Mapping tree canopies in urban environments using airborne laser scanning (ALS): a Vancouver case study

  Background  The distribution of forest vegetation within urban environments is critically important as it influences urban environmental conditions and the energy exchange through the absorption of solar radiation and modulation of evapotranspiration. It also plays an important role filtering urban water systems and reducing storm water runoff.

  Methods  We investigate the capacity of ALS data to individually detect, map and characterize large (taller than 15 m) trees within the City of Vancouver. Large trees are critical for the function and character of Vancouver's urban forest. We used an object-based approach for individual tree detection and segmentation to determine tree locations (position of the stem), to delineate the shape of the crowns and to categorize the latter either as coniferous or deciduous.

  Results  Results indicate a detection rate of 76.6% for trees > 15 m with a positioning error of 2.11 m (stem location). Extracted tree heights possessed a RMSE of 2.60 m and a bias of -1.87 m, whereas crown diameter was derived with a RMSE of 3.85 m and a bias of -2.06 m. Missed trees are principally a result of undetected treetops occurring in dense, overlapping canopies with more accurate detection and delineation of trees in open areas.

  Conclusion  By identifying key structural trees across Vancouver's urban forests, we can better understand their role in providing ecosystem goods and services for city residents.