As an example, county surveyors might be digitizing the location of underground water mains in an area where you are planning to excavate. Their standards might require no more than three chances out of 1000 that an excavation planned to be 10 feet from the main might actually hit the water main. In other words, you can be 99.7 percent sure of not hitting the water main.
In this case, the equation is
so
See Also
Digitizing Accuracy and Acceptable Error