GPS For GIS : Getting the Appropriate Combination

Abstract

The concept of using GPS as a 'real-world' digitiser for GIS applications has attracted the attention of many GIS users. The apparent ease-of-use of GPS, however, has meant that many users are unaware of the limitations of the system. Depending on the type of system (hardware and software) and data collection procedure employed, GPS technique can give positioning accuracies ranging from few millimetres to a hundred metres or so. On the other hand, different GIS applications require different levels of data accuracy, mainly a function of the "scale" of the database. The question then is: which GPS system should one use for their GIS need? This paper describes a variety of GPS systems which give different accuracies. An emphasis is given to systems based on low-cost GPS instrumentation ("sensors" and "engines" - C/A-code single frequency receivers) which may be more economically appropriate to use for most GIS applications. More importantly, issues regarding system accuracy, which GPS users need to be aware of, are addressed. Among the important ones are; (1) What level of reliability is the accuracy associated with? (2) What observation conditions or constraints need to exist for this accuracy to be guaranteed? In this paper, C/A-code-based systems are tested and their performance characteristics are discussed. In addition, limitations of the systems with regards to observation conditions are discussed.