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But in this case when we are only 5000 m from our origin our depth error at our presumed position is 2m/1000m x 5000m = +/-10m. Traveling to our target position would be to over/under shoot the vertical by 8 m and over/under shoot the horizontal by 6 m creating a positional error of 10 m on the axis shown above.
Figure 76b: Calculating positional error.
The effect of the inclination error can similarly be calculated using our simple rule. Inclination only affects the vertical plane and we are 5000 m from the origin in this plane so the positional error will be at right angles to our space vector and will have a magnitude of 0.6% of 5000 m or 30 m.
Figure 76c: Calculating positional error.
The third axis will be in the horizontal plane and is due to the azimuth error of 1°. This will produce an ellipse axis across the wellbore of approximately 2% of 3000 m which is 60 m. It is quite common for the azimuth error to be the dominant error in a 3D ellipse of uncertainty. These ellipses are not to scale but show the orientations and relative sizes.
Figure 77: Plotting the ellipse uncertainty.
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