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991.
992.
我国现代高程测定关键技术若干问题的研究及进展 总被引:5,自引:0,他引:5
李建成 《武汉大学学报(信息科学版)》2007,32(11):980-987
阐述了大地水准面在大地测量学及相关地球科学中的科学意义、作用及其应用,提出了利用GPS技术结合高精度似大地水准面代替我国高程测定的思路,分析了目前地球重力场长波、中波和短波在计算cm级大地水准面中具备的条件,论述了确定1 cm精度城市和5 cm省级似大地水准面的若干关键技术问题及其必要性和可行性,提出了我国cm级大地水准面确定中需解决的主要理论问题和若干关键技术,评述了我国局部大地水准面的主要进展和取得的成果。 相似文献
993.
基于广义点理论的多基线影像钣金件3D重建与尺寸检测 总被引:1,自引:0,他引:1
采用沿CAD模型在影像中投影的外轮廓边缘点统计与最邻近直线广义点平差迭代相结合的方法,实现了钣金件初始方位的2D-3D自动定位。提出利用影像上的直线信息进行广义点平差,并基于多基线影像对工业钣金件进行高精度三维重建和尺寸误差检验,给出了带端点约束条件的多基线影像广义点摄影测量平差模型,并验证了其有效性。 相似文献
994.
We report on bedload transport observations using piezoelectric bedload impact sensors (PBIS), an indirect method of estimating the volume of bedload transport of coarse sediment. The PBIS device registers vibrations produced by bedload (particle diameter >~20 mm) and records the signal as a sum of the number of impulses per time. Sediment transport at the Erlenbach stream has been continuously monitored with a PBIS array starting in 1986. The sensor array spans the width of an entire cross‐section and is mounted flush with the surface of a check dam immediately upstream of a sediment retention basin. We compare PBIS data with long‐term sedimentation records obtained from repeated surveys of material stored in the sediment retention basin, with artificial sediment input under controlled conditions in the field, and also with laboratory experiments. The rate of bedload transport is proportional to the number of impacts on the sensor per unit time. The reliability of the calibration relationship increases with the length of the observation period, e.g. for higher numbers of impacts and larger bedload volumes. Sediment volumes for individual flood events estimated with the PBIS method are in agreement with volumes estimated using an independent empirical method based on the effective runoff volume of water, the peak water discharge, and the critical discharge for the onset of sediment transport. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
995.
George S. Constantinescu Witold F. KrajewskiCelalettin E. Ozdemir Talia Tokyay 《Advances in water resources》2007
Wind is responsible for systematic errors that affect rain gauge measurements. The authors investigate the use of computational fluid dynamics (CFD) to calculate airflow around rain gauges by applying a high-resolution large eddy simulation (LES) model to determine the flow fields around a measuring system of two rain gauges. The simulated air flow field is characterized by the presence of massive separation which induces the formation and shedding of highly unsteady eddies in the detached shear layers and wakes. Parts of these detached structures occur over the orifice of the rain gauges and may substantially affect the dynamics of the raindrops in this critical region. Non-dissipative LES methods used with fine enough meshes can successfully predict these eddies and their associated fluctuations. The authors compare statistics from LES with steady-state Reynolds averaged Navier–Stokes (RANS) simulations using the k–ε and shear stress transport k–ω turbulence models. They find that both RANS and LES models predict similar mean velocity distributions around the rain gauges. However, they determine the distribution of the resolved turbulent kinetic energy (TKE) to be strongly dependent on the RANS model used. Neither RANS model predictions of TKE are close to those of LES. The authors conclude that the failure of RANS to predict TKE is an important limitation, as TKE is needed to scale the local velocity fluctuations in stochastic models used to calculate the motion of raindrops in the flow field. 相似文献
996.
龙门山断裂带的性质与活动性研究 总被引:38,自引:7,他引:31
沿龙门山构造带两侧,在9个100~500m深的钻孔中进行了水压致裂地应力测量,获得了断裂带两侧地表浅部构造应力的大小、方向和分布特征。据这些实测的应力资料,用库仑摩擦滑动准则分段研究了断裂带的性质和活动性。结果表明:试验得到的摩擦强度(0.6~1.0)似乎适用于浅部断层构造,地壳浅部地应力的大小和方向与断裂的性质及活动性密切相关,应力状态与断裂带的逆冲性质相一致;在断裂带东段的广元地区.中段的平武、汶川和茂县地区,实测的最大水平主应力值达不到逆断层活动要求的临界状态;而西段与鲜水河断裂带、南北向构造带复合部位的二郎山地区,现今构造应力状态比较复杂.实测的最大水平主应力值满足逆断层活动要求的临界状态。 相似文献
997.
998.
Using data sets assembled within the WMO Solid Precipitation Measurement Intercomparison, the paper deals with the wind related losses in national methods of solid precipitation measurement. Fourteen different types of precipitation gauges are included in the Harzgerode/GDR intercomparison field. Initial results of the comparisons between the Valdai double fence reference method and the other gauges in terms of dependence on wind speed and type of precipitation are presented. These results reveal that the national unsheltered HELLMANN gauge catches only 24 per cent to 70 per cent of the daily totals of solid precipitation as compared with the reference gauge. Results of the analysis of monthly totals and individual events reveal a similar dependence of gauge deficiencies on wind speed and type of precipitation. Case studies of single snow falls and the wind field conditions around and within the double fence reference are also described. 相似文献
999.
Measurement of air temperature in the presence of a large radiant flux: an assessment of passively ventilated thermometer screens 总被引:1,自引:0,他引:1
Evyatar?ErellEmail author Vítor?Leal Eduardo?Maldonado 《Boundary-Layer Meteorology》2005,114(1):205-231
The energy balance of small temperature sensors was modelled to illustrate the effects of sensor characteristics, particularly size, on the accuracy of readings in the presence of strong shortwave or longwave radiant loads. For all but extremely small sensors, radiant exchange may lead to unacceptable errors. The common practice of using passively ventilated instrument screens was evaluated in a series of comparative measurements. The differences resulting from the use of different models of shields may be an order of magnitude greater than the error resulting from sensor calibration. In the absence of technological innovation capable of reducing the error due to radiant exchange to negligible proportions, it is suggested that a standard methodology for calibrating and labelling the error resulting from the characteristics of the screens be adopted, to allow comparison of new data with long-established records. 相似文献
1000.
Based on the high-accuracy data obtained from the GPS measurements carried out in 1992, 1995 and 1996, the isochronous active
units with different kinematic property inside the North China area have been distinguished, 4 active units and 1 transition
zone with distinct differential movement have been determined. They are Ordos-Yinshan unit, Yanshan unit, Shanxi-Hebei-Shandong
(Jin-Ji-Lu) unit, Jiaodong-Liaoning-Shandong (Jiao-Liao-Lu) unit and Yanshan-Hebei (Yan-Ji) transition zone. The relative
movements among the neighboring units in this period have been given. 1 The compressive movement between Ordos-Yinshan unit
and Yanshan unit is not obvious with an amount of 0.4±1.3 mm/a. 2 Jin-Ji-Lu unit moves E40°S off the Ordos-Yinshan unit and
the magnitude is 4.4±1.0 mm/a. 3 Relative to the Yan-Ji transition zone of differential movement, Yanshan unit shifts W38°N
with a value of 2.4±1.3 mm/a and Jin-Ji-Lu unit moves eastward 35° by south with an amount of 2.3±0.9 mm/a. 4 Jin-Ji-Lu unit
has a tensional left-lateral movement of 4.7±1.4 mm/a in the direction of E37°S relative to Yanshan unit. 5 Some area near
Tanlu belt which is located in the southern part of Jin-Ji-Lu block has a southward movement 14° by west with a magnitude
of 1.5±1.1 mm/a off the Jin-Ji-Lu unit. 6 Relative to Jin-Ji-Lu unit, Jiao-Liao-Lu unit has a trend of clockwise movement
with a tensional right-lateral motion at the north end which neighbors Yanshan unit and a compressive motion at the south
end. It should be noted that the errors given in the paper are obtained based on the divergence among the displacements of
the sites in the unit, rather than the value calculated from the displacement error of the sites. The analyzed results indicate
that: 1 Shanxi tectonic zone and Yan-Ji transition zone are the major tectonic active zones to show the frame and magnitude
of interior relative movement in North China area, and others are the secondary tectonic active zones; 2 The complete horizontal
deformation in the North China area is not homogeneous nor successive; 3 The kinetic model of North China area might be "mantle
dragging plus boundary coupling".
Foundation item: The National Key Basic Research Project Mechanism and Prediction of Continental Earthquakes (G1998040700). 相似文献