A Clearer Picture of D.C.’s Tree Canopy Using LiDAR
Washington D.C.’s Urban Forestry Administration (UFA) has been keeping a close eye on the District’s tree canopy for the last ten years. Tree canopy cover has been mapped in the District about every five years including 2006, 2011, and the latest in 2016. The 2016 study built off of continual long-term urban tree canopy (UTC) work, allowing UFA to strategically evaluate and monitor progress towards the District?s 40% tree canopy cover goal including on newly acquired school and park properties under UFA management. Areas of canopy gain and loss were identified to help UFA continue its mission to serve citizens, visitors, and dozens of partners in the District with a robust, equitably distributed urban forest. Plan-It Geo, a company specializing in trees and technology, created the land cover data using several remote sensing data sources: 1-meter resolution aerial imagery from the National Agriculture Imagery Program (NAIP), flown in July 2015; 8-centimeter aerial imagery from Sanborn and the DC Office of the Chief Technology Officer (OCTO) flown in April 2015; and a LiDAR point cloud (eight points per square meter) from Sanborn and (OCTO), flown in April 2015. Using SAGA GIS, a normalized digital surface model (nDSM) was created from the LiDAR point cloud by extracting first and last return elevations and subtracting the last return from the first return to create a height model of D.C.?s landscape. Then, using the NAIP imagery and nDSM, an object-based image analysis (OBIA) software program called Feature Analyst was used to classify land cover types through an iterative approach, analyzing spectral signatures across four bands (red, green, blue, and near-infrared) as well as elevation (LiDAR), texture, and spatial patterns. This process resulted in five initial land cover classes including tree canopy, non-canopy vegetation, impervious surfaces, bare soil, and water bodies. This land cover map allows UFA to monitor and measure the extent of their tree canopy at various geographic scales. It also provides spatial information on the space that is available for planting new trees. In fact, UFA and other city agencies and nonprofits plant more than 10,000 trees throughout the District annually in an effort to change the past downward trends in tree canopy coverage and make D.C. a greener, more sustainable, and more livable city.