Assalam Alaykom warahmato Allah. This is my first participation in this astonishing GIS online Club, may Allah make us all benifit from it for the best of all. The following is part of a technical paper I presented last November at the First National GIS Symposium in Saudi Arabia that was held in Khobar. This part is right on the subject in concern here which Dr. Al-Ghamdi has thankfully explained. However, "man nzeed 3ala assamin illa 3asal" as we say:
Surveying and engineering applications generally demand mapping at a precision level higher than that which is commonly accepted in a GIS system. In order to realize this higher level of mapping, it is fundamental to understand the terms used to describe the quality of measurements, and consequently, the quality of maps
Several terms are used to describe the quality of measurements and information derived from them. The terms accuracy and precision, although widely misunderstood to mean the same, are terms most commonly used in surveying and engineering
The term accuracy is the nearness between measurements and their true value. The further a measurement is from its true value, the less accurate it is. As opposed to accuracy, the term precision is the degree in which several measurements provide similar results. In other words, it pertains to how repeatable measurements are
The grouping of rifle shots on a target is an example often used to demonstrate the difference between the two concepts. Figure 1 shows four different groupings that are possible to obtain. Based on the preceding definitions, both terms are clearly distinguishable in this example (SEE ATTACHMENT)
It must be noted that while the precision of any measurement is only affected by the measuring instrument, the accuracy of any measurement is the product of accuracies of the components of that measurement. For example, a GPS measurement is only as accurate as the sum of the instrument accuracy, the control accuracy, and the map projection and datum transformation accuracies
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