Chain-based polyline tessellation algorithm for cartographic rendering     

Mingguang Wu  Peibei Zheng  Guonian Lu  A-Xing Zhu 《制图学和地理信息科学》2017,44(6):491-506

ABSTRACT

Although the extrusion of 1D polylines into a 2D polygon and the tessellation of the polygon into primitives has been performed in the field of computer graphics, the efficient, robust, and high-quality tessellation of thick polylines that supports various cartographic styles for polyline rendering remains a practical challenge. We examined and compared existing vertex-, segment-, and polygon-based algorithms, and present a chain-based tessellation algorithm for cartographic rendering: points in the polyline are classified into abrupt and smooth points according to their point spacing, angle and thickness. The polylines are then divided into abrupt and smooth chains by clustering points that are adjacent and of the same type. An optimal algorithm is also presented to continuously tessellate the smooth chains into minimum geometric primitives and efficiently tessellate the abrupt chains into geometric primitives. We implemented the proposed algorithm and present the results of robustness, efficiency, quality, and cartographic styling tests conducted on real-world polylines. The results indicate that this approach has substantial advantages when considering all four requirements (robustness, efficiency, quality, and the support for various cartographic styles).  相似文献   

海面风的特征分析     

张淮水  刘安国  宋珊  吴术礼 《中国海洋大学学报(自然科学版)》1989,(2)

作者利用在渤海进行的不同时距的平均风速观测值,经统计分析求得1秒、2分和10分钟平均风速间的线性关系。同时根据风速梯度观测值给出风速指数廓线的指数值,并且给定风速对数廓线的特征值:海面摩擦速度和海面粗糙度。此外,作者还提出了海面风应力的经验公式。  相似文献   

南海潮汐的伴随同化数值模拟   总被引：21，自引：2，他引：21       下载免费PDF全文

吴自库  田纪伟  吕咸青  王丽娅 《海洋与湖沼》2003,34(1):101-108

把利用正交潮响应方法对 2 4 8个周期超过 6年的南中国海的TOPEX/Poseidon卫星高度计资料进行潮波分析提取的沿轨分潮调和常数同化到二维非线性潮汐数值模式中去 ,优化模型中的开边界条件和底摩擦系数 ,模拟了南海m1 和M2 分潮的潮汐。所用的同化方法是伴随同化。根据计算结果给出了m1 和M2 分潮的同潮图。计算结果与 5 9个验潮站资料的比较结果是 :m1 分潮的振幅和迟角的平均绝对误差分别是 4.8cm和 8.7°;M2 分潮的振幅和迟角的平均绝对误差分别是 4.3cm和 1 1 .0°,表明计算结果与验潮站资料符合良好。研究结果表明 ,利用伴随同化方法把TOPEX/Poseidon资料同化到潮汐数值模式中去对模式进行校正是有效的  相似文献   

The nearshore current along a high-rainfall,trade-wind coast-Nicaragua     

Stephen P. Murray  Myron Young 《Estuarine, Coastal and Shelf Science》1985,21(5):687-699

Runoff from abundant rainfall on the watersheds along the east coast of Nicaragua results in a well-defined nearshore current, extending 20–40 km out from the coast. Important terms in the controlling dynamical balance are the Coriolis forces, and eddy frictional forces. Calculations of the diabathic surface slope 0(10^?5) show a smooth setup of 4–5 cm from the outer edge of the current to the shoreline. A longshore surface slope of 0(10^?8) appears to be set up by the longshore wind stress, and further computations allow an estimate of ～ 6 gr cm^?1 s^?1 for the dynamic eddy viscosity. An analytical expression including diabathic surface slope and density gradient, parabathic surface slope, wind stress, and quadratic bottom friction reproduces the salient observed features of the nearshore current. These include the pronounced maximum in the parabathic (along-shelf) current about 10 km off the coast, a complex diabathic velocity structure, and a shelf countercurrent just seaward of the outer edge. Further calculations suggest that the dominant driving arises from the freshwater-induced density gradients, accounting for upward of 80% of the flow velocity. As suggested by Royer (1982), the prevailing trade wind exerts an onshore wind stress that serves the important role of maintaining the integrity of the density gradients via the convergence of a surface Ekman layer toward the coast.  相似文献   

南极绕极流线性理论的一个摄动解     

张庆华  刘娜  潘增第 《海洋科学进展》2003,21(4):377-386

为了模拟南极绕极流区的流动，首先将环形条带区从Drake海峡处分开，并展成以经纬度为坐标的矩形区域。将模拟区划分为边界区和内部区。与Munk的大洋环流理论不同．在我们所考虑的线性化的涡度方程中保留了经向摩擦作用。这是基于绕极流流轴处的强剪切产生的经向摩擦项与β效应同等重要的判断。另外．内部区的北边界取为流线，而2个侧边界区的北边界视为南北水体交换的通道。基于巴西暖流穿过北边界流入和秘鲁寒流穿过北边界流出，以及常年有大于100Sv(平均约134Sv)的水体自西向东穿过Drake海峡的观测事实，引入了净通量条件。计算出了不同于Sverdrup流的更集中的内部区带状流的流线分布，这与观测大体一致。在Drake海峡处引入匹配条件，从而得到了完整的边界解。计算结果与理论分析指出，通过Drake海峡的流量远小于南极绕极流所带动水体的纬向通量是因为Drake海峡偏离绕极流流轴(也是南大洋西风带的主轴)。受南美大陆的阻挡，另一部分水体通过秘鲁寒流流出该海区。  相似文献   

Wind speed scaling and the drag coefficient   总被引：2，自引：0，他引：2  

WEN Fan  GAO Zhiyi  WU Ziku  L&#; Hongmin 《海洋学报(英文版)》2005,24(4):29-42

[1]Banner M L, Chen W, Walsh E J, et al.  相似文献   

Formula for predicting bedload transport rate in oscillatory sheet flows     

Yu-Hai Wang 《Coastal Engineering》2007

At high bed shear stress sheet flows often occur in coastal waters in which high-concentration bedload sediments are transported in a thin layer near the bed. This paper firstly constructs a theoretical model (partial differential equations, PDEs) for the intense transport of non-cohesive bedload sediments by unidirectional currents and then seeks a special solution to the PDEs to determine the thickness of the bedload particle–water mixture, which could serve as the “reference height” that is often invoked in numerical computation and simulation of suspended sediment transport in turbulent flows. Moreover, a modified formula is presented to determine the “reference concentration”. Using a “uch” approach the present study derives a 1D formula for predicting bedload transport rate in sheet flows driven by asymmetric waves, with the help of a novel formula for evaluating wave friction factor. The new bedload formula can generically take into account slope angle (positive and negative), wash load concentration in the driving water flow and other factors that affect bedload transport rate. It compares well with measured data in a large-scale wave flume [Dohmen-Janssen, C.M., Hanes, D.M., 2002. Sheet flow dynamics under monochromatic non-breaking waves. Journal of Geophysical Research, 107(C10), 1301–1321], a large-scale oscillatory water tunnel [ Hassan, W.N., Ribberink, J.S., 2005. Transport processes of uniform and mixed sands in oscillatory sheet flow. Coastal Engineering, 52, 745–770] and in a swash zone of natural beach [Masselink, G., Hughes, M.G., 1998. Field investigation of sediment transport in the swash zone. Continental Shelf Research, 18, 1179–1199].  相似文献   

Residual Currents Through the Dover Strait Measured by H.F. Radar     

D. Prandle  R. Player 《Estuarine, Coastal and Shelf Science》1993,37(6)

Residual (i.e. non-tidal) components of flow in the Dover Straits are determined from measurements recorded by the OSCR H.F. Radar system. The data are divided into 10 monthly sets, obtained from 5 months of dual radar deployments on the English side of the Straits and a corresponding 5 months from the French side. For each of these sets the tidal component was removed by subtracting constituents derived from separate (monthly) harmonic analyses.In each deployment, surface currents were measured at 700 locations at 20-min intervals, providing spatial resolution on a grid as fine as 660 m. This fine spatial resolution reveals distinct patterns of monthly-mean residual circulations never previously recognized. In particular, a residual gyre is shown to be a dominating feature along the French side, with currents exceeding 20 cm s^-1 and a diameter of 20 km. The previous obscurity of this feature may be attributable to the large (> 1·5 m s^-1) and strongly spatially variable tidal currents. In these conditions, tidal advection will rapidly smear any surface signature obtained from satellite observations. Likewise, fine resolution (grid spacings of an order of 1 km) is required to reproduce these gyre dynamics in numerical models.The time-varying residual currents were correlated with wind recordings. Significant correlations were obtained for an 'open-sea' response (as represented by Ekman theory), with the wind-driven surface current veering at angles of up to 45° to the right of the wind direction. This 'open-sea' response is modified close to the coast where the generation of surface gradients force the currents to align with the topography.Larger-scale residual motions were identified from modal analyses. The primary modes, on both sides, involved large-scale flows through the Straits partially correlated with local winds. However for winds aligned approximately north-south, the components of these flows on the English and French sides are in opposition. These modes also revealed large oscillatory motions with periods of between 20 and 40 h.Estimates of net residual flows through the Straits from the radar measurements confirm earlier calculations of the range of variability in this parameter. However the increasing complexity of the flow patterns revealed in this study emphasizes the difficulty in quantifying the long-term net flow.  相似文献   

Study of long-wave breaking     

田纪伟  李鸿雁  胡敦欣  张发高  侯一筠 《中国海洋湖沼学报》1994,12(2):136-143

This paper reports study on the mechanism of long-wave breaking in a one-dimensional open chan-nel,without bottom friction,or with linear bottom friction.In general,the wave breaking phenomenon is related to the notion of"blow-up of solution"and can be studied by using the blow-up theory of par-tial differential equations(PDE).A pair of Riemann invariants are introduced to transform the governingequations into ordinary differential equations(ODE)along the characteristic directions of the governingequations.From the properties of the solutions of the ODE,the time and location of wave breakingare estimated and the criterion of long-wave breaking for the case with linear bottom friction is obtained.  相似文献   

渤海湾盆地东营凹陷现河矿区古近系断裂体系及形成演化     

邓路佳  吴孔友  焦红岩  董方  赵海燕  刘煜磊  张法冰 《地质力学学报》2022,28(3):480-491

受双重动力背景控制,渤海湾盆地东营凹陷现河矿区古近系伸展与走滑作用叠加明显,明确断裂体系叠加关系对油气勘探意义重大。文章运用高精度地震资料及钻井资料,通过井震结合解释、构造样式识别、断裂活动性定量表征及平衡剖面恢复等技术手段,从断裂类型与组合样式、断裂叠加关系、断裂活动及演化、区域动力学背景等方面开展了深入研究。结果表明:现河矿区古近系发育伸展与伸展-走滑两套断裂体系,伸展-走滑断裂基于先期伸展断裂叠加走滑作用形成。古近系主干断裂存在沙三中期和东营早期两次活动高峰,受控于伸展-走滑应力叠合作用,现河矿区古近系共经历初始伸展阶段（Ek—Es₄）、强伸展阶段（Es₃下—Es₂下）、走滑-弱伸展阶段（Es₂上—Es₁）、再活化伸展阶段（Ed）和定型保留阶段（N—Q）5个主要的断裂演化阶段。   相似文献