An approach for an object-based positional error model

C. Tinnachote and X. Chen

Space Technology Application and Research Program

School of Advanced Technology

Asian Institute of Technology

Pathumthani 12120 THAILAND

Chanin.Ti@chula.ac.th

xychen@ait.ac.th

Abstract

Rigorous and yet flexible models for positional errors of geometric features in GIS are crucial to the modeling of error propagation and the appropriate handling of these errors. Existing error models were derived and described based only on point features. This paper presents a new approach for extending those point-based error models towards an object-oriented one. For this purpose, the classical concept of the object-based “Minkowski operations” is employed. Multiple line and polygon datasets of the same area were considered as observations from which the weighted-average position of the features could be calculated. The resulting weighted-average features can be used as the reference or “true” values in measuring the error of each observation dataset. Results of the experimentation have proven that the technique is very useful in extending and implementing existing conceptual error models into practical work.

Geodetic GPS Analysis of Land Victoria’s GPSnet

N. Penna

Department of Spatial Sciences

Curtin University

(Now at the University of Newcastle upon Tyne UK)

nigel.penna@newcastle.ac.uk

J. Lo

Department of Spatial Sciences

Curtin University

G. Luton

Space Geodesy Analysis Centre

Geohazards Division

Geoscience Australia

The first published results of the geodetic GPS analysis of the data from the base stations that comprise Land Victoria’s GPSnet are presented.  Nineteen GPS base stations distributed across the state of Victoria (Australia) make up the network, whose principal purpose aims to reduce the reliance for positioning across the state on ground marks, and satisfy the ever increasing number of GPS users desiring access to GPS base stations.  This paper provides zero order ITRF2000 coordinates for the base stations and details the method used in their computation and subsequent transformation to GDA94, for use by surveyors and for reference for future geodetic studies such as deformation monitoring.  Three weeks of data were analysed from April 2003, plus a supplementary one week of data from June 2002, with day-to-day coordinate repeatabilities of 1-2mm in the horizontal components and <6mm in height from both data sets.  After transformation of both the mean April 2003 and June 2002 ITRF2000 coordinates to GDA94, coordinate differences were sub-centimetre for both horizontal and vertical components for all twelve stations present in both data sets, demonstrating the high quality of the data from the GPSnet base stations and the stability of the network.  The potential of the GPSnet stations for atmospheric research and GPS meteorology is also demonstrated.

How well does the Virtual Reference Station (VRS) System of GPS Base Stations Perform in Comparison to Conventional RTK?

G Ong Kim Sun and P Gibbings

Faculty of Engineering and Surveying

University of Southern Queensland

Toowoomba QLD 4350

Peter.Gibbings@usq.edu.au

In response to the recent growth in multiple reference station networks throughout the world, a pilot project of the Virtual Reference Station (VRS) network has been established in south-east Queensland.  Independent testing of this network was required to establish its performance in post processed and real-time positioning, and its reliable coverage area.

Tests were conducted at several sites, both inside and outside the network.  GPS data from a single antenna were used to simultaneously record real-time positions derived from both the VRS base stations and a conventional base station.  These data were analysed in terms of accuracy, precision and initialisation times.  At the same time, raw data were logged for later analysis of the post processing capabilities of the VRS.

Accuracy and precision estimates from the data collected showed that the VRS is at least comparable to, and in some instances may be considered superior to, conventional RTK.  For example, during tests when low numbers of satellites were visible, the VRS-RTK was able to initialise in shorter times than conventional RTK.

In general, the VRS-RTK proved to be a reliable substitute for conventional RTK using a single base station.  In fact, VRS-RTK was shown to be more reliable and robust than conventional RTK, and in many instances was able to produce results where conventional RTK failed.  VRS also showed great potential for post processing that, until now, has been largely ignored.

Inner constraints for 3-D survey networks

W. Tan

Department of Building

School of Design and Environment

National University of Singapore

Kent Ridge Crescent, Singapore 117566

bdgtanw@nus.edu.sg

This paper provides a simple and accessible derivation of the inner constraints for 3-D survey networks. There are three positional constraints, three rotational constraints, and one scale constraint. These constraints are often used in minimally constrained free net adjustments where error ellipses reflect the geometry of the network rather than the arbitrary imposition of constraints.