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GIS Guide to Good Practice
Section 3 - Spatial data types

3.4 Precision and Accuracy

In incorporating any spatial data source it is crucially important to consider the issues of precision and accuracy.

"Precision implies that the degree of measurement of an attribute is refined; accuracy that the measurement taken is correct within the degree of precision indicated" (Richards and Ryan 1985: 20).

This can best be illustrated with respect to the example of a highly detailed topographical survey undertaken using a total-station survey instrument and based upon reference points taken from a 1:2500 scale base-map. This is an example of a potentially very precise method using potentially very inaccurate data. This is because the source data, i.e. the fixed point locations derived from the map, are not sufficiently accurate to justify the precision employed in the method. To summarise, accuracy relates to the correctness of a result, whereas precision is essentially a measure of the units used.

As an aside, such issues of precision and accuracy are of particular interest in the context of GPS, where the quoted accuracy of co-ordinates varies commonly from sub-centimetre to around ±50 or so metres. Although this varies according to where on the globe you are referring, one second is approximately equal to thirty metres, yet, in terms of decimal degrees, is represented by a value of 0.0002777778. Thus a typical GPS readout in decimal degrees of, say, 52.005N is only within about 500 metres - making the general GPS error rate of ±50 metres trivial!

There is a temptation to believe the apparent accuracy with which computer-based GIS report the co-ordinate locations of objects. This information is, however, only as good as the initial data source, and errors made at this stage can be perpetuated through the lifetime of the data set.


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© Mark Gillings, Peter Halls, Gary Lock, Paul Miller, Greg Phillips, Nick Ryan, David Wheatley, and Alicia Wise 1998

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