Impact of orthorectification and spatial sampling on maximum NDVI composite data in mountain regions

Topography and accuracy of image geometric registration significantly affect the quality of satellite data, since pixels are displaced depending on surface elevation and viewing geometry. This effect should be corrected for through the process of accurate image navigation and orthorectification in order to meet the geolocation accuracy for systematic observations specified by the Global Climate Observing System (GCOS) requirements for satellite climate data records. We investigated the impact of orthorectification on the accuracy of maximum Normalized Difference Vegetation Index (NDVI) composite data for a mountain region in north-western Canada at various spatial resolutions (1 km, 4 km, 5 km, and 8 km). Data from AVHRR on board NOAA-11 (1989 and 1990) and NOAA-16 (2001, 2002, and 2003) processed using a system called CAPS (Canadian AVHRR Processing System) for the month of August were considered. Results demonstrate the significant impact of orthorectification on the quality of composite NDVI data in mountainous terrain. Differences between orthorectified and non-orthorectified NDVI composites (ΔNDVI) adopted both large positive and negative values, with the 1% and 99% percentiles of ΔNDVI at 1 km resolution spanning values between − 0.16 < ΔNDVI < 0.09. Differences were generally reduced to smaller numbers for coarser resolution data, but systematic positive biases for non-orthorectified composites were obtained at all spatial resolutions, ranging from 0.02 (1 km) to 0.004 (8 km). Analyzing the power spectra of maximum NDVI composites at 1 km resolution, large differences between orthorectified and non-orthorectified AVHRR data were identified at spatial scales between 4 km and 10 km. Validation of NOAA-16 AVHRR NDVI with MODIS NDVI composites revealed higher correlation coefficients (by up to 0.1) for orthorectified composites relative to the non-orthorectified case. Uncertainties due to the AVHRR Global Area Coverage (GAC) sampling scheme introduce an average positive bias of 0.02 ± 0.03 at maximum NDVI composite level that translates into an average relative bias of 10.6% ± 19.1 for sparsely vegetated mountain regions. This can at least partially explain the systematic average positive biases we observed relative to our results in AVHRR GAC-based composites from the Global Inventory Modeling and Mapping Studies (GIMMS) and Polar Pathfinder (PPF) datasets (0.19 and 0.05, respectively). With regard to the generation of AVHRR long-term climate data records, results suggest that orthorectification should be an integral part of AVHRR pre-processing, since neglecting the terrain displacement effect may lead to important biases and additional noise in time series at various spatial scales.     

基于Hadoop平台的K-means聚类算法

传统的K-means算法虽然具有很多优点,但聚类准则函数对簇密度不均的数据集分类效果较差.文中在加权标准差准则函数的基础之上,增加了收敛性判定,并在Hadoop平台上提出了一种基于MapReduce编程思想设计与优化的K-means并行算法.与传统的K-means算法相比,设计的并行算法在聚类结果的准确性、加速比、扩展性、收敛性等方面都有显著的提高,降低了因簇密度不均引起误分的概率,提高了算法的聚类精度,并且数据规模越大、节点越多,优化的效果就越明显.     

飞行器纵向通道姿态模糊控制器设计

飞行器姿态控制系统结构极其复杂,难以建立准确的数学模型。为了实现姿态的精确控制,将模糊控制方法应用于飞行器纵向通道姿态控制中,在模糊逻辑理论的框架下,根据姿态控制系统的输入输出量,提出了一种模糊规则优化设计方案,通过模糊推理,实现了对飞行器姿态的模糊控制。仿真结果表明,所设计的模糊控制器性能稳定,系统响应的调整时间在1s以内,超调量不超过3%,且小扰动情况下仍能保持较好的控制效果,有效提高了系统的稳态精度与动态品质。     

高精度智能质子磁力仪的最新发展

本文从质子磁力仪核心技术的角度对高精度智能质子磁力仪的最新发展进行了科学的论述,总结了我国及当今该仪器领域的最新成果,为我国广大地磁勘探工作者选购高质量的质子磁力仪从事物探工作提供科学指南。     

滑动频率估计算法在微波测量液位中的应用

在微波测量液位的系统中,正弦信号的频率估计是实现液位测量的关键.比较并分析了离散FFT变换的两种正弦信号频率估计方法:Rife比值法和三角形法,提出了一种滑动频率估计方法,通过实时确定比例因子,得到了滑动频谱校正算法.实验结果表明,该方法频率估计精度较高,估计性能优于单一的Rife法、三角形法,适合于低信噪比条件下的实...     

基于STM8的红外与超声波测距仪设计

介绍了一种基于STM8单片机的红外和超声波相结合的距离继电器的应用设计。阐述了该距离继电器的测距原理。经实际应用证明,该距离继电器的测量距离远大于10 m,克服了普通超声波测量距离只能达到5 m左右的问题,并且消除了测量的盲区。测量精度可以精确到1 cm,符合并超过实际应用要求。     

DSP系统中PWM通道实现D/A转换精度的分析方法

以PWM作为D/A的功能接口,分析了不同电路对转换精度的影响,提出了以PWM接口输出经滤波电路获得的精度与电路结构及参数之间关系的分析方法,初步确定了理论上分析转换精度的步骤与方法.并以TMS320F2812为例对其PWM通道实现D/A转换扩展功能的精度做了实例计算,验证了理论分析的可行性与准确性.为磁悬浮数字控制系统的设计与预知其设计精度之间的关联提供了理论基础,可以为相关设计提供借鉴与参考.     

蒙古文整词计算机生成理论研究

采用面向对象方法,模拟传统蒙古文整词各种形式构成机理,提出了几种蒙古语整词计算机生成数据模型。文章主要依据整词计算机生成三种模型,探讨了传统蒙古文整词计算机最优化生成理论所涉及的精确度、时间复杂度、空间复杂度三项基本要素以及最优化生成必须考虑的整词复杂特征载荷与一体化合一计算知识表示方法和计算结构,证明了“B - J - T= W”数据模型是传统蒙古文整词计算与生成最优化对象模型。     

高精度继电保护采样系统设计

对继电保护采样系统的精度进行分析,重点分析互感器的电路设计,信号通道上滤波电路的设计和AD7865外围电路的设计．以及FFT算法在信号采样中的应用。     

C语言中“/”影响运算精度问题的探讨

使用C语言编程进行计算的时候常常会因为不注意类型转换产生问题,尤其是在使用除法运算符"/"的时候,经常因为没有注意运算数据类型转换,从而导致运算结果不正确或着说是精度不够的问题。以下就C语言中"/"影响运算精度的问题做简单的探讨,希望助于初学者甚至是有一定编程水平的人更好地使用C语言。