The error magnitudes discussed above are essentially the standard deviations of the range of values that error source can be expected to take. Many of these error sources will have zero mean and hence the standard deviation is all that is required.
However, the formulation of the model also allows for error sources which have a non-zero mean. These are known as bias terms and lead to error ellipses which not be centred on the survey point, but are offset in a
particular direction.
The most obvious application for this would be for measured depth errors, where it is well known that drill pipe is measured on surface but when downhole expands with temperature and stretches under tension from the weight the drill-string. So the true measured depth will be longer than the sum of the drillpipe lengths on surface.
However, after discussion at ISCWSA meetings, the committee took the view that biased models should are not encouraged. If biases are present these should be corrected where possible, and if not the error ellipses should be large enough to encompass the expected actual location of the wellbore.
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