For first responders entering into a post-disaster situation, there is usually a severe lack of up-to-date ground truth. The initial period of time has multiple sources of conflicting information coming in and creating confusion about the situation. The most important immediate requirement is to create a traversal map, highlighting navigable paths to victims of the disaster and possible hazardous locations. Due to infrastructure damage, it is hard for existing centralized geospatial portals to quickly update and provide this information, which has become outdated. IoT solutions that can be deployed without extensive preparation provide the capability to quickly acquire and disseminate essential information to rescue teams. In this paper, we present a decentralized system, named DeimosBC, that is able to provide such a mapping service faster and more reliably, utilizing the work of volunteers and relying on a blockchain backend that is based on an IoT system. Our solution utilizes the availability of modern smartphones with GPS receivers and processing capabilities to collect sequences of GPS locations and chain them into trajectories. These trajectory data are submitted as entries into a blockchain after cleaning them through a purpose-built smart contract. DeimosBC relies on the inherent robustness and distributed nature of a blockchain to make collating and assembling a map from these paths more accurate and less susceptible to disruption. We describe how DeimosBC would work for a hypothetical disaster scenario of a Category 5 hurricane striking an area of the Gulf of Mexico.