This report presents guidance for practitioners on the use of positional, or spatial, data in Geographic Information Systems (GIS) for transportation applications. As GIS applications become more common in transportation system management and decision making, concerns have grown about the accuracy of the data used to populate them. Transportation spatial data usually describes the location of features on the highway system using a one-dimensional linear referencing system. The level of accuracy varies by data source and is affected by the precision of the measurement system used to collect it. Agencies need a way to understand the errors that may result when using these data in GIS applications and how these errors may be compounded when combining data from various sources. This project reviewed the linear referencing systems used by state departments of transportation, examined the limitations of typical data sources used in these systems, and developed a model to evaluate the effects of varying data accuracy and provide an assessment of the level of confidence in the system outputs. This report will provide valuable information to transportation practitioners who need to understand and account for the level of precision in GIS-based transportation decision tools. The accompanying CD-ROM contains a prototype error model that can be used to understand the effects of using and combining typical data sources. The outputs of the model list the confidence values at various probabilities associated with specific data sources for a wide range of transportation applications. The error model is implemented as a software program called GISError. It was developed in Visual Basic with a graphical user interface. A user guide for GISError is included in NCHRP Report 506 as Appendix A. (Author/publisher) This report may be accessed by Internet users at http://gulliver.trb.org/publications/nchrp/nchrp_rpt_506.pdf
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