The use of relative arrival time residuals in teleseismic tomography means that only relative velocity variations are constrained beneath the study region. While absolute arrival times may be more useful, contributions from source origin time uncertainty and structure outside the model region may negate any potential advantage. One possibility is to embed a local model in a large regional or global model and jointly invert for local, regional and even global structure (e.g. Burdick et al., 2014). This requires a large regional or global dataset and irregular parameterization to deal with the different resolving power of the various datasets.