Parallel Index lines are mentioned in Annex to IMO res.A893(21)-Guidelines For Voyage Planning.
Section 4- Appraisal
"Additional information which should be marked on the charts
include: ..... Parallel index lines should also be drawn where appropriate."
Section 6- Monitoring, point (j)
"Radar can be used to advantage in monitoring the position of the vessel by the use of parallel indexing, which is a simple and most effective way of continuously monitoring that a vessel is maintaining its track in restricted coastal waters. Parallel indexing can be used in any situation where a radar-conspicuous navigation
mark is available and it is practicable to monitor
continuously the vessel’s position relative to such an object. It also serves as a valuable check on the vessel’s progress when using an electronic chart."
IMO requirement is good reason to have this tool included in OCPN tools, I think
PI line is the line perpendicular to EBL (point of rotation is ODraw EBL End point) and for user is defined by distance from the boat and direction (from north or relative to heading). Length is not defined- the line fill whole screen
(depends on the range/scale).Sometimes I use radar/ECDIS EBL with offset from the boat as substitute.
If jongough decide to extend ODraw EBL for PI lines, I suggest to include option show/hide for VRM,EBL,PI (now show/hide is for VRM) and abitity to offset EBL with relation to moving boat.
attached are screenshoots of radar screen
with 2 PI lines, 2 EBL and VRM showing relation between them and ECDIS screen with 2 PI lines, 1 EBL+VRM with offset from the center and 1 EBL centered.