Journal of Surveying Engineering

The orientation of a plane table at a new station is carried out with the help of three well-defined visible points available on the ground along with their corresponding plot on the plan using Bessel’s graphical solution. There are seven regions $(R1–R7)$ in and around the area bounded by the three well-defined ground points and the plane table may be stationed in any one of them for orientation. While carrying out such field work, a completely opposite orientation was obtained for the plane-table station in reference to the ground points and corresponding plotted plan points for some of the regions, hinting at possible limitation of the method. This paper makes an effort to understand and analyze the limitation of Bessel’s graphical solution. The orientations were carried out on an AutoCAD worksheet and cross checked through field works. It was observed that the position of the plane table station and the third point used to draw the ray in reference to the line joining the first two points used for sighting played a critical role in obtaining the correct orientation of the plane table. When the position of the plane-table station and the third point were on the same side of the line joining the first two points in $R2$ , $R3$ , and $R4$ regions, it resulted in a completely opposite orientation of the table. This necessitated modification in Bessel’s method to obtain correct orientations, but the orientations obtained had a very high variation in the magnitude of errors in the distances at a lower scale of 1:500. The field orientations carried out at a higher scale (1:100) yielded a lower magnitude of errors in the distances. Alternatively, the limitation could be overcome by interchanging the third point with any of the first two points used for sighting such that the plane-table station point and the third points were on the opposite side of the line joining the first two points used for sighting. This alternative also yielded correct orientations with much lesser errors in distances. However, when the plane table was positioned within the area bounded by the three well-defined ground points (i.e., $R1$ region) and orientation was carried out as per the method, it yielded a correct orientation even though the plane-table station and the third point were on the same side of the line joining the first two points used for sighting. When the position of the plane-table station and the third points were on the opposite side of the line joining the first two points in $R5$ , $R6$ , and $R7$ regions, Bessel’s method yielded correct orientations.

Selecting the appropriate receiver is an issue when a major portion of global positioning system (GPS) data collection is below forest canopies. This study compares four low-cost GPS receivers, in order to determine the most suitable receiver for position assessment under different forest canopy covers, in terms of ease of use, accuracy, and reliability. A total of 33 positional assessments were gathered per receiver, plot, and method, in 18 forest locations. Data were described and analyzed through a sample comparison analysis at 95% confidence level (Mann–Whitney nonparametric test), in order to determine the existence of differences in accuracy and precision in positioning between receivers. Results showed that there were significant differences between the receivers regarding accuracy and precision measuring coordinates; moreover, accuracies were different depending on the canopy cover and forest characteristics. Therefore, practical recommendations for each case were settled in order to help foresters to select the most suitable receiver. Moreover, key forest variables regarding GPS performance were identified, so that forest environments could be effectively clustered by them.

In rapid global positioning systems (GPS) positioning one of the key problems is to quickly determine the ambiguities of GPS carrier phase observables. Since carrier phase observations are generally collected only for a few minutes in the mode of rapid GPS positioning, the least squares floating solution of the ambiguities will be highly correlated and the decorrelation approach has often been used in order to reduce the search space of integer ambiguities. In this paper we propose a regularized algorithm as an alternative approach to decorrelation, and compute the regularization parameter by minimizing the trace of mean squared errors. Since regularization has been essential to solve inverse ill-posed problems and shown to be very significant in reducing the condition number of normal matrices, we will explore possible applications of regularization for improving the high correlation of the estimated float ambiguities. Numerical experiments with 50 epochs of single frequency observations show that the condition number after regularization reduces to half of that of the floating solution if the ambiguities could be known to $2–3\text{cycles}$ . If better knowledge about the ambiguities could be obtained to within $1\text{cycle}$ , further improvement can be achieved. The results indicate that regularization could be used for fast GPS ambiguity resolution. Our experiments also demonstrate that a scale factor of about 8 is needed to multiply the estimated variance of unit weight for obtaining a reasonable estimator for the accuracy of float ambiguities.

In this paper, a new algorithm is employed in global positioning system (GPS) rapid positioning using several-epoch single-frequency phase data. First, we define the double- $k$ -type ridge estimator (DKRE) based on the structure characteristics of multicollinearities of the normal equations matrix in the double-difference (DD) model, and prove when the DKRE is not worse than the least-squares estimator (LSE) in the sense of a reduced MSEM. Taking into account how the ridge parameter in the ordinary ridge estimator is confirmed based on the generalized ridge estimator (GRE), we propose a method of estimating two ridge parameters for the DKRE. Second, we improve the LAMBDA method through replacing the cofactor matrix computed by the LSE with the cofactor matrix computed by the DKRE. The ambiguities-fixed solution is found by the sequential LSE. A theorem stating that the success rate of the improved LAMBDA method is bigger than the original LAMBDA method is given. Finally, through the GPS positioning tests, it is shown that the present method is highly efficient and reliable, and some very valuable conclusions are obtained by analyzing the computation results.

A Leica System 5000 dual frequency global positioning system receiver System 500 rover operating from a fixed base station was mounted in a drogue made from a $5\text{-}\mathrm{gal}$ (19 L) fisherman’s bucket and used to measure ebb and flood tidal currents in Absecon Inlet, Atlantic City, N.J., to provide data for the design of a marina. The drogue was deployed from a boat in the inlet channel during times of maximum ebb and flood currents. Nine deployments were made around the time of maximum ebb flow and six deployments around the time of maximum flood flow. The drogue was tethered to a boat by a slack line to prevent its loss. Drogue positions were determined at $3\text{-}\mathrm{s}$ intervals and used to calculate flow velocities which were then spatially averaged over travel distances of approximately $50\mathrm{ft}$ (15.2 m). No position had more than $0.54\mathrm{ft}$ (0.16 m) reported horizontal error, and the average of all errors was $0.06\mathrm{ft}$ (0.02 m) for the ebb tide and $0.08\mathrm{ft}$ (0.024 m) for the flood tide.

A method for extracting streetlight poles from orthophotos is presented. The main challenge of the research undertaken was to recognize streetlight poles which are narrow, vertical objects that have very limited width in the orthophotos. The proposed method was successful in extracting such poles by observing their shadows. The method is based on modifying the orthophotos using a sequence of image filters and a cross correlation operation to extract streetlight-pole locations and positions. The method was applied and validated using a road network in the Durham Region, Ont., Canada. The proposed method, which allows verification and enhancement of streetlight pole databases, should be of interest to highway agencies interested in simple and effective methods for road feature extraction.