ABSTRACT
The third transformation stage in our model of GIS as a spatial data processing system is that of manipulation and analysis. We have now seen how data about the world are collected and entered into the computer, and the ways in which they can be organized into a model of geographic reality. The very first manipulation operations are those concerned with the verification and correction of spatial data after input, and these were introduced in Chapter 5. If the process of data input were perfectly ‘clean’ and digitizer operators never grew tired or made mistakes (!), these operations would be unnecessary, so they have been separated from the main body of manipulation and analysis operations considered here. The majority of GIS software systems make the assumption that the data they are processing are entirely accurate, although this is rarely the case. There is an urgent need for users of GIS to understand the implications of the likely errors in their databases and for software which is able to take this into account to some extent (Veregin, 1989; Chrisman, 1991).
