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

Damen, M. C. J.

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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 derivation of a simplified reflectance model for the estimation of LAI. J. G. P. W. Clevers

Document type:: Multivolume work

Structure type:: Chapter

217 absorption for nproved and srical solution Lthin nine rete layer, fas described 5d on a theory the transfer Lffusing media, fard fluxes Allen et al. îclude the Duntley Lytical model rvation id is an i and his tes plant fication) and is are carried plifications Lk, 1981). aits model action factions of not introduce netry to anents as set of 3 model is Ltrarily a three- adiative dividing the s usually iaracteristies nd optical ractical tances for oasis is of relation- .ni-empirical atroduced 1 background. correction verified by 1. ETATION eflectance AI; between physically nd in order to be done; (preferably in a visible istance, the soil in the reason soil sor geometry n canopy, soil 1 projection erequisite for new definition ith the soil should be la). Further, r to be able fraction of . soil that is sun sensor — 0 - Figure 2: Schematic presentation of a simplified reflectance model for vegetation and soil combined. Figure 1: Schematic presentation to illustrate aspects of the new definition of soil cover. C = covered soil; N = soil not covered or illuminated, a: Soil illuminated by the sun. b: Soil visible to a sensor, c: Illuminated soil visible to this sensor. illuminated by the sun as well as directly detectable by the sensor) will be classified as the fraction of soil that is not covered (figure lc). The complemen tary fraction will now be defined as soil cover ("apparent soil cover"). In the special situation of the sensor looking vertically downwards, this definition of soil cover is equivalent to the relative vertical projection of green vegetation, the relative area of the shadows included. In order to ascertain whether there is a useful relationship between infrared reflectance and LAI for green vegetation, the former should be corrected for soil background, because it may influence infrared reflectance independently of the LAI. The infrared reflectance is then calculated for the situation of the visible background being completely black and not reflecting any radiation. This corrected infrared reflectance value is then used to estimate LAI. Let us consider the simple situation of a surface, partly covered with green vegetation and partly bare (figure 2). The fraction of the surface covered with vegetation is called soil cover, B. If the reflectance of the soil is called r and the reflectance of the s vegetation r , then the total measured reflectance, r, will equaY: r = r v . B + r s . (1-B) (1) A green band will be denoted by the subscript g and equation (1) is then written as: r = g v,g s, g (1-B) ( 2) The reflectance of vegetation in a green band (r ) may be regarded as being independent of the numbed of leaf layers, because leaf transmittance in the green is assumed to be negligible. Hence equation (2) describes the linear relationship between the reflectance in a green band and soil sover, if the soil reflectance can be considered to be constant (constant soil moisture content). Analogously, by attaching the subscript r to a red band reflectance, we have: r r r v,r B + r s,r (1-B) ( 3) Because r can also be regarded as being independent of the number of leaf layers, this equation describes the linear relationship between red reflectance and soil cover. For an infrared band the subscript ir will be used and equation (1) is then written as: r. xr r . . B + r v,ir s,ir (1-B) ( 4) In this equation r . is not independent of tne number of leaf layers, so V ife r may not be regarded as a con stant. This means that the reflectance measured in an infrared band (r. ) is not a linear function of • i lr soil cover. For estimation of LAI the corrected reflectance r' could be used. It is (according to equation 1) defined as: r' = r - r . (1-B) = r . B (5) s v The corrected reflectance is the reflectance one would have obtained with a black background. In order to obtain the corrected infrared reflectance equation (5) first has to be applied to the infrared band: r ! ir r. xr r s, ir (1-B) ( 6) B can be ascertained by means of equation (2) or (3) . However, the reflectance of bare soil and of vegeta tion in a green or red band should be known. Although it is quite often possible to ascertain a good estimate for the reflectance of vegetation (complete cover), estimating the reflectance of bare soil poses greater difficulties. The reflectance of a soil may change very rapidly, according to soil moisture content. Also, very large local differences in soil moisture content may occur. At low soil cover this may cause large inaccuracy if neither the soil moisture content nor the actual reflectance of the soil are known. To obtain an accurate estimate of LAI one either has to know or to measure the reflectance of the bare soil, or one has to derive a relation that is less dependent on differences in soil moisture content. For many soil types, reflectance in the different spectral bands does not differ very much (e.g. Condit, 1970); often there is a slight increase in reflectance with increasing wavelength. However, often the ratio of the reflectance in two spectral bands is independent of the soil moisture content:

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