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Email: Jim Smith

Survey Review 42, No. 316 April 2010

1. An examination of network RTK GPS services in Great Britain
S. J. Edwards, P. J. Clarke, N. T. Penna and S. Goebell

As of March 2009, network real-time kinematic (RTK) GPS surveying is available in Great Britain with the aid of two commercial service providers, Leica’s “SmartNet” and Trimble’s “VRS Now”, both of which rely largely on the Ordnance Survey’s “OS Net” network of around 120 continuously operating reference stations. With the aim of testing the performance of Network RTK under both ideal and less-ideal conditions (greater distances and elevation differences from the nearest reference stations, proximity to the edges of OS Net, and increased susceptibility to ocean tide loading effects), we have tested the positional accuracy of both commercial Network RTK systems by comparison with precise coordinates determined using the Bernese scientific GPS processing software, at six representative locations spanning England and Wales. We find that the coordinate quality measures provided by the Network RTK solutions are overall representative of the actual coordinate accuracy, which is typically 10-20 mm in plan and 15-35 mm in height, and can be successfully used to identify outliers. Positional accuracy tends to be poorest outside of the bounds of OS Net and at greater elevation differences from nearby reference stations. Averaging of coordinates over two short windows separated by 20-45 minutes can be used to achieve moderate improvements in coordinate accuracy without the need for single long occupations of sites.

2. Height determination f Qom0langma Feng (Mt Everest) in 2005
Chen Junyong, Zhang Yanping, Yuan Janli, Guo Chunxi and Zhang Peng

Since the 1960s China has carried out several geodetic campaigns for measuring the height of Qomolangma Feng-Mt. Everest independently or in cooperation with foreign countries. Large scale geodetic field work and data processing have been done in the campaigns, dealing with positioning, height determination, gravimetry, astronomical measurement and atmospheric reflection observation etc. Some survey tasks were done in order to improve the accuracy and reliability for the height determination of Mt. Everest. These tasks include setting up of a survey target on the summit of Mt. Everest (MES) in the 1975, 1992 and 2005 campaigns, joint use of GPS, laser ranging and trigonometric levelling in the 1992 and 2005 campaigns, exploration of the thickness of the ice-snow layer on the summit and refinement of the local gravity field including the geoid in the northern slope area with new surface ground gravity data, DTM and astro-gravity levelling or GPS levelling results in the campaigns mentioned. It is the first time in China that the thickness of the ice-snow layer on the summit was measured by ground penetrating radar integrated with GPS in the 2005 campaign. The orthometric heights of the snow surface and rock surface of the summit were determined as 8847.93m and 8844.43m respectively in the 2005 campaign. The rate of decrease of the snow summit of Mt. Everest between 1992 and 2005 was 1.8cm/a.

3. Performance assessment of a long range reference station ambiguity resolution algorithm for network RTK GPS positioning
Weiming Tang, Xiaolin Meng, Chuang Shi and Jingnan Liu

The authors firstly propose a new ambiguity resolution (AR) method for processing long range reference station baselines in Network Real-Time Kinematic (NRTK) GPS positioning. This method has the following advantages: no need for observational equation linearization and resolution, satellite by satellite based ambiguity resolution and long range baseline ambiguity resolution of up to a few hundred kilometres. In order to assess the performance of this method, several sub-networks of Ordnance Survey’s OSNet(TM) in which the baseline lengths vary from 200km to 400km are formed. The corresponding initialization times and Double-Differenced (DD) biases are estimated. The results show that even if the inter-station distances are up to 400km the proposed long range AR algorithm can quickly and accurately resolve the ambiguities. According to the performance of ambiguity resolution and the character of DD biases between reference stations, the authors conclude that it is possible to implement highly accurate NRTK GPS positioning based on much sparse OSNetTM with inter-station distances up to 400km and this will significantly reduce the investment in the establishment and maintenance of a network of reference stations.

4. An extended adaptive Kalman filtering in tight coupled GPS/INS integration
Wu Fu-mei and Yang Yuan-xi

In tight coupled GPS/INS integration by using Kalman filtering, the main error includes not only the dynamic model error (INS error), but also the errors caused by the poor geometry of GPS satellites or short of GPS satellites. In this paper, an extended adaptive Kalman filtering algorithm is presented based on the adaptive filter. The new algorithm can not only resist the influence of the dynamic model errors but also control the influence of the errors caused by the poor geometry of GPS satellites by adjusting the coefficient matrix of the predicted states. An actual computation example shows that the new algorithm can degrade the influence of the two kinds of errors and improve the precision of navigation.

5. Performance of precise point positioning with ambiguity resolution for 1- to 4-hour observation periods
J. Geng, X. Meng, F. N. Teferle, A. H. Dodson

Recent progress in integer ambiguity resolution at a single station has made it possible to achieve high positioning accuracy in static precise point positioning (PPP) using a short period of observations. In this paper, 12 stations across Europe are used to conduct short-period (i.e. one, two, three and four hours) static PPP with ambiguity resolution from Day 245 to 251 in 2007. It is demonstrated that, when over three hours of observations are used, PPP can achieve a success rate of 100% for ambiguity resolution, a 3D positioning accuracy of about 1.0 cm and an occurrence of less than 1.0% for degraded solutions. Moreover, for the fixed solutions, increasing the observation period hardly improves the horizontal positioning accuracy while still improving the vertical one. Therefore, it is proposed that at least three hours of observations should be used in the ambiguity-fixed static PPP if a reliable millimetre positioning accuracy is required in the engineering applications.

6. Direct transformation from geocentric Cartesian coordinates to geodetic latitude and ellipsoidal height
Yanxing Li, Jinghua Zhang, Junqing Zhang, Zhongfu Zhang and Xuesong Du

In analysing the geometric relation between the geocentric coordinates and the geodetic latitude and ellipsoidal height, a tangent (or cotangent) quadratic equation of geodetic latitude is created, following which the geodetic latitude can be solved directly. We have found in the calculation of latitude from the tangent equation that the larger the absolute value of latitude is, the higher the accuracy will be, but the calculation from the cotangent equation is just the opposite. Similar phenomenon also occurs in the calculation of ellipsoidal height. Therefore, we take corresponding tactics to ensure the high accuracy and stability in the calculations of geodetic latitude and ellipsoidal height. With the method in the paper, the geodetic latitude and ellipsoidal height of any point in space from the Earth’s centre to the height of Earth synchronous orbit satellite can be uniquely determined. The maximum errors for the calculations of geodetic latitude and ellipsoidal height are smaller than 4×10(-12) deg and 4×10 m(-8), respectively.

7. Retrieval of Airborne LIDAR Misalignments Based on the Stepwise Geometric Method
Xiaohong Zhang and Rene Forsberg

In this paper, a new simple method for airborne LIDAR system misalignment calibration is described in detail. The method is especially designed for environmental surveys, such as beach mapping or glacier surveys. The proposed method, termed stepwise geometric misalignment determination is based on the relationship between the point clouds on a regular object, e.g. a flat top building, and the ground truth of the objects used for calibration. In order to extract the footprints on the objects, filtering is implemented before the calibration. The tests verify that the proposed method is feasible, and very effective. The calibration flight should be designed beforehand to ensure the accuracy of the retrieved boresight corrections. We outline some examples of application of the method by using airborne laser scanning data from Greenland, acquired by a Riegl laser scanning system.

8. Correlation analysis of multipath effects in GPS-code and carrier phase observations
H. Nahavandchi and G. Joodaki

Multipath error is a limiting factor for successful ambiguity resolution in precise real-time kinematic GPS positioning, both in the carrier phase and pseudorange measurements. Understanding the temporal characteristics of the multipath errors enables the stochastic modelling of these errors. To do this, dual frequency data as well as data from short baselines of several metres apart are observed and analyzed under different multipath conditions. Further, a covariance model (first order Gauss-Markov in this study), accounting for the multipath errors can be created if the temporal characteristics of the multipath errors are derived. The correlation time parameter is an important factor in the analysis of a random process hence this study. Different baselines are observed to estimate the correlation time of the multipath errors. Longer correlation times are obtained in the carrier phase observations due to the double difference process. Not accounting for the long correlation time usually leads to an overestimation of the accuracy and may affect the determination of the ambiguity parameters. This has been shown through a simple numerical investigation.

Survey Review 42, No. 315 January 2010

1. The perspective from Asia concerning the impact of Compass/Beidou-2 on future GNSS
Kai-Wei Chiang, Yu-Sheng Huang, Meng-Lung Tsai and Kwo-Hwa Chen

The next decade promises drastic improvements to global navigation satellite systems. The USA is modernizing GPS, Russia is refreshing GLONASS, Europe is moving ahead with its own Galileo system, and The People’s Republic of China is expanding its Beidou-1 system from a regional navigation system to a full constellation global navigation satellite system known as Compass/BeiDou-2, which consists of thirty five satellites including geostationary satellites, MEO satellites, and geosynchronous satellites in the coming year. Extra satellites will improve performance for all applications, and especially where satellite signals can be obscured, such as in urban canyons, under tree canopies, or in open-cut mines. The benefits of the expected extra satellites and their signals are increased availability, accuracy, continuity, and reliability.
The advent of hybrid GNSS constellation has made the study of compatibility and interoperability increasingly important. A number of performance analyses have been conducted on a global scale with respect to availability, reliability, accuracy, and integrity in different simulated scenarios (such as open sky and urban canyons) for each system individually as well as for all possible combinations.
Since the Compass/BeiDou-2 has received increasing attention from GNSS communities, the main objective of this study is to investigate the performance of Compass/BeiDou-2 in comparison with GPS, GLONASS, and Galileo in the greater Asia region; and also to explore whether combining Compass/BeiDou-2 with another GNSS would yield performance improvements in the region.
Performance analysis can be conducted using either signals or geometrical conditions. However, the scope of this study is limited to the impact of current and future GNSS based on geometrical conditions. Therefore, the satellite visibility and DOP values of each system or their possible combinations are used as the major indices for the performance evaluation with an emphasis on the addition of Compass. In addition, those indices are analyzed in terms of their spatial and temporal distributions with an emphasis on the greater Asia region. Moreover, the spatial performance analyses are conducted as both global and regional scales to provide more insightful comparisons to illustrate the importance of Compass for users in the greater Asia region.

2. An integral for geodesic length after derivations by P D Thomas
Craig M. Rollins

Using a change of variable suggested by P. D. Thomas (1952), the arc length of a segment of a geodesic curve on an ellipsoid becomes an integral having the same form as arc length on an ellipse, a simpler problem. This leads to a succinct theoretical solution to the Direct and Indirect Problems of geodesics. With modern mathematical software, it is also a practical solution

3. Land subsidence using absolute and relative gravimetry: a case study in central Taiwan
Cheinway Hwang , Tze-Chiang Cheng, C.C. Cheng, and W.C. Hung

We experiment with absolute and relative gravimetry to determine land subsidence. A gravity network in the Yunlin County of central Taiwan is established to determine gravity variations that are largely due to land subsidence. Every 6 months, gravity values at two absolute gravity stations were measured by a FG5 gravimeter and those at 7 relative stations by a Graviton-EG and a Scintrex CG-5 relative gravimeter. A weighted constraint network adjustment was carried out by holding fixed gravity values at the absolute stations. Correction models for temporal gravity changes are developed and applied to raw gravity measurements. The adjusted gravity values oscillate, but in general increase with time, showing signature of land subsidence. An empirical gravity-to-height admittance factor is determined using gravity change (from FG5) and height change (from levelling) at the two absolute gravity stations. At most gravity stations, there is good agreement between subsidence rates from gravimetry and levelling. Some large discrepancies (> 1 cm/year) also exist and are caused by uncertainties in relative gravity accuracy, hydrological effects and the admittance factor. This study suggests that gravimetry has the potential to determine land subsidence to cm accuracy and is more efficient and economic than tools such as levelling and GPS.

4. Space resection in photogrammetry using collinearity condition without linearization
Said M. Easa

Space resection in photogrammetry involves determining the spatial position and orientation of a photo based on image measurements of ground control points that appear in the photo. Since space resection is a nonlinear problem, existing methods involve linearisation of the collinearity condition and the use of an iterative process to determine the final solution using the least-squares method. The process also requires initial approximate values of the unknown parameters, some of which must be estimated by another least-squares solution. This paper presents an optimisation model for space resection with or without redundancy that requires no linearisation, iterations, or initial approximate values. The model, which is nonlinear and noncovex, is solved using advanced Excel-based optimisation software that has been recently developed. Application of the model is illustrated using two numerical examples. The proposed model is simple and converges to the global optimal solution very quickly. The presented concept has many potential applications in solving nonlinear land surveying problems without the need for linearisation, and as such should be of interest to surveying and geomatics engineers.

5. Semi-automatic building extraction in dense urban settlement areas from high-resolution satellite images
S.D. Mayunga , D. J. Coleman and Y. Zhang

Recent availability of high-resolution satellite images provides a new data source for geospatial data acquisition. This high-resolution data source has made it possible to extract man-made features such as roads and buildings, which are required for varieties of applications including urban planning, creation of GIS databases, and environmental monitoring. In this paper, we have developed a new method to extract buildings in urban informal settlement areas using high-spatial resolution panchromatic imagery. The proposed method uses radial casting algorithm to initialize snakes contours, and the fine measurement of building outlines is automatically carried out using snakes models. The building extraction results are satisfactory with an extraction rate of 94 percent as demonstrated by examples over a variety of selected test areas. The potential and limitations of applying this method to extract buildings is also discussed.

6. The least-squares estimation of adjustment model constrained by some non-negative parameters
Yingchun Song, Jianjun Zhu and Zhiwei Li

This paper studies the least-squares estimation of the adjustment model constrained by some non-negative parameters from an entirely novel point view. The non-negative parameters-constrained least-square problems are first translated to convex quadratic programming problems and then searched for optimal solutions. The paper gives the necessary and sufficient conditions on the solvability of the optimization problem, which consequently gives the general form of the least-squares estimation of the adjustment model constrained by non-negative parameters. A simple and straightforward algorithm is designed to resolve the problem. Comparative calculations on a simulation example indicates that the proposed methods are advantageous for conventional models, ease of implementation, and can be readily applied to adjustment computations of practical measurements.

7. Integrated GPS and Pseudolite Positioning for deformation monitoring
Guang Yang, Xiufeng He, Yongqi Chen

The pseudolite-augmented GPS technique can overcome the limitations of GPS-only surveys in unfavourable environments, like urban areas, deep open-pit mines and valleys. However, several problems need to be solved before pseudolites can be employed in precise surveys. This paper discusses the potential use of pseudolites for deformation monitoring in unfavourable survey environments. An analysis was conducted to illustrate how pseudolite measurements can improve the strength of the positioning geometry at a hydro-power dam. The methodology for the integration of GPS and pseudolite measurements has been developed, in particular the method/strategy to calibrate the pseudolite multipath effect, one of the severe sources of errors in pseudolite measurements. A set of real GPS pseudolite observations based on a 10×20 m network was processed to demonstrate the usefulness of the developed methodology.

8. Network code DGPS positioning and reliable estimation of position accuracy
M. Bakula

A network reference station approach instead of a single reference station improves the efficiency and reliability of precise GPS positioning. GPS reference stations exist in some countries, and GPS observations are available for users in real-time mode and for post-processing. This paper presents DGPS post-processing positioning with the use of three simultaneous reference stations. The paper shows that when a reference station network is used, it can create pseudo-range corrections for a virtual reference station, located in the vicinity of an unknown station, and improve the accuracy of determined positions. Network reference stations give redundant observations and reduce the impact of some measurement errors. Practical calculations and analysis of accuracy in code DGPS positioning and the validation of standard deviations determined from adjustment are presented for medium-long distances between a rover and reference stations.

9. Measurement of cross-slope of roads: evaluations, algorithms and accuracy analysis
G. Caroti and A. Piemonte

The Topography and Photogrammetry section of the Department of Civil Engineering, Pisa University, Italy, in cooperation with the Geonetlab Research Centre of Trieste University, is developing several methods for the evaluation of road cross-slope. The measurements are performed by means of instrumentation integrated on the Mobile Mapping Systems vehicle GIGI One (GPS Integrated with Glonass and INS One). Two different approaches are followed. The first one involves the use of a low cost monoaxial laser scanner IBEO Ladar Digital Automotive (LD-A), synchronised with an Applanix Position and Orientation System for Land Vehicles (POS LV). For the second approach, the cross-slope is computed only from inertial system data, with a simplified algorithm that describes vehicle dynamics. The paper presents an accuracy assessment of both methods. The tests are carried out on two different datasets, namely on the national road SS58 “Strada Nuova per Opicina”, joining Trieste to Opicina and the national road SS1 “Aurelia” between Rosignano and Campolecciano, near Livorno. The control measurements involve several single point checks carried out with conventional surveying instrumentation. The paper shows that the new proposed algorithm allows to compute cross-slope with an accuracy of 0.4° or better.