How to assess the capacity of urban airspace: A topological approach using keep-in and keep-out geofence

•In the low-altitude urban airspace, usable airspace is not equal to open airspace.•Airspace availability is defined as free, usable, and unusable.•Topological framework is proposed and applied to hypothetical and real environments.•Effects of keep-out and keep-in geofence are not symmetric and exhibit tradeoffs.•It is a first step to assess airspace capacity and is applicable to airspace design.