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When magnetic north is included (see next chapter) we have three different North References to contend with.
These can be in any order with several degrees of variation between them so a clear North Arrow is essential on all well plans and spider maps.
When correcting from one reference to the other it is common practice to set the company reference North straight upwards and plot the others around it. In figure 22a & 22b, Grid North is the preferred company reference, so all quoted azimuths would be referenced to Grid and the North arrow is centred on Grid North.
Figure 22b: The three north references.
In this example (Figure 22c), the well is heading 60 o Grid with magnetic North at 6 o west of True and Grid North 2 o East of True.. Depending on the reference used, the azimuth can be expressed three different ways, so it is easy to see how confusion can occur.
Figure 22c: The Convergence angle calculation.
Worked examples follow on the next pages…..
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