Our understanding of large-scale mantle dynamics depends on accurate models of seismic velocity variation in the upper mantle transition zone (400--1000 km depth). With the Mode Branch Stripping technique (MBS) of van Heijst and Woodhouse <1997> it is possible to extract the dispersion characteristics of overtone surface wave signals from single source-receiver overtone waveforms. Such data provide new global transition zone constraints. We combined more than a million measurements of path-average overtone phase velocity with normal-mode splitting functions and body wave travel times to construct model S20RTSb of shear velocity heterogeneity throughout the mantle. We discuss in detail the resolution of structural heterogeneity in the transition zone. The main observations are the following: (1) Large-scale shear velocity variations (15%) in the upper 250 km of the mantle are at least 5 times larger than deeper in the mantle. High-velocity keels of Archean cratons extend to about 200 km depth. Low velocities related to mid-ocean ridge upwelling are confined to the upper 150 km of the mantle. (2) The 220-km discontinuity in PREM cannot be reconciled with Rayleigh wave dispersion, especially in oceans. (3) The velocity below the oceanic lithosphere (350--400 km depth) is 1--1.5% lower than beneath the continental lithosphere. (4) High-velocity slabs of former oceanic lithosphere are conspicuous structures just above the 670-km discontinuity. They extend to about 1100 km depth in the South American, Indonesian, and Kermadec subduction zones, indicating that slabs penetrate through the 670-km phase transition in several subduction zones. (5) We observe lower-than-average shear velocity below the lithosphere at eight hot spots (including Hawaii, Iceland, Easter, and Afar). It is, however, difficult to accurately estimate their depth extent in the transition zone because of the limited vertical resolution.