GIS users and professionals are aware that the accuracy of GIS results cannot be naively based on the quality of the graphical output. Data stored in a GIS will have been collected or measured, classified, generalised, interpreted or estimated, and in all cases this allows the introduction of errors.; With the processing of translation of this data into the GIS itself further propagation or amplification or errors also occur. It is essential that GIS professionals understand these issues systematically if they are to build ever more accurate systems.; In this book the authors decade of study into these problems is brought into focus with an account of the development, application and implementation of error propagation techniques for use in environmental modelling with GIS. Its purpose is to provide a methodology for handling error and error propagation.