Surveying and Land Information Science

vol. 65, no. 1, 2005

 

What Does Height Really Mean? Part II: Physics and Gravity

Thomas H. Meyer, Daniel R. Roman, and David B. Zilkoski

 

This is the second paper in a four-part series considering the fundamental question, “what does the word height really mean?” The first paper in this series explained that a change in National Geodetic Survey’s policy, coupled with the modern realities of GPS surveying, have essentially forced practicing surveyors to come to grips with the myriad of height definitions that previously were the sole concern of geodesists. The distinctions between local and equipotential ellipsoids were considered, along with an introduction to mean sea level. This paper brings these ideas forward by explaining mean sea level and, more importantly, the geoid. The discussion is grounded in physics from which gravitational force and potential energy will be considered, leading to a simple derivation of the shape of the Earth’s gravity field. This lays the foundation for a simplistic model of the geoid near Mt. Everest, which will be used to explain the undulations in the geoid across the entire Earth. The terms geoid, plumb line, potential, equipotential surface, geopotential number, and mean sea level will be explained, including a discussion of why mean sea level is not everywhere the same height; why it is not a level surface.

 

 

Analysis of the Design Parameters of Multi-Reference Station RTK GPS Networks

Ahmed El-Mowafy

 

In this study, the main design parameters of multi-reference station RTK-GPS networks have been analyzed, and their impact on the achieved positioning performance and phase ambiguity resolution is discussed. The main parameters considered are: the distance separating reference stations, network configuration, communication between the computing center and the user, and network algorithm. A number of tests have been conducted in a case study utilizing the Dubai Virtual Reference Network System (DVRS) in the United Arab Emirates. First, the real-time performance of the existing network was assessed. Next, the performance of the network was evaluated in a post-mission processing after making changes to its design, addressing the above parameters, by adding data from three auxiliary reference stations. Different network configurations were examined, consisting of variable combinations of the DVRS and the auxiliary stations. In addition, positioning results of two network algorithms (FKP/VRS and the Multiref methods) were compared. Finally, recommendations concerning the selection of baseline lengths, network architecture and station redundancy have been given.

 

 

Delaunay Triangulation Structured Kriging for Surface Interpolation

Yaron A. Felus, Alan Saalfeld, and Burkhard Schaffrin

 

Surface interpolation is an essential tool in surveying and geographical information systems projects. For example, given a list of observations (e.g. elevations, gravity or magnetic field values, and underground-water levels), a prediction of a value at an unobserved location is made.  Surveyors and engineers commonly use Triangulated Irregular Network (TIN) based linear interpolation for surface interpolation. TIN interpolation is computationally very efficient, utilizing a Delaunay triangulation algorithm and simple mathematical function. However, the TIN method uses only three local data points. Therefore, it is often less accurate and will yield a higher Mean Square Prediction Error (MSPE). Kriging is a relatively new, accurate interpolation method which yields a smaller Mean Square Prediction Error (MSPE). Nevertheless, kriging is computationally inefficient and requires the inversion of an n n matrix where n is the number of data points. A unique approach is presented here that combines these two techniques such that the Delaunay triangulation data-structure is used to determine the interpolation neighborhood of a kriging prediction process. The new TIN-based kriging algorithm is used to interpolate aeromagnetic data for a geographical information system developed in West Antarctica. A comparison is made between global kriging, TIN linear interpolation, and the TIN-structured kriging.

 

KEYWORDS:  Kriging, Delaunay triangulation, interpolation, aeromagnetic data

 

Development of Pattern Recognition Algorithm for Automatic Bird Detection from Unmanned Aerial Vehicle Imagery

Amr Abd-Elrahman, Leonard Pearlstine, and Franklin Percival

 

In this study a multi-stage pattern recognition algorithm was developed to identify individual birds using images captured by a small unmanned aerial vehicle (UAV) equipped with a progressive scan video recorder. The developed algorithm utilizes correlation matching in addition to spectral characteristics of photographed birds. Individual images of flights over a wildlife management area and an agricultural area in southern Florida were tested. Omission and commission errors were mostly less than 20 percent in tested images and frequently less than 10 percent. Further efforts to develop the algorithms and automatically determine the thresholds used in the algorithm are suggested. A visual C++ Graphical User Interface (GUI) was developed to facilitate the implementation and use of the developed algorithm.

 

 

Establishment of National Grid Based on Permanent GPS Stations in Israel

G. Steinberg and G. Even-Tzur

 

The ultimate goal of the Survey of Israel (SOI) is to be capable of restoring and defining cadastral boundaries in Israel with an accuracy of 5 cm, at a 95 percent confidence level. In order to achieve this goal, we should be able to measure accurate and reliable horizontal control points and define their rectangular coordinates. Measuring new control points at this accuracy in a homogeneous network can be done by using GPS techniques underpinned by reliable, permanent GPS reference stations. However, integrating measurements performed during the last decade in classical networks (which were characterized by lower accuracy levels) with measurements based on permanent GPS reference stations poses a problem. The Israeli Geodetic Control Network will be based on an array of permanent GPS reference stations. A new datum, called ILGD05 (Israeli Geodetic Datum 2005), was defined for these stations. The transformation of geographic coordinates in ILGD05 to an improved national grid was adopted in a manner that would minimize changes in the existing rectangular coordinates values. This paper describes how the Survey of Israel met the demands and capabilities of the innovative technology. Following a more detailed description of the problems, we discuss the options for improving geodetic control, as well as the reasons for the decisions that were made. The paper suggests a strategy for the implementation of the proposed change in Israel, which will lead us into a new era in geodesy and surveying.