The urban forest is becoming increasingly important in the contexts of urban green space, carbon sequestration and offsets, and socio-economic impacts. This has led to a recent increase in attention being paid to urban environmental management. Tree biomass, specifically, is a vital indicator of carbon storage and has a direct impact on urban forest health and carbon sequestration. As an alternative to expensive and time-consuming field surveys, remote sensing has been used extensively in measuring dynamics of vegetation and estimating biomass. Light detection and ranging (LiDAR) has proven especially useful to characterize the three dimensional (3D) structure of forests. In urban contexts however, information is frequently required at the individual tree level, necessitating the proper delineation of tree crowns. Yet, crown delineation is challenging for urban trees where a wide range of stress factors and cultural influences affect growth. In this paper high resolution LiDAR data were used to infer biomass based on individual tree attributes. A multi-tiered delineation algorithm was designed to extract individual tree-crowns. At first, dominant tree segments were obtained by applying watershed segmentation on the crown height model (CHM). Next, prominent tree top positions within each segment were identified via a regional maximum transformation and the crown boundary was estimated for each of the tree tops. Finally, undetected trees were identified using a best-fitting circle approach. After tree delineation, individual tree attributes were used to estimate tree biomass and the results were validated with associated field mensuration data. Results indicate that the overall tree detection accuracy is nearly 80%, and the estimated biomass model has an adjusted-R2 of 0.5.
Airborne Light Detection and Ranging (LiDAR) is used in many 3D applications, such as urban planning, city modeling,
facility management, and environmental assessments. LiDAR systems generate dense 3D point clouds, which provide a
distinct and comprehensive geometrical description of object surfaces. However, the challenge is that most of the
applications require correct identification and extraction of objects from LiDAR point clouds to facilitate quantitative
descriptions. This paper presents a feature-level fusion approach between LiDAR and aerial color (RGB) imagery to
separate urban vegetation and buildings from other urban classes/cover types. The classification method used structural
and spectral features derived from LiDAR and RGB imagery. Features such as flatness and distribution of normal vectors
were estimated from LiDAR data, while the non-calibrated normalized difference vegetation index (NDVI) was
calculated by combining LiDAR intensity at 1064 nm with the red channel from the RGB imagery. Building roof tops
have regular surfaces with smaller variation in surface normal, whereas tree points generate irregular surfaces. Tree
points, on the other hand, exhibit higher NDVI values when compared to returns from other classes. To identify
vegetation points an NDVI map was used, while a vegetation mask was also derived from the RGB imagery. Accuracy
was assessed by comparing the extraction result with manually digitized reference data generated from the high spatial
resolution RGB image. Classification results indicated good separation between building and vegetation and exhibited
overall accuracies greater than 85%.
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