RouteZero employs a hybrid approach, prioritising activity-based data - collecting detailed information on flight segments, hotel nights, rail legs, and vehicle type wherever feasible. Where granular activity data is missing, spend-based emission factors fill the gap, in line with GHG Protocol guidance. Activity-based methods are more accurate, relying on specifics like aircraft type, distance, and travel class. Spend-based methods are less precise but broaden data coverage. Methodological selection is dictated by data availability and client reporting requirements, not convenience. Accuracy is contingent on the granularity of input data and the relevance and transparency of the underlying emission factors.