Exploring the Effects of Target Location Size and Position System Accuracy on Location Based Applications ⋆ Cliff Randell 1 , Erik Geelhoed 2 , Alan Dix 3 and Henk Muller 1 1 Department of Computer Science, University of Bristol, UK 2 Hewlett-Packard Laboratories, Bristol, UK 3 Computing Department, Lancaster University, UK cliff, henkm@cs.bris.ac.uk erik geelhoed@hp.com alan@hcibook.com Abstract. We describe an examination of various physical and human factors which influence the effectiveness of location-based applications. By varying both the target location size and position system accuracy, and hence the ease of use of an application, we are able to identify physi- cal constraints which apply as well as quantifying performance and eval- uating human factors. A movement analysis is proposed which allows us to formulate a set of equations that relate the time to find the target to the target location size, distance and positioning system accuracy. We validate our work using a game based application, digital hopscotch, in which the location size and the accuracy of the positioning system are varied. A further set of tests is performed outdoors using a GPS-based application. We show that the results from these experiments concur with the results from our equations. This work may be usefully embed- ded in software packages that allow designers to build location-based applications. 1 Introduction Location-based applications for mobile computing have become of increasing in- terest to the research community, and are entering daily usage by the general public. Examples of this range from GPS-based navigation systems for vehi- cles and pedestrians, ‘where’s my nearest’ applications for mobile phones and, increasingly, Geographic Information Systems (GIS) which associate informa- tion with geographic co-ordinates. Emerging applications include gaming and location-based entertainment. In this paper we explore physical aspects which affect the performance of applications, in particular the parameters of the lo- cation or target, and also the specification of the position sensing system. As part of this continuing research we are seeking to contribute to methods for the objective evaluation of such systems. ⋆ Funding for this work is received from the U.K. Engineering and Physical Sciences Research Council, Grant No. 15986, as part of the Equator IRC