Abstract
The data acquisition from normalized digital surface models (nDSM), based on regularly conducted aerial image surveys, represents an efficient opportunity for forest enterprises to ensure topicality as well as quality for tree height information. Two of such image data sets—L60 (60% along-track/30% across-track) and L80 (80/40%)—were each used to create an nDSM for a 23.5 km\(^{2}\) large study area in Saxony. The objective of the present study was to assess the accuracy of both nDSMs and to examine the influence of image overlap, tree type and terrain slope on the results of this accuracy assessment. Point clouds were initially extracted from the corresponding images using semi-global matching and were eventually used to create a digital surface model. An nDSM was produced by subtracting a digital terrain model to get absolute tree heights. The comparison between this nDSM and terrestrial measurements (n = 51) resulted in root mean square errors between 2.1 m and 2.2 m. Although opposing median values were found for the nDSMs based on L60 (−0.3 m) and L80 (0.5 m), no significance was found between the two models (Wilcoxon test, p = 0.16). However, large overlapping areas and lower terrain slopes have a positive effect on the completeness of the nDSM (L60: 92.9%, L80: 99.2%). Although the small number of measurements does not allow for more than the identification of a tendency, these results show the potential of image-based nDSMs for the regular update of tree height information in Saxony and beyond.
Zusammenfassung
Bewertung der Genauigkeit der Baumhöhenbestimmung in Sachsen auf der Basis von normierten Oberflächenmodellen. Die Datengewinnung aus normierten Digitalen Oberflächenmodellen (nDOM) auf Basis regelmäßig durchgeführter Luftbildaufnahmen stellt für Forstbetriebe eine effiziente Möglichkeit dar, Aktualität und Qualität der Baumhöheninformationen zu gewährleisten. Unter Verwendung zweier derartiger Luftbilddatensätze—L60 (60% Längsüberlappung/30% Querüberlappung) and L80 (80%/40%)—wurde jeweils ein nDOM für ein 23.5 km\(^{2}\) großes Untersuchungsgebiet in Sachsen generiert. Ziel der vorliegenden Studie war es, die Genauigkeit der beiden nDOMs zu untersuchen und den Einfluss der Bildüberlappung, der Baumart sowie der Geländeneigung auf das Resultat zu analysieren. Unter Verwendung des Semi-Global Matching wurden Punktwolken aus den entsprechenden Luftbildern extrahiert und diese zur Erzeugung eines Digitalen Oberflächenmodells genutzt. Durch Subtraktion eines Digitalen Geländemodells entstand jeweils ein die Baumhöhen wiedergebendes nDOM, dessen Genauigkeit durch den Vergleich mit terrestrischen Referenzmessungen (n = 51) untersucht wurde. Im Rahmen dieser Evaluation ergaben sich Root Mean Square Errors von 2.1 m bis 2.2 m. Obwohl sich für L60 (−0.3 m) und L80 (0.5 m) gegensätzliche Medianwerte ergaben, wurde keine Signifikanz zwischen den beiden Modellen festestellt (Wilcoxon-Test, p = 0.16). Allerdings haben große Überlappungsbereiche sowie eine geringe Hangneigung einen positiven Einfluss auf die Vollständigkeit des Höhenmodells (L60: 92.9%, L80: 99.2%). Obwohl die geringe Anzahl an Referenzmessungen nur Rückschlüsse auf Genauigkeitstrends zulässt, zeigen diese Resultate das Potential von Luftbild-basierten nDOMs für die regelmäßige Aktualisierung von Baumhöheninformationen in Sachsen und vergleichbaren Regionen.
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Zimmermann, S., Hoffmann, K. Accuracy Assessment of Normalized Digital Surface Models from Aerial Images Regarding Tree Height Determination in Saxony, Germany. PFG 85, 257–263 (2017). https://doi.org/10.1007/s41064-017-0021-4
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DOI: https://doi.org/10.1007/s41064-017-0021-4