Measuring stem diameters with TLS in boreal forests by complementary fitting procedure

Timo P. Pitkänen, Pasi Raumonen, Annika Kangas

    Research output: Contribution to journalArticleScientificpeer-review

    26 Citations (Scopus)
    72 Downloads (Pure)

    Abstract

    Point clouds generated by terrestrial laser scanners (TLS) have enabled new ways to measure stem diameters. A common method for diameter calculation is to fit cylindrical or circular shapes into the TLS point cloud, which can be based either on a single scan or a co-registered combination of several scans. However, as various defects in the point cloud may affect the final diameter results, we propose an automatized processing chain which takes advantage of complementing steps. Processing consists of two fitting phases and an additional taper curve calculation to define the final diameter measurements. First, stems are detected from co-registered data of several scans using surface normals and cylinder fitting. This provides a robust framework for localizing the stems and estimating diameters at various heights. Then, guided by the cylinders and their indicative diameters, another fitting round is performed by cutting the stems into thin horizontal slices and reassessing their diameters by circular shape. For each slice, the quality of the cylinder-modelled diameter is evaluated first with co-registered data and if it is found to be deficient, potentially due to modelling defects or co-registration errors, diameter is detected through single scans. Finally, slice diameters are applied to construct a spline-based taper curve model for each tree, which is used to calculate the final stem dimensions. This methodology was tested in southern Finland using a set of 505 trees. At the breast height level (1.3 m), the results indicate 5.2 mm mean difference (3.2%), −0.4 mm bias (-0.3%) and 7.3 mm root mean squared error (4.4%) to reference measurements, and at the height of 6.0 m, respective values are 6.5 mm (3.6%), +1.6 mm (0.9%) and 8.4 mm (4.8%). These values are smaller compared to most of the corresponding contemporary studies, and outperform the initial cylinder models. This indicates that the applied processing chain is capable of producing relatively accurate diameter measurements, which can, at the cost of computational heaviness, remove various defects and improve the modelling results.

    Original languageEnglish
    Pages (from-to)294-306
    Number of pages13
    JournalISPRS Journal of Photogrammetry and Remote Sensing
    Volume147
    Early online date8 Dec 2018
    DOIs
    Publication statusPublished - 2019
    Publication typeA1 Journal article-refereed

    Keywords

    • Circle fitting
    • Cylinder fitting
    • Stem extraction
    • Taper curve
    • Terrestrial laser scanning

    Publication forum classification

    • Publication forum level 1

    ASJC Scopus subject areas

    • Atomic and Molecular Physics, and Optics
    • Engineering (miscellaneous)
    • Computer Science Applications
    • Computers in Earth Sciences

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