This paper provides the first direct measurement of how real-time population presence generates economic transactions by merging high-frequency mobile phone location data with card transaction records in Lyon metropolitan area, France’s second-largest area after Paris. Using a Poisson Pseudo-Maximum Likelihood estimator, we estimate the elasticity of transactions with respect to population presence, uncovering three key findings: (1) substantial heterogeneity in consumption responsiveness, with elasticities ranging from 1.08 during peak periods to 0.84 in peripheral areas; (2) a systematic goods-services dichotomy, where food retail exhibits near-unity elasticity (1.08) while experiential services show significantly lower responsiveness (0.64); and (3) persistent spatial frictions, with transaction flows declining by about 2% for a 1% increase in distance. By integrating real-time population presence and transaction data, we advance the urban economics literature by quantifying previously unobserved heterogeneities in how population presence translates into economic activity and demonstrating that spatial constraints remain significant even at micro-geographic scales.