Accuracy of spatial databases in a geographic information system (GIS) has become a recent focus of GIS research. Two types of errors exist in spatial data: positional and attribute. Methods to measure accuracy, reduce input error and track error propagation are required to ensure valid output. Sources of error have to be recognized and a level of accuracy assigned to the data. Often data layers are combined to produce new data with little regard to the accuracy of the input data. Data in a GIS are often represented as having a much higher precision than that at which they were collected. Users have to be made aware of the accuracy of the data they are working with and the precision of their GIS. Hydrologic modelling with GIS must take into account the accuracy of the data to produce accurate output. Scale must also be considered, i.e. single watershed or an entire drainage basin, as this affects the resolution of data collection. Examples are used when possible and relevant.
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