摘要:

A radiative transfer model was used to explore how the error in inferring spectral hemispherical reflectance (pλ) from nadir reflectance values varies as a function of wavelength, solar zenith angle, leaf area index and leaf orientation distribution. Secondly, a technique using multiple spectral nadir reflectance values to inferpλfor a single wavelength was tested using field data. In addition, several techniques that use multiple off-nadir view angles taken in azimuth planes (calledstringsof data) were tested using field data. These latter techniques were very accurate (with errors less than 4 percent of the true value)and are ideally suited to present and future sensor systems that scan in a known azimuth plane (e.g. Advanced Very High Resolution Radiometer (AVHRR) and other scanning radiometers) or view fore and aft in a known azimuth plane (e.g. Advanced Solid-State Array Sensor (ASAS)Moderate Resolution Imaging Spectrometer (MODIS)High Resolution Imaging Spectrometer (HIRIS)), a brief analysis was performed to explore the effects of errors in hemispherical reflectance on terrestrial energy budget and productivity calculations.

ISSN:0143-1161    

DOI:10.1080/01431168708954813

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

A three-dimensional tree canopy model of scenes which consist of a discontinuous tree layer and a continuous field layer is described. ‘Scene’ or spatial average tree canopy layer transmissions are defined for direct solar radiation, diffuse sky radiation and emission of reflected radiation from the ground and field layers. Hemispherical canopy photographs are used to measure these three types of transmission and the measurements provide the parameters for the model which can be inverted to give estimates of the spatial average properties of a scene. The model requires only the frequency distribution of tree canopy diameters to be specified and these can be measured on the ground or on aerial photographs. The effect of each canopy is additive and so the model can be applied to any pattern and density of trees. Hemispherical photographs can be reinterpreted for any solar azimuth and zenith angle and are therefore applicable for long periods of time. Measurements in two areas of savanna woodland in Kordofan, Sudan, showed that canopy cover measured in the traditional way, which treats the canopy as opaque, overestimated interception by approximately half. Comparisons betweenAcacia Senegaland other species indicate that there are some significant differences in transmission characteristics between savanna tree species. The relevance of these observations to remote sensing of savanna field layer primary production is discussed.

ISSN:0143-1161    

DOI:10.1080/01431168708954814

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

AVHRR-LAC thermal data and Landsat MSS and TM spectral data were used to estimate the rate of forest clearing in Mato Grosso, Brazil, between 1981 and 1984. The Brazilian state was stratified into forest and non-forest. A list sampling procedure was used in the forest stratum to select Landsat MSS scenes for processing based on estimates of fire activity in the scenes. Fire activity in 1984 was estimated using AVHRR-LAC thermal data. Slate-wide estimates of forest conversion indicate that between 1981 and 1984, 353966 ha ±77 000 ha (0·4 percent of the state area) were converted per year. No evidence of reforestation was found in this digital sample. The relationship between forest clearing rate (based on MSS-TM analysis)and fire activity (estimated using AVHRR data)was noisy (R2= 0·41). The results suggest that AVHRR data may be put to better use as a stratification tool rather than as a subsidiary variable in list sampling.

ISSN:0143-1161    

DOI:10.1080/01431168708954815

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Landsat MSS data were evaluated for mapping forest cover types in the northern Lower Peninsula of Michigan. The study examined seasonal variations, interpretation procedures and vegetation composition/distribution and their effect on overall classification accuracy and ability to identify individual pine species. Photographic images were used for visual interpretations while digital analysis was performed using a common (ERDAS) microcomputer image processing system. The classification schemes were evaluated using contingency tables and were ranked using the KAPPA statistic. The various classification schemes were ranked differentially according to study site location. Visual interpretation procedures ranked best, or least accurate, depending on the spatial distribution and complexity of the forest cover. Supervised classification techniques were more accurate than unsupervised clustering over all sites and seasons. Maximum likelihood classification of June data was superior to any digital classification technique of February data. The study indicates that classification accuracy is more dependent on the composition and distribution of forests in the northern Lower Peninsula of Michigan than on the selection of a particular classification scheme.

ISSN:0143-1161    

DOI:10.1080/01431168708954816

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Hand-held MARK-II radiometric measurements were used to monitor levels of water stress in buffelgrass (Cenchrus ciliaris L.) plots. Three levels of irrigation treatments were applied: dry, medium and wet. The radiometric RED (630 to 690 nm) and NIR (760 to 900 nm) measurements obtained from the plots were converted to reflectance factors and to perpendicular and normalized difference vegetation indices (PVI and NDVI respectively). Reflectance and vegetation indices were monitored over the growing season to trace buffelgrass development and response to rainfall and irrigation treatments. Higher than normal rainfall in 1986 moderated the water stress effects. However, we found that a range manager, using a hand-held radiometer, could detect (1) initial plant cover development due to increases in chlorophyll amounts at first of season in RED band, (2) continuing plant structure development in the NIR band, (3) maximum biomass in both RED and NIR bands, (4) initial onset of water stress due to plant structure changes in NIR band and (5) prolonged water stress effects due to chlorophyll concentration changes in the RED band. Water stress onset was detected in the NIR and PVI 24 days before being apparent in the RED or to visual observations; stress might have appeared sooner in the RED band if rainfall had been below normal or for soil with a lower water holding capacity. The NIR and PVI vegetation index provided better separation among the three levels of water stress treatments due to the effect on plant structure than did the RED and NDVI. These results indicated that hand-held radiometers may be useful tools that range managers can use to monitor water stress in grasses.

ISSN:0143-1161    

DOI:10.1080/01431168708954817

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Field spectroscopy involves the study of the interrelationships between the spectral characteristics of objects and their biophysical attributes in the field environment. It is a technique of fundamental importance in remote sensing, yet its full potential is rarely exploited. In this article the principles of the subject are explained and its historical development reviewed with reference to the instruments and methods adopted. Field spectroscopy has a role to play in at least three areas of remote sensing. Firstly, it acts as a ridge between laboratory measurements of spectral reflectance and the field situation and is thus useful in the calibration of airborne and satellite sensors. Secondly, it is useful in predicting the optimum spectral bands, viewing configuration and time to perform a particular remote sensing task. Thirdly, it provides a tool for the development, refinement and testing of models relating biophysical attributes to remotely-sensed data.

ISSN:0143-1161    

DOI:10.1080/01431168708954818

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The maximum likelihood and the nearest neighbour classification algorithms are reviewed, particularly from the point of view of user/analyst requirements. The two algorithms were put to use for the classification or Landsat TM data of agricultural scenes and accuracy with respect to ‘ground truth’ was evaluated using different parametric settings. Results show that within the maximum likelihood classification, accuracies and errors can vary to a considerable degree depending on the formation of the statistical classes from the training data. More interestingly, it was found that the nearest neighbour algorithm produced higher accuracies and was judged to be more robust, but it has computer implementation problems with high data dimensionality.

ISSN:0143-1161    

DOI:10.1080/01431168708954819

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Although the idea of using a satellite as a source of data for settlement studies is not new, actual measurements of the areal extent of settlements on a large scale has been meagre. A digital analytical approach has been used to identify and measure the areal extent of settlements in the Oyo-Ogbomoso-Ilorin area of Nigeria. Using 95 sampled pixels selected from Ilorin, Ogbomoso and Oyo as training areas, an area of about 249 km2has been classified as settlements. It was observed that settlements with built-up areas of about 300 h were identifiable from Landsat. Despite this observation, there is a need for further research to determine the level of accuracy of settlement identification and areal measurements which is possible from Landsat MSS data. The procedure of selecting subscenes to define known settlements is suggested as being a more accurate way of measuring the areal extent of settlements.

ISSN:0143-1161    

DOI:10.1080/01431168708954820

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

ISSN:0143-1161    

DOI:10.1080/01431168708954822

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

ISSN:0143-1161    

DOI:10.1080/01431168708954823

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor