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801.
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The separation between the reference surfaces for orthometric heights and normal heights—the geoid and the quasigeoid—is typically
in the order of a few decimeters but can reach nearly 3 m in extreme cases. The knowledge of the geoid–quasigeoid separation
with centimeter accuracy or better, is essential for the realization of national and international height reference frames,
and for precision height determination in geodetic engineering. The largest contribution to the geoid–quasigeoid separation
is due to the distribution of topographic masses. We develop a compact formulation for the rigorous treatment of topographic
masses and apply it to determine the geoid–quasigeoid separation for two test areas in the Alps with very rough topography,
using a very fine grid resolution of 100 m. The magnitude of the geoid–quasigeoid separation and its accuracy, its slopes,
roughness, and correlation with height are analyzed. Results show that rigorous treatment of topographic masses leads to a
rather small geoid–quasigeoid separation—only 30 cm at the highest summit—while results based on approximations are often
larger by several decimeters. The accuracy of the topographic contribution to the geoid–quasigeoid separation is estimated
to be 2–3 cm for areas with extreme topography. Analysis of roughness of the geoid–quasigeoid separation shows that a resolution
of the modeling grid of 200 m or less is required to achieve these accuracies. Gravity and the vertical gravity gradient inside
of topographic masses and the mean gravity along the plumbline are modeled which are important intermediate quantities for
the determination of the geoid–quasigeoid separation. We conclude that a consistent determination of the geoid and quasigeoid
height reference surfaces within an accuracy of few centimeters is feasible even for areas with extreme topography, and that
the concepts of orthometric height and normal height can be consistently realized and used within this level of accuracy. 相似文献
804.
805.
通过分析遥感影像中直角型房屋特点,本文提出一种新的房屋提取方法:利用Freeman编码推导出常见直角型房屋代码,使得每一种形状房屋都具有惟一编号,且编号不随房屋大小、方位及位置改变而改变,利用Hough变换提取房屋角点并剔除伪角点,最后利用模式匹配方法获得各个角点之间的连接关系。实验结果表明该方法具有较强的抗噪性,能有效提取影像中直角型房屋。 相似文献
806.
Currently,the OGC GML(Geography Markup Language) specification has been the de facto standard for GIS data sharing and exchanging and spatial interoperation.Adopting nested association expression approach of XML data,GML data documents can store both spatial information and semantic relationship information of geographical elements.To improve the efficiency of path query on GML two type information,the paper describes a holistic index method for GML data,which we call EKR+(optimized R+-tree base on Extend R... 相似文献
807.
808.
Geomorphic characteristics of hillslope and channelized debris flows: A case study in the Shitou area of central Taiwan 总被引:1,自引:0,他引:1
The data on the hillslope and channelized debris flows in the Shitou area of central Taiwan occurred during Typhoons Toraji and Nali in 2001 were applied in this paper. The geomorphic parameters, including the flow length, gully gradient, drainage area and form factor of the debris flows were determined by spatial analysis using a Geographic Information System (GIS) based on the data derived from field investigation, aerial photographs, and topographical maps. According to such determined geomorphic parameters, the threshold conditions and empirical equations, such as the relationship between the gully gradient and drainage area and that between gully length and drainage area and topographic parameter, are presented and used to distinguish the geomorphic characteristics between the channelized and hillslope debris flows. 相似文献
809.
本文针对电子航海图(ENC)使用过程中面临的数据安全问题,结合S-57标准下CRC校验和完整性认证需求,提出了一种顾及点位篡改定位特性的电子航海图零水印算法。该算法在分析ENC物标(要素)中以经纬度表示的离散点位坐标数据分布规律的基础上,构造离散点经纬度坐标数据校验码生成模型,结合离散点经纬度坐标数量对ENC单元格网进行二叉树分块处理,并分块解算基于离散点经纬度坐标校验码的二值序列,通过该二值序列与置乱加密后的水印图像进行异或操作构造零水印图像。算法中离散点的经纬度坐标校验码和格网分块技术的引入,在保证ENC数据精度和满足CRC校验的同时,具备点位篡改定位的特性。试验结果表明,该水印算法具有良好的稳健性、不可见性和一定的篡改定位精度,适用于电子航海图数据的版权保护。 相似文献
810.
震相识别对获取深部速度结构至关重要.横穿加瓜海脊的T1测线东段,地形高差达3 km,沉积厚度相差约2 km,沉积基底变化复杂,对海底地震仪(OBS)地震剖面的震相识别增加了难度.本文以该测线东段OBS数据为例,采用地形校正、基底校正、多次波识别、正演模拟、走时投影等多种手段,开展了复杂构造的OBS震相识别研究,获得了测线下方深部速度结构模型.结果表明,采用的方法对复杂地质结构下OBS震相识别是行之有效的,相比传统的方法提供了更多且准确的震相走时信息,获得的速度模型更加可靠. 相似文献