摘要:

An airborne laser profiling altimeter was used to measure surface features and properties of the landscape during the HAPEX-Sahel Experiment in Niger, Africa in September 1992. The laser altimeter makes 4000 measurements per second with a vertical resolution of 5 cm. Airborne laser and detailed field measurements of vegetation heights had similar average heights and frequency distribution. Laser transects were used to estimate land surface topography, gully and channel morphology, and vegetation properties ( height, cover and distribution). Land surface changes related to soil erosion and channel development were measured. For 1 km laser transects over tiger bush communities, the maximum vegetation height was between 4-5 and 6-5 m, with an average height of 21 m. Distances between the centre of rows of tiger bush vegetation averaged 100 m. For two laser transects, ground cover for tiger bush was estimated to be 225 and 301 per cent for vegetation greater than 0-5m tall and 190 and 25-8 per cent for vegetation greater than 10m tall. These values are similar to published values for tiger bush. Vegetation cover for 14 and 18 km transects was estimated to be 4 per cent for vegetation greater than 0-5 m tall. These cover values agree within 1-2 per cent with published data for short transects (⩾ 100 m) for the area. The laser altimeter provided quick and accurate measurements for evaluating changes in land surface features. Such information provides a basis for understanding land degradation and a basis for management plans to rehabilitate the landscape.

ISSN:0143-1161    

DOI:10.1080/01431169608949179

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The remote sensing data of a rugged terrain always suffer from topographic effects and, in such cases people frequently perceive valleys as ridges, and vice versa in the optical imagery of Sun-synchronous satellite. The appearance of inverse topography can be termed as false topography perception ( FTP) FTP has been observed in various satellites images such as Landsat, IRS, and SPOT The problem of FTP becomes more severe in high altitude areas of hilly terrain, such as Himalayan terrain. FTP is usually caused by the combination of various interrelated factors; among them are topographic relief, Sun elevation, the azimuth angle, viewing angle, and hatching or engraving features present on the slopes. Once satellite data of rugged terrain are acquired, then all FTP-influencing factors are fixed, except for the viewing angle of the observer. Hence, the correction of FTP becomes straightforward. There are two ways to correct satellite dala for FTP effects: ( a) rotate the image by between 135° -180° i.e changing the viewing angle of the observer; or (b) inverse the image data by making a negative of the whole image (255-image) which will bring change in the Sun' s illumination by 1807deg; The present paper discusses the FTP phenomena and its causes, and suggests methods of correction.

ISSN:0143-1161    

DOI:10.1080/01431169608949180

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Measurement error is an important component of variation in most measured variables and also, therefore, in the sample variogram of field-based reflectance. The variogram of the underlying variation in reflectance is regularized and, therefore, must be continuous through the origin. The variogram of measurement error in reflectance, however, is unlikely to pass continuously through the origin. Therefore, the sample semivariance at a lag of just grealer than zeroyv(0++ ) is likely to be some positive value due solely to the error in measuring reflectance. We recommend that where possibleyv(0++ ) should be computed by repeated measurement of reflectance al the same location, x, and over the same support, v. If repeated measurement is not possible then in certain circumstances the nugget variance of the variogram model may be used to estimate measurement error in the sample variogram denotedyvME, IFyvis not estimated and measurement error is not separated from the underlying variation then geostatistical techniques that depend on yv( h) and which are currently being applied in remote sensing may be affected. We demonstrate these ideas with a simple example involving regularizing the variogram of the Normalized Difference Vegetation Index (NDVI) of a field of pasture measured with a field radiometer.

ISSN:0143-1161    

DOI:10.1080/01431169608949181

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Optimal filters provide minimum variance interpolation and smoothing of noisy data, but the form of the noise power spectrum must be known for their design. We analyse data from the Microwave Limb Sounder (MLS) experiment onboard the Upper Atmosphere Research Satellite (UARS) to quantify parameters describing the noise spectra. MLS views space as part of its calibration sequence providing regular samples of the radiometer output. From these samples we determine breakpoint frequencies at which the spectral power density of the low frequency l/fequal to that of the white noise. Breakpoint frequencies were in the range from 0005 to 002Hz for the 90 filter channels of the MLS. A method was developed to distinguish between 11/fand systematic disturbances caused by cyclic changes in the operating environment as the spacecraft traverses its orbit.

ISSN:0143-1161    

DOI:10.1080/01431169608949182

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

ISSN:0143-1161    

DOI:10.1080/01431169608949183

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

ISSN:0143-1161    

DOI:10.1080/01431169608949184

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor