Public Snap Map Activity as an Opportunistic Urban Sensor: Spatial Concentration, Temporal Rhythms, and Measurement Requirements Across Three Saudi Cities

Public Snap Map activity, a short-lived stream of stories pinned to places, can serve as an opportunistic urban sensor, and this study measures what it reports and what it requires to report reliably. From more than 1.18 million public snaps collected on a 1 km grid across three contrasting Saudi cities in 2024, activity is highly concentrated, to degrees that differ by city. Over the full windows, Makkah leads on every measure (Gini 0.71 [0.68, 0.73]), with one Grand Mosque tile holding 2.7% of its snaps, and its lead survives a spatial-block bootstrap. Riyadh (0.59 [0.57, 0.60]) and Dammam (0.56 [0.54, 0.59]) are close in degree and differ in shape: Dammam concentrates in a strong head of busy tiles, Riyadh in a long active tail. Over the week that all three windows share, Makkah does not lead, because its estimate rises with accumulated observation. Two operating requirements follow. Concentration estimates stabilized after 23 to 27 days in the two campaigns observed long enough to confirm it and after 12 to 14 days once collection start-up had passed. Under binomial thinning with item-independent visibility, the Gini is biased downward, Makkah’s lead survives at 5% retention, and the Riyadh–Dammam ordering inverts at 10%.