ISSN:0143-1161    

DOI:10.1080/01431168808954846

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The spatial resolution of the next generation of sensors for the global monitoring of vegetation is assessed with particular reference to the proposed Moderate Resolution Imaging Spectrometer (MODIS). The main innovative use of such instruments will lie in their ability to monitor land transformations at global and continental scales. Reliable monitoring is shown to rely on the success with which the changes in the phenomena being analysed can be separated from other temporal changes. Depending on the type of spatial change being monitored, sensor properties such as accuracy of registration, resolution and radiometric sensitivity are shown to have greatest importance.

ISSN:0143-1161    

DOI:10.1080/01431168808954847

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The aim of this paper is to investigate the effect of solar zenith angle on the normalized difference vegetation index (NDVI) derived from the Advanced Very High Resolution Radiometer (AVHRR) channel 1 (red) and channel 2 (near-infrared) data. The NDVIs as a function of solar zenith angle are simulated for a nadir-looking sensor. Simulation is essentially the inverse of atmospheric correction of remotely-sensed data. Calculations are made for four surface cover types, namely, the high, moderate and low green-leaf vegetation densities and bare soils. The results show that the NDVI for bare soil remains constant for solar zenith angles up to about 60°, then decreases for solar zenith angles above this. This constant value of NDVI is found to be about 1 per cent larger than the correct (surface) value. The NDVIs for high, moderate and low green-leaf vegetation densities remain constant up to about 30° solar zenith angle. However, for larger solar zenith angles these NDVIs decrease smoothly and significantly. With the exception or the bare soil case, the relative deviation of the NDVI from its constant value has been fitted to a curve as a function of solar zenith angle. The equation of this curve is found to make solar zenith angle corrections to the NDVI with a maximum possible error of about 9 per cent for solar zenith angles up to about 70°. This error decreases as the vegetation leaf density decreases. Since the global vegetation index (GVI) data are usually from the near-nadir view (≤ 10°) and since within this view angle there is negligible off-nadir view effect, the relation obtained in this work may be used to correct GVI data for solar zenith angle effect. But it cannot be used for correcting the data which correspond to larger off-nadir view angles used by the AVHRR. Finally, the results of this investigation show that the multitemporal AVHRR data from about 4 months are needed for producing one GVI image.

ISSN:0143-1161    

DOI:10.1080/01431168808954848

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The estimation of canopy vegetative parameters from reflected radiation would provide valuable information for assessing crop stress. Vegetative and optical properties of cotton, soya beans, grain sorghum and sunflower were measured at three growth stages at different solar zenith angles. Scattering and absorption coefficients and asymptotic reflectance in thematic mapper bands TM 1, 2, 3 and 4 were determined from measured reflectance, transmittance and absorptancc. Plant height, ground cover, leaf area index, leaf overlap index, foliage density and leaf angle were measured at each sampling date. Increasing leaf area increased the scattering coefficient value in band TM4 while the band TM 3 coefficient value remained unchanged. Absorption coefficient value changes in bands TM 4 and TM 3 with leaf area were opposite to those of scattering coefficients. Increases in solar zenith angle caused increased scattering in TM 4 for all crops; however, in TM 3 increases occurred only in cotton and soya beans. Asymptotic reflectance in band TM 3 was reached in all crops at LAIs ≥ 30. Band TM 4 asymptotic reflectance occurred in cotton and sunflowers at LAIs ≥ 30, but for soya beans and grain sorghum LA Is ≥ 6·0 were required. Canopy optical properties were related to Sun angle and canopy vegetative parameters which were classified into three types representing Sun angle, leaf quantity and canopy architecture. Solar zenith angle, leaf area index and foliage density produced the best estimates of canopy optical properties for all crops.

ISSN:0143-1161    

DOI:10.1080/01431168808954849

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The geometrical structure of a vegetation canopy determines the amount of foliage presented to a sensor and the form of the relationship between reflectance and vegetation amount. The aim of this study was to develop a practical measure of vegetation amount that was sensitive to canopy geometry. This measure was termed the radiometric leaf area index (RLAI) and comprised measurements of leaf area index (LAI), leaf inclination or curvature and the area of the canopy visible to the sensor. RLAI, evaluated on simulated and laboratory-derived data, was sensitive to canopy geometry but, like LAI, suffered from a high measurement error. Primarily as a result of error in these data sets reflectance was similarly correlated to both RLAI and LAI and therefore RLAI offered no advantage over LAI for the measurement of vegetation amount. It was concluded that future formulations of RLAI should be more complex and accurate.

ISSN:0143-1161    

DOI:10.1080/01431168808954850

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The remote sensing of agricultural crops has concentrated on the use of red and near-infrared radiance. The increasing availability of middle and thermal infrared radiance data has opened up a new source of spectral information. In grassland areas middle and thermal infrared radiance are usually negatively related to green leaf area index (GLAI). These data can be used in vegetation indices (in addition to red and near-infrared radiance data) to model the GLAI-radiance relationship empirically. The accuracy of GLAI estimation was significantly increased using such indices rather than a red/near-infrared based index. These increases were masked when applying a methodology to allow for sampling error and it is suggested that this was due to this section of the methodology rather than insufficient spectral information from the middle and thermal infrared wavebands.

ISSN:0143-1161    

DOI:10.1080/01431168808954851

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A simple model based on the Kubelka-Munk theory of scattering is used to describe a relationship between reflection ratios (ρ1/ρ2) in two contrasting wavebands and fractional light absorption by a canopy. The analysis reveals that whilst the relationship between ratio and vegetation is curvilinear, it varies linearly with the fraction of photosynthetically active radiation absorbed by vegetation.

ISSN:0143-1161    

DOI:10.1080/01431168808954852

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

This paper describes digital crop classification techniques developed for Australian conditions. Using extensive ground truth data, supervized maximum likelihood was used to classify Landsat data from five dates across approximately half a Landsat scene into winter crops and other land cover types. Inaccuracies in the prediction of total crop hectarage and in the classification of an independent test sample of pixels indicated confusion between wheat and barley for all combinations of dates. However, when wheat, barley and oats were considered together against all non-winter crops, the predictions were very accurate and demonstrated the applicability of this method of classification.

ISSN:0143-1161    

DOI:10.1080/01431168808954853

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The design requirements for a broad-band red and near infrared radiometer for monitoring vegetation from a light aircraft are discussed and an instrument which incorporates these characteristics, called an integrated camera and radiometer (ICAR), is described. It consists of two downward-looking, spectral radiometers and a solar radiometer coupled with a 35 mm camera and a data-logger in a convenient payload for mounting in a light aircraft. The distinctive features of the ICAR arc the synchronization of the camera and radiometers, the equivalence of their fields of view and the integral microcomputer which controls the instrument and acts as a programmable data-logger. The simultaneous aerial photographs are used to locate the data geographically, to select the fields of view which are of the desired target, to interpret the radiometer data, and to act as a record of the terrain conditions for visual interpretation. ICAR data are compared with data from an Exotech 100BX radiometer mounted in an aircraft and also with satellite imaging radiometer data for Ihc same locations.

ISSN:0143-1161    

DOI:10.1080/01431168808954854

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

For monitoring crop growth and development, remote sensing techniques have proved to offer great possibilities. In agricultural field trials a high temporal and spatial resolution are strict requirements. These can be satisfied by using an airborne multispectral photographic (MSP) system. This system and its calibration are described. Results proved the validity of the applied procedure for atmospheric correction and radiometric calibration, resulting in information about crops in field trials with greater precision than by conventional field sampling methods.

ISSN:0143-1161    

DOI:10.1080/01431168808954855

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor