Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Measure of Quality Assurance

One of the values of a GIS is its capability to combine different types of information that have the same geographic base. For example a user might need a map displaying a single variable derived from a combination of soil type, bedrock type, and depth to bedrock. An important aspect of the defining information of a report combining these three variables is the confidence one can place in the accuracy of the resultant information. Each of the constituent individual variables is stored in the database and each has its own characteristics of precision and accuracy—measured in terms of geographic coordinates and the values of the variable. Therefore, just as the values of the three individual variables are combined to produce a single variable, the precision and accuracy attributes of these variables should be combined—according to appropriate numerical techniques—determine the accuracy of the final result. Statements of accuracy should be included as part of the resultant report.

The process of assessing accuracy is not always easy, nor are the results always encouraging. For example, in a study done by the Australian Commonwealth Scientific Industrial Research Organization a number of years ago, involving three variables, an analysis was made of the output. When particular points on the Earth’s surface were picked out and “ground truthed,” it was found that at least one of the constituent variables was incorrect more than 60 percent of the time. The ...