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Incidence Angle Dependency of Leaf Vegetation Indices from Hyperspectral Lidar Measurements

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We have studied the effect of incidence angle on the spectral content of leaf measurements from hyperspectral light detection and ranging (lidar) data. New results obtained for different ornamental plant leaves indicate that their backscatter properties do not follow the Lambert scattering law, especially in the visible wavelength range: specular reflections were observed near the normal incidence. Also the vegetation spectral indices, such as normalized difference vegetation index (NDVI), or even the simple ratios may change with the laser incidence angle to the target. The reason for this is the difference in their backscatter vs. intensity behaviour between visible and near-infrared (NIR) wavelengths. In comparison with earlier results it turns out that this phenomenon seems to depend on the internal structure and surface properties of leaves. Further information on the extent and role of this effect for different leaves is needed, but our results indicate that the nature of laser reflection in tree canopies may vary between species. The calibration of hyperspectral lidar vegetation reflectance measurements must be further studied by rigorous experiments and modelling.



Abhängigkeit von Vegetationsindices für Blätter vom Inzidenzwinkel aus hyperspektralen Laserscanner-Messungen. In diesem Beitrag wird der Einfluss des Einfallswinkels auf spektrale Indices, welche aus hyperspektralen Laserscanner-Messungen von Blättern abgeleitet werden, untersucht. Neue Ergebnisse für Blätter von verschiedenen Zierpflanzen zeigen, dass deren Rückstreuverhalten vor allem im sichtbaren Bereich des elektromagnetischen Spektrums nicht dem eines Lambertschen Strahlers entspricht: Bei genähert senkrechter Einfallrichtung wurde gerichtete Reflexion beobachtet. Auch spektrale Vegetationsindices wie z. B. der Normalized Difference Vegetation Index (NDVI) oder auch nur einfache Verhältnisse können sich mit dem Einfallswinkel des Laserstrahlsändern. Der Grund dafür ist ein unterschiedliches Rückstreuverhalten im sichtbaren Bereich bzw. im nahen Infrarot. Im Vergleich mit früheren Ergebnissen scheint dieses Phänomen von der inneren Struktur und den Oberflä - cheneigenschaften der Blätter abzuhängen. Während tiefergehende Untersuchungen zum Ausmaß und zur Rolle dieses Effekts noch ausstehen, weisen unsere Ergebnisse darauf hin, dass die Art der Laserreflexion für unterschiedliche Spezies variieren könnte. Die Kalibrierung von Reflexionsgraden aus multispektralen Laserscanner-Messungen erfordert weitere Studien in Hinblick auf eine strenge Modellierung und experimentelle Validierung.
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Keywords: HYPERSPECTRAL; LIDAR; VEGETATION INDICES

Document Type: Research Article

Publication date: 01 May 2016

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  • Photogrammetrie - Fernerkundung - Geoinformation (PFG) is an international scholarly journal covering the progress and application of photogrammetric methods, remote sensing technology and the intricately connected field of geoinformation processing.

    Papers published in PFG highlight new developments and applications of these technologies in practice. The journal hence addresses both researchers and student of these disciplines at academic institutions and universities and the downstream users in both the private sector and public administration.

    PFG places special editorial emphasis on the communication of new methodologies in data acquisition, new approaches to optimized processing and interpretation of all types of data which were acquired by photogrammetric methods, remote sensing, image processing and the computer-aided interpretation of such data in general.

    PFG is the official journal of the German Society of Photogrammetry and Remote Sensing.
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