Remote sensing for resources development and environmental management: Remote sensing for resources development and environmental management

Damen, M. C. J.

Bibliographic data

Multivolume work

Persistent identifier:: 856342815

Title:: Remote sensing for resources development and environmental management

Sub title:: proceedings of the 7th international Symposium, Enschede, 25 - 29 August 1986

Year of publication:: 1986

Place of publication:: Rotterdam; Boston

Publisher of the original:: A. A. Balkema

Identifier (digital):: 856342815

Language:: English

Additional Notes:: Volume 1-3 erschienen von 1986-1988

Editor:: Damen, M. C. J.

Document type:: Multivolume work

Volume

Persistent identifier:: 856343064

Title:: Remote sensing for resources development and environmental management

Sub title:: proceedings of the 7th international Symposium, Enschede, 25 - 29 August 1986

Scope:: XV, 547 Seiten

Year of publication:: 1986

Place of publication:: Rotterdam; Boston

Publisher of the original:: A. A. Balkema

Identifier (digital):: 856343064

Illustration:: Illustrationen, Diagramme

Signature of the source:: ZS 312(26,7,1)

Language:: English

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Editor:: Damen, M. C. J.

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Volume

Collection:: Earth sciences

Chapter

Title:: 3 Spectral signatures of objects. Chairman: G. Guyot, Liaison: N. J. J. Bunnik

Document type:: Multivolume work

Structure type:: Chapter

Chapter

Title:: The canopy hot-spot as crop identifier. S. A. W. Gerstl, C. Simmer & B. J. Powers

Document type:: Multivolume work

Structure type:: Chapter

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 The canopy hot-spot as crop identifier S.A.W.Gerstl, C.Simmer* & B.J.Powers Los Alamos National Laboratory, Theoretical Division, N.Mex., USA * Present address: Institut für Meereskunde an der Universität Kiel, Maritime Meteorologie, FR Germany ABSTRACT: Illuminating any reflective rough or structured surface by a directional light source results in an angular reflectance distribution that shows a narrow peak in the direction of retro-reflection. This is called the Heiligenschein or hot-spot of vegetation canopies and is caused by the absence of mutual shading of leaves. The angular intensity distribution of the hot-spot, its brightness and slope, are therefore indicators of the plant's geometry. We propose the use of hot-spot angular characteristics as crop identifiers in satellite remote sensing because the canopy hot-spot carries information about plant stand architecture that is more distinctive for different plant species than for instance their spectral reflectance characteristics. A simple three-dimensional Monte Carlo/ray tracing model and an analytic two-dimensional model are developed to estimate the angular distribution of the hot-spot as a function of thesize of the plant leaves. The results show that the brightness-distribution and slope of the hot-spot change distinctively for different leaf sizes indicating a much more peaked maximum for the smaller leaves. All rough and structured surraces illuminated by a directional light source with a wavelength considerably smaller than the size of the constituents of the surface show a local maximum of the reflected radiation within a cone around the direction of retro-reflection. If the sun is considered as the only radiation source that illuminates a vegetated surface (e.g. a grass lawn, an agricultural field, or a forest), and the angular distribution of the reflected sunlight around the shadow of the observing instrument is measured, a narrow intensity peak is observed in the reverse solar direction when the observer's shadow can be eliminated. This effect is called the canopy hot-spot in agricultural remote sensing 1 the Heiligenschein in atmospheric optics 2 , and the opposition effect in planetary physics 3,4 , and is caused by the absence of mutual shading of the surface's constituents. When observed precisely in the direction of the incident radiation, only the illuminated parts of the surface structures (e.g. leaves) are seen, while in all other view directions the shadowed parts are also observed which leads to a reduced reflected light intensity. Since this effect is purely based on shadowing, no colors are produced, which is a definite distinction from the glory that also appears in the solar retro-direction . Figure 1 shows a photographed canopy hot-spot, taken from an airplane at about 500 ft. above a coniferous forest. In Figure 2 a hot-spot is shown of grassland as photographed from about 2000 ft. with standard panchromatic film. In Fig. 3 an infrared photograph of a hot-spot over a coniferous forest is shown. In this figure the hot-spot is less prevalent because the canopy reflectance in the near IR is larger than in the visible, which leads to reduced contrast. The airplane's shadow in the center of the hot-spot is no longer visible due to the large distance of the shadow from the airplane (penumbra effect) and the competing brightness of the hot-spot retro-reflection. The angular intensity distribution of the hot-spot, its brightness and slope, over unknown surfaces of planetary bodies, has been used to estimate the roughness of the moon's surface 3 , the size of the particles that make Saturn's rings, and other planetary surface characteristics 4 . However, the use of observable hot-spot angular Figure 1. Canopy hot-spot (Heiligenschein) over coniferous forest in the visible. characteristics tor crop identification has not yet been exploited. We propose such use for satellite remote sensing because the canopy hot-spot carries information about plant stand architecture that is more distinctive for different plant species than for instance their spectral reflectance characteristics. Bunnik et al 5 have studied the usefulness of spectral indices measured in the hot-spot direction (exact retro-reflection) for biomass determination of agricultural crops. We developed a simple three-dimensional Monte Carlo/ray tracing model to estimate the angular distribution of the canopy hot-spot as a function of the size and orientation of the plant leaves that constitute the vegetation canopy. The reflecting canopy is assumed to be a layered medium with square or circular horizontally oriented leaves. For simplicity, take the direction of the incident radiation to be perpendicular to the layers and leaves. This may be considered a simple representation of a planophile and heliotropic

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