TreesCount!2015 engaged over 2,200 volunteers in a citizen science project to create a spatially accurate digital inventory of all New York City street trees. Data is collected using an innovative method that combines site-surveying methods with geospatial technology to derive the location of street trees with a high degree of accuracy. The user-centric mapping method was developed by a local nonprofit, TreeKIT, in combination with a web platform application created by Azavea. In addition to supporting volunteer data collection, TreesCount! also provides an online training module, event registration and management, and provides live metrics and a progress map on the status of the data collection. This application illustrates how citizen science can support the collection of high quality spatial data for municipal urban forest management as well as short-term and long-term citizen engagement. The complete dataset has been given back to the public as open data, and was also uploaded into an operational database for daily municipal use where it will be constantly updated and visible to the public through an interactive tree map.