To assist gas distribution companies to effectively monitor their infrastructure and locate gas leaks, this paper considers the use of an Unmanned AerialVehicle (UAV) carrying a methane sensor to survey a region for gas leaks. As the UAV collects in situ measurements, it gathers evidence regarding the presence or absence of leaks in the region. To relate the UAV measurement to a region on the ground, we propose to use Upwind Survey Regions (USRs). If the UAV collects a measurement of high gas concentration, the USR represents a region that is likely to contain the leak. Likewise, if the UAV collects a measurement of low gas concentration, the USR represents a region that is likely to be clear of gas leaks. We propose a framework to process the measurements and produce a survey map indicating areas that can be reliably cleared of gas leaks and areas that may contain gas leaks. Our framework is composed of two steps: (1) mapping UAV measurements into USRs on the ground; and (2) fusing the various mapped USRs to produce the survey map.We discuss how USRs can be estimated and we test our framework using both simulated and real UAV flights.