Physical address, which have been used by humans for centuries to locate places in the physical world, are not sufficient to identify buildings or places in a digital world. Difficulty in joining data from disparate sources in a single location, due to the different ways of identifying buildings using addresses or local numbering systems, becomes a major obstacle to information and knowledge exchange. We developed a grid reference system-based Unique Building Identifier (UBID). The method converts a building footprint on a map to a unique code. Unlike geographic coordinates (latitude and longitude), which require high precision and can only locate points on the earth, the UBID code records two-dimensional information about a building footprint in a concise and practical manner. The UBID itself is neither a database nor a data schema. Rather, it acts as an external common key between databases to facilitate data mapping. The developed ruleset ensures that individual implementers can reach the same conclusion utilizing publicly available digital maps. It can also automatically resolve discrepancies, caused by physical space or structure changes over time, during data exchange. The method is also flexible and scalable to identify a portion of a building or a group of buildings.