Travelling cylinders are not maps, they are relative movement plots done on the prescribed scan interval of the software; each point is then simply connected according to the well for clarity. The scan interval is usually 100ft (30m) but in some circumstances this needs to be much finer.
Travelling cylinder diagrams are conventionally plotted in polar coordinates and when used correctly should include a series of tolerance lines calculated from the approved survey program of the ‘subject well’ and the definitive surveys of the ‘offset wells’ (including planned wells); these constitute the anti-collision ‘allowable departures from plan’ towards every other well on the plot thus eliminating tedious field calculations of separation factors etc.
The angular values plotted should be normalized by the current wellbore azimuth; thus the reference azimuth for the plot is high-side + azimuth. This keeps the plot essentially reference to the North Azimuth of choice (true or grid).
In this way, the azimuth plotted is the sum of the angle of the point relative to the local high side and the high-side azimuth of the planned well at the depth of interest. Thus, a point which is 153° to the right of high side and at a depth that has an azimuth of 40° is shown with a bearing of 193°.
If the planned well consists of a simple buildup followed by a tangent section, then the high side will always be located at the angle corresponding to the borehole target azimuth. By contrast, a well with 20° of right-hand walk will start with a high-side arrow 10° to the left of target azimuth and will finish 10° to the right.
In cases where high-side azimuth varies over the section represented by the travelling-cylinder plot, depth-annotated high-side arrows may be used to show this variation.
It is not recommended to use the local borehole high-side direction as the diagram's zero reference, although a convenient aid to appreciating the diagram's meaning in simple build-and-hold cases, leads to difficulties because plots involving complex well shapes with large azimuth changes at low inclinations will exhibit discontinuities.
If a directional driller wants a plot to a high-side reference then give him/her a drawing pin to fix the plot to the wall at the ‘bullseye’ then rotate the plot to whatever azimuth is being drilled but remember to correct to Grid or True North not Magnetic.
In vertical wells where high side is indeterminate, the angle is the horizontal bearing of the point from the planned well.