b

Email: Jim Smith

Survey Review 42, No. 318 October 2010

1. The correction of the pseudoscopic effect on Quickbird satellite imagery
M. Gil, L. J Ortiz, T.Rego and L.Gelpi

Orthoimages have become a useful tool for photointerpretation and they are currently replacing conventional vector maps in many applications. Very high-resolution satellite images such as QuickBird, provide an important source of data for the creation of such cartography. However, the inverted perception of relief in optical sensor data significantly diminishes the visual quality of these orthoimages. This problem is caused by illumination from the southeast during data collection.
In the present study, the pseudoscopic effect is resolved by means of a DEM and simple image processing tools available in commercial software. The superimposition of the orthoimage and the properly shaded DEM will provide the correct three-dimensional feature visualization of relief. In the absence of stereo pairs, the limitations of the two-dimensional surface of the satellite image are overcome for the proper perception of three-dimensionality. The map obtained is improved in quality to better understand the earth's surface and provide important information about relationships between land shape and slopes.

2. Quality analysis of global geopotential models at 1542 GPS/levelling benchmarks over the Hellenic mainland
C. Kotsakis and K. Katsambalos

The aim of this study is to present the results of several ‘external’ quality tests for the most recent (at the time of writing this paper) global geopotential models (GGMs) using precise GPS and leveled orthometric heights over the area of Greece. The tested GGMs include the GRACE-based combined model GGM03C, the latest EIGEN-type combined models EIGEN-GL04C and EIGEN-GL05C, the ultra-high resolution model EGM08 that was released last year by the US National Geospatial-Intelligence Agency, and also the older NASA/NIMA/OSU’s EGM96 model. The evaluation tests are based on comparisons of absolute and relative geoid undulations that are computed from the selected GGMs and the external GPS/levelling data. The test network covers the entire part of the Hellenic mainland and it consists of more than 1500 benchmarks which belong to the Hellenic national triangulation network, with direct levelling ties to the Hellenic vertical reference frame. The spatial positions of these benchmarks have been recently re-determined at cm-level accuracy (with respect to ITRF00) through a nation-wide GPS campaign that was organized in the frame of the HEPOS project. Our results show that the EGM08 model offers a remarkable improvement for the agreement among geoidal, ellipsoidal and orthometric heights in the mainland part of Greece, compared to the performance of other combined GGMs over the same area. Finally, our study gives a preliminary (yet realistic) accuracy assessment for GGM/GPS-aided orthometric height determination, over different baseline lengths, throughout the Hellenic mainland.

3. Geodetic basis of the longest tunnel in the Republic of Croatia
M. Zrinjski and M. Džapo

The tunnel ''Mala Kapela'', 5760 m long, is the longest tunnel in the Republic of Croatia. This paper gives the presentation and the analysis of the geodetic basis in the tunnel ''Mala Kapela''. The tunnel geodetic basis consists of the overhead and underground part.

The overhead geodetic basis consists of:
1. two geodetic rectangles (one in the vicinity of the northern, and the other in the vicinity of the southern portal)
2. a precise traverse connecting positionally these two micro-networks
3. a precise levelling figure connecting vertically the micro-networks of the northern and southern portals.

The underground geodetic basis consists of:
1. two networks in the right tunnel tube in the form of a triangle chain (one of them at the northern, and the other at the southern tunnel portal)
2. two traverses in the left tunnel tube (one of them at the northern, and the other at the southern part of the tunnel) connecting the underground networks in the right tunnel tube and the overhead geodetic basis at the northern, i.e. the southern portal of the tunnel
3. two closed precise levelling figures (one at the northern and the other one at the southern part of the tunnel).

This is the first time that an underground geodetic basis in the form of a triangle chain has been applied in the Republic of Croatia. It was chosen because it best meets the set criteria related to the accuracy of cutting the tunnel ''Mala Kapela''. The error sources in direction measurement were processed with special attention given to the error of the initial grid bearing and direction orientation. The article gives an outline of formulas used to calculate corrections and reductions of the lengths measured with electro optical distance meter. At the end, there is an illustration of the accuracy of cutting the tunnel ''Mala Kapela''. The transverse deviation for the cutting point in the right tunnel tube was sQ = 0.8 cm and the longitudinal deviation sL = 2.1 cm. The transverse deviation for the cutting point in the left tunnel tube was sQ = 1.0 cm and the longitudinal deviation sL = 2.2 cm. The vertical deviation for the cutting point in the right tunnel tube was sH = 1.9 cm, and for the cutting point in the left tunnel tube sH = 2.1 cm.

4. Estimating geometric parameters of highways and railways using least-squares adjustment
Xiaohua Tong, Xiaolin Meng and Keliang Ding

This paper introduces a least-squares based approach that is developed for the estimation of horizontal geometric parameters of highways and railways. Curve recovery forms an important procedure in the establishment of the spatial database of a Geographic Information System for Transportation (GIS-T), which is especially essential nowadays when different mobile mapping systems such as those using Global Positioning System (GPS) are widely employed for data acquisition. Comparing with the existing methods that estimate the geometric parameters of highway and railway alignments such as straight-lines, circular curves or spiral curves individually, this study discusses how to extract the entire set of parameters of combined alignments simultaneously with an integrated estimation method. Two case studies were used to show the feasibility of the developed technique. The first example investigates the possibility for recovering the geometric parameters of expressway curves whilst the second experiment entails realignment of existing railway curves. The results show that the proposed method has the advantages of high accuracy in the estimation of highway and railway curve parameters, and of less track lining work comparing with the traditional method in railway track realignment.

5. Using areal-scale as opposed to the linear-scale for map projections
H. Helali , J.L. Awange, and E. Omidi

Maps are made to scale. Scale represents the ratio of distance on the map to distance on a projected coordinate system, i.e., Universal Transverse Mercator (UTM). This specification is, however, limited by the fact that different lines produce different scales. This often has disadvantages for users who may require an optimal scale; one scale representing the entire map. Scale variations clearly show that measurements on maps are deteriorated by biases. It is, therefore, desirable to have a unique scale independent of linear measurements to enhance the accuracy of further data processing. In this contribution, an optimal scale based on the relationship between the areas on a map and a reference ellipsoid is proposed to reduce the distortions of the projected coordinate system. The motivation behind the area approach is the fact that as the number of lines approaches infinity on a map, a surface is built which is accurately represented by an area as opposed to linear features. Using several map projections, this paper demonstrates that linear-scale optimization is achieved through areal-scale. Almost all of the commercial software measure the linear-scale based on one line. The linear scale remains unchanged even if the projection is changed or map view moved. Therefore, this contribution can pave the way for GIS industry to present a better indication of scale and more accurate data processing results.

6. State survey and real estate cadastre in Serbia development and maintenance strategy
I. R. Aleksic, O. R. Odalovic and D. M. Blagojevic

The paper provides an overview of the strategic objectives for the development and maintenance of the State survey and real estate cadastre in Serbia for the years 2006-2015. We described project design and strategic objectives in details. Special attention was dedicated to the integrated strategy planning process. Effects of the achievement of the strategic objectives such as protection and security of real right, efficient, economical, quality and security of real estate market also were shown.

7. A quality management system for positioning with GNSS technology
I. Silva, A. Pantoja and J. C. Toledo

This paper presents a proposal for a Quality Management System for a generic GNSS Surveying Company as an alternative for management and service quality improvements. As a result of the increased demand for GNSS measurements, a large number of new or restructured companies were established to operate in that market. Considering that GNSS surveying is a new process, some changes must be performed in order to accommodate the old surveying techniques and the old fashioned management to the new reality. This requires a new management model that must be based on a well-described procedure sequence aiming at the Total Management Quality for the company. The proposed Quality Management System was based on the requirements of the Quality System ISO 9000: 2000, applied to the whole company, focusing on the productive process of GNSS surveying work.

8. A Methodology for creating RTK positioning coverage maps via a radio modem link to CORS stations
J. A. Armenteros and A. J. Gil

The Andalusian Positioning Network (RAP) is a real-time kinematic (RTK) GPS-based network that covers the Autonomous Community of Andalusia (southern Spain) homogeneously and offers, amongst other services, radio modem-based differential corrections for accurate real-time positioning (RAP-RTK). The institutions responsible for permanent GPS networks do not currently provide information regarding the radio coverage of their stations. In this communication we therefore present a methodology for creating RTK positioning maps via a radio modem link to continuously operating reference stations (CORS). We have applied this methodology to the University of Jaén (UJAE)’s permanent GPS station and present, for the first time, two coverage maps for this service. One of these maps is based on field data and the other on a combination of field data and computerized applications that generate viewshed maps, thus offering relevant information regarding the areas where users can make use of this service. This type of application is shown to be an excellent tool for estimating beforehand the possibility of using the radio modem-based differential correction service without having to visit the site.

Survey Review 42, No. 317 July 2010

1. Ocean tide loading and relative GNSS in the British Isles
P. J. Clarke and N. T. Penna

Ocean tide loading (OTL) affects all parts of the British Isles to varying degree, causing peak-to-peak vertical displacements of up to 13 cm in South-West England over semi-diurnal and diurnal timescales. Lateral displacements are typically around one-third of the magnitude of vertical displacements at a point, so are also considerable for carrier phase GNSS surveying. Using a recent numerical ocean tide model, we predict that widespread residual displacements up to ~1 cm remain in kinematic or short-occupation static relative GNSS positions computed with respect to the nearest continuously operating reference station. Even if OTL is not modelled explicitly, these errors will be mitigated by network GNSS to an extent dependent on the number and location of reference stations used, and the adjustment or error interpolation scheme adopted in the processing. For a selection of error interpolation algorithms, we predict that throughout most inland regions of Great Britain and Ireland, network processing reduces the residual OTL error to within the expected kinematic GNSS system noise. However, pockets of OTL error exceeding 1 cm may remain, especially in coastal locations and in South-West Ireland, South Wales, South-West England, and the islands off the west coast of Scotland. Residual OTL error at semi-diurnal periods dominates that at diurnal periods. We derive a simple heuristic for estimating the magnitude of this error from a sample of observations at a locality, valid within a short window of the spring/neap tidal cycle, and show how the residual error may, if necessary, be mitigated by a suitable averaging scheme.

2. On the geodetic datums in Sri Lanka
P.G.V. Abeyratne, W.E. Featherstone and D.A. Tantrigoda

The geodetic datums in Sri Lanka (formerly Ceylon) for latitude and longitude, height and gravity are described and discussed, because this information is not available in the open literature. It is recommended that most of these datums require some upgrading.

3. C/A code multipath mitigation at GPS base stations for improved DGPS corrections using wavelets
T. L. Dammalage, C. Satirapod, S. Kibe and C. Ogaja

Differential GPS (DGPS) correction technique is one of the most popular and accurate techniques to enhance GPS positioning accuracies by minimizing most of the common mode errors as a collective process. Several DGPS processing techniques can be used, for instance, single differencing and double differencing which are very common in practice. Irrespective of the DGPS processing techniques used, the ultimate accuracy of the user location depends on the existence of non-common mode (site-dependent) errors, which occur at the points of observation and the reference. Of these errors, the most common site-dependent error is the multipath. Several researchers have proposed different strategies and approaches to minimize the multipath effect on the final GPS position estimates. This manuscript evaluates the potential of applying wavelets transform to C/A code multipath mitigation at GPS reference stations, for improved differential GPS corrections.
An experiment was conducted making C/A code DGPS observations at three precisely known ground control stations assuming that two of them were permanent reference stations and the third as a user station. Four segments of about 24-hour continuous static observations were made by introducing four different multipath environments at one of the reference stations, each segment using a different type of artificial signal reflector. By making use of the known GPS receiver-reflector configuration, pseudo-range multipath was precisely calculated for each observation segment. In a subsequent analysis, different wavelet families and decomposition levels were applied to extract multipath from the pseudo-range residuals. By comparison, the best wavelet and level of decomposition were identified. The accuracy of this process is highly dependent on selecting the best suiting wavelet family and level of decomposition. The C/A code DGPS positioning accuracies prior to and after multipath correction are also presented.

4. Nothing but blind pitiless indifference: boundary monuments, deferral and the public interest
B. Ballantyne and S. Rogers

Boundary monuments in Canada have long been asserted to be a public good. Such goods, however, be they monuments or water and sewerage systems, are only in the public interest if they are reliable. Some 800 boundary monuments in 26 residential subdivisions in the province of Alberta were closely inspected (using metal detectors and shovels) for their reliability. Four findings resulted. First, monuments established immediately upon survey, but before servicing and construction, are reliable only 60% of the time. Second, deferring establishment for 4.5 months increases the reliability of the monuments by only 10%; they are reliable 70% of the time. Third, the practice of not deferring establishment until house construction is the reason that deferral is ineffective at significantly enhancing the reliability of monuments. Fourth, although enhanced deferral is in the public interest (if boundary monuments are a public good), land surveyors are reluctant to embrace a longer deferral period. This reluctance is partly a function of wanting to appease clients who prefer to locate house foundations from boundary monuments, and partly a function of viewing deferral as the slippery slope to the wide-spread use of coordinates in place of monuments to define boundaries. This reluctance, however, leads to a logical contradiction: If monuments are a public good, then their reliability only 60 – 70% of the time is intolerable. Conversely, if monuments are not a public good, then their current use is questionable.

5. Precision Estimation of Mobile Laser Scanning System
Jian Wang and Feng-xiang Jin

A mobile laser scanning system for mobile mapping is developed in our lab. In this paper, an overview of it and its observation model is given. Based on the analysis of error sources, possible factors affecting its positioning accuracy, such as scanning range error, scanning angle error, mounting error, synchronization error, interpolation error, coordinate conversion error, GPS positioning and attitude error are discussed in detail. Its precision estimation method is presented and some experiments are made to get the evaluation of its positioning accuracy.

6. Rapid and accurate INS alignment for land applications
Kai-Wei Chiang, Yun-Wen Huang, and Xiaoji Niu.

INS has been widely used in applications such as aerospace guidance and land vehicle navigation. Position, velocity and attitude are navigation parameters which need to be determined on a moving platform by processing the measurements of inertial sensors including specific forces and angular rates. Generally speaking, the initial attitude angles between the body and navigation frames play an important role, because they could affect the accuracy of the navigation solutions. Therefore, those angles have to be estimated accurately prior to switching the INS into navigation mode using a technique known as INS alignment. In the fine alignment, an optimal estimator such as the Kalman filter is used to perform the alignment process. Due to measurement errors, the Kalman filter takes about 10 to 15 minutes to converge. Those errors increase the alignment time and deteriorate the overall accuracy of the initial attitude angles estimated. In this article, a hybrid alignment scheme which integrates an ANFIS with the Kalman filter is proposed to obtain a better accuracy of the initial attitude angles with the emphasis on the heading angle. This method reduces the time required to achieve reasonable results for fine alignment. A tactical grade inertial measurement unit is used to verify the performance of the proposed scheme in this study. The preliminary results presented in this article indicate that a faster alignment procedure with higher accuracy can be achieved through the use of proposed scheme.

7. My years with Survey of Kenya
D. D. McCormack

The author began his career in the New Zealand Lands & Survey Department in 1952 and, after qualifying as a cartographer, was recruited to Survey of Kenya in early 1958, spending five and a half years on cadastral, topographical, township, atlas, land consolidation and miscellaneous mapping, also other duties relating to famine relief and new administrative boundaries leading up to Independence. These years and activities are described in relation to the huge political changes in Kenya at that time.

8. Virtual archaeological sites modelling through low-cost methodology
D. G. Aguilera and J. G. Lahoz

The recording and 3D virtual modelling of large and complex cultural heritage sites is currently receiving much attention due to the emergence of a wide array of new technologies which greatly support the efficient generation, administration and analysis of such models. Nevertheless, continues to be a problem the huge effort and costs that have to be invested to obtain realistic models. Besides on-site measurements, much time is often spent in manually rebuilding the whole site with a CAD package or a 3D-modelling tool.
In this paper a low-cost and flexible system has been developed for virtual archaeological sites modelling, starting from the acquisition system and ending with the generation of a virtual model in three dimensions. Various efforts have been made to increase the level of automation without losing accuracy and reliability. Finally, in order to demonstrate its capabilities some examples applied in archaeological sites are reported.

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.