the buffer effect L as unbuffered minus buffered AUC on identical test units while matching training size and class counts, then vary only whether those units are scattered or contiguous. Removing near-test training samples costs +.0693 AUC for scattered street-view units at r=100 m (95% CI [+.0357,+.1028]; 7/7 cities). Pooling two disjoint corrected So2Sat LCZ42 city sets gives +.0252 at r=200 m ([+.0136,+.0369]; 29/41 cities; p<10^-4). Compact blocking produces two distinct cases: in 28/48 primary cities the realised buffered and unbuffered training multisets are identical, so L_B=0 by construction; among the remaining active cities, mean L_B ranges from -.0012 to +.0004 and each collection-level 90% interval lies inside a prespecified +/-.005 AUC tolerance, although one city exceeds that tolerance. A coordinate-only three-state rule separates provably inert, certified-bounded, and must-measure cases for spatially local learners, with no violations in 1,368 full-pool checks. Redundancy is therefore a property of the realised geometry and training intervention, not of the label "spatial CV."