OPTIMAL SEGMENTATION OF WALLS BUILT ON SLOPES

Walls built on slopes are often designed with horizontal segments requiring the digging of ditches in which the depth increases with the length of the segment, while the effective (lowest) height of the wall above the ground decreases. The optimal length of the segment is determined by minimizing the total costs—consisting of the fixed cost per segment, cost of digging and cost of builidng for a given required effective height. Models with linear and non-linear digging and building costs for slopes having constant angle of inclination are developed.     

The derivation of vegetation indices from AVHRR data

Abstract

The reliability of a 1-weck composited normalized difference vegetation index has been evaluated by using a cloud-screening algorithm applied to the visible and near-infrared data from the Advanced Very High Resolution Radiometer on NOAA-9. It is found that in some areas ofthe U.S. Great Plains this satellite sensor product may not be reliable due to the high frequency of cloud occurrence. Using the example of day-to-day variation in (he observed clear-sky radiances for one target, the vegetation index is shown to have maxima at high off-nadir and low solar zenith angles: this behaviour has been examined in detail. Some recommendations to improve the compositing technique are given.     

Local Aromaticity in Benzo- and Benzocyclobutadieno-Annelated Phenanthrenes

The effects of benzo- and benzocyclobutadieno-(BCBD-)annelation on the local aromaticity of the central ring (X) of phenanthrene were examined. The local aromaticity of the ring X was estimated using a graph-theory-based index and two geometry-based indices. DFT calculations of phenanthrene congeners showed that many of these molecules are nonplanar, with significant steric strain. All three indices indicate that the effect of benzo-annelation is opposite to that of BCBD-annelation. The deviation of some phenanthrene derivatives from planarity does not influence the effects of annelation. This finding is in accord with some previously reported studies on the dependence of the aromaticity of the ring(s) on distortion from planarity.     

Maintenance policy of aircraft according to multiple criteria

A model has been developed to derive scheduled replacement operating-ago maintenance policies for the critical parts of aircraft based on multiple criteria : the lowest possible operating cost per unit of time, minimal mission reliability, and minimal expected inflight failure cost. An example has been given, using the Weibull distribution.     

A Case of Breakdown of the Kekulé–Structure Model

For a long time, Kekulé structures have been used to predict and rationalize the stability, geometry, and π-electron properties of polycyclic conjugated molecules, especially hydrocarbons. We now point out an example, demonstrating that the Kekulé–structure model is not generally applicable. Namely, the molecule of dibenzo[cd,mn]indeno[123, gf]pyrene is perfectly planar and strain-free. Yet, its geometry (determined by means of an unrestricted symmetry–broken UB3LYP/6-311G(d,p) DFT method) is in complete disagreement with what one would expect on the basis of its Kekulé structures.     

Root clustering of a real matrix in an algebraic region

Necessary and sufficient conditions are given for the eigenvalues of a real matrix to lie in an algebraic region (of the complex plane). These conditions are given in terms of rational functions of the matrix coefficients.     

Global land monitoring from AVHRR: potential and limitations

Global Vegetation Index ( GVI) time series of visible, near-IR and thermal IR Advanced Very High Resolution Radiometer (AVHRR)weekly composite data with a 015^° spatial resolution collected from NOAA-9 and -11 satellites have been used to develop a prototype global land monitoring system. The system is based on standardized anomalies of the Normalized Difference Vegetation Index (NDVI) and channel 4 brightness temperature ( T₄ )for the period April 1985-September 1994. Processing included: post-launch updated calibration; cloud screening; filling in the cloud induced data gaps by monthly averaging and spatial interpolation; suppressing residual noise by smoothing; calculating 5-year monthly means and standard deviations of NDVI and T₄and their standardized anomalies. The derived anomalies show potential for detecting and interpreting the seasonal cycle and statistically significant interannual variability. Yet, discontinuities and residua! trends can be traced in time series of NDVI and T4. Discontinuities result from the switch from NOAA-9 to NOAA-11 in 1988, and the Mount Pinatubo eruption in 1991. Trends are a combined effect of satellite orbit drift and a possible persistent error in post-launch calibration of solar channels. The orbit drift affects the solar and thermal IR channels through systematic variation of illumination geometry and diurnal heating/cooling of the surface and atmosphere, respectively. Examples are given to illustrate the magnitude of these effects, which reduce the ability to monitor small year-to-year surface changes. The present system yields more accurate results in geographic regions, where atmospheric, angular and diurnal variability effects have a lesser impact on the derived anomalies, i.e. over vegetated areas outside the tropics during local summers. For global-scale monitoring, angular, atmospheric and diurnal variability corrections must be incorporated in the present system.     

Cloud screening for determination of land surface characteristics in a reduced resolution satellite data set

Abstract

A spatial coherence method is developed for cloud screening of a reduced resolution data set over the U.S. Great Plains, The method is based upon a comparison of the spatial standard deviation of scene radiance for a particular observation time with that for clear sky conditions (the background variability). The procedure relies on the assumption that the presence of partial cloudiness increases spatial variability. The standard deviation thresholds are tabulated functions of space and time and are presented as the background variability maps. Complete cloud covers are screened by testing the computed skin temperatures and albedos against constant thresholds, whereas night-time cirrus and low levels of cloudiness are delected using the 3.7-11 μm brightness temperature differences. The cloud-screening scheme is tested on daytime and night-time data of July 1986.     

On the relationship between monthly mean and maximum-value composite normalized vegetation indices

Abstract

A relationship between the maximum-value composite and monthly mean normalized difference vegetation index (NDVI) is derived statistically using data over the U.S. Great Plains during 1986. The monthly mean NDVI is obtained using a simple nine-day compositing technique based on the specifics of the scan patterns of the NOAA-9 Advanced Very High Resolution Radiometer (AVHRR). The results indicate that these two quantities are closely related over grassland and forest during the growing season. It is suggested that in such areas a monthly mean NDVI can be roughly approximated by 80 per cent of the monthly maximum NDVI, the latter being a standard satellite data product. The derived relationship was validated using data for the growing season of 1987.