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Old 18-02-2019, 04:59   #1
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Parallel Indices -- Still Relevant?

It may be little known that it is possible to use PI's without doing any actual chart work, on modern MFD's --


Just use the horribly misnamed "set offset EBL" tool. Once offset, of course, it is no longer an EBL . But it can be used to set a perfectly usable PI.


If you use the radar in HU, as I do, then this "offset EBL" is automatically created parallel to your heading.



PI's are incredibly useful -- they show graphically and at a glance, whether you are being set towards some obstacle and whether you are correctly following your track.


They show graphically, the relative motion of everything your radar sees, in relation to your HEADING (and if you use Course Up orientation, then in relation to your course).



Do we need this anymore? After all, we can set a waypoint and have a track line represented on the screen, so that we immediately see if we are drifting off the track. We can even read out XTE in real time from our instruments.


Being oriented to a safe track line is OK for most purposes, I guess, but I find that when sailing hard on the wind, for example, a PI is incredibly useful to show the limit. I often use them to determine where to tack, for example.
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Old 18-02-2019, 06:15   #2
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Re: Parallel Indices -- Still Relevant?

Normally there is no need, but if GPS drifts this is good knowledge to have.

In the last few months for example there have been a number of incidents with Russia interfering with GPS reception in Northern Norway. This had a lot of consequences, as GPS coordinates are not only used for navigation, but also for example for the planning and building of roads.
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Old 18-02-2019, 06:20   #3
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Re: Parallel Indices -- Still Relevant?

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Normally there is no need, but if GPS drifts this is good knowledge to have.

In the last few months for example there have been a number of incidents with Russia interfering with GPS reception in Northern Norway. This had a lot of consequences, as GPS coordinates are not only used for navigation, but also for example for the planning and building of roads.

Now that GLONASS and Galileo are up and running, I wonder if it is as easy to interfere with GNSS navigation which is receiving all three (like my system does)? Anyone know?


If it is really that easy to interfere with GNSS, then absolutely agree with you -- this certainly does indicate the need for other means of doing pilotage. Orientation along a PI would be a key tool for those with radar.




It goes without saying, that a radar overlay on the chart, is a great way to check your position accuracy, or at least, aggregated position accuracy error with chart error. I spent last summer in the Arctic, where the charts were basically useless except to give a general idea of the shape of land masses. We used radar as the primary navigation tool
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Old 18-02-2019, 06:40   #4
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Re: Parallel Indices -- Still Relevant?

I use them - safety PIs going into unfamiliar channels or when passing obstacles offshore. IMO, XTE is useless when sailing; I tend to use waypoints to sketch out the rough track, then let the wind decide the actual track - moving WPs as we go along. If then coming up on a fixed object (ODAS, oil platform, anchored vessel, etc) I will drop the offset EBL on it (based on GPS-derived CMG; I prefer N-up orientation), and monitor progress off that.
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Old 18-02-2019, 07:01   #5
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Re: Parallel Indices -- Still Relevant?

Parallel Indexing is a valuable tool to judge the drift but it should always be used together with bearings or other means of position fixing.

In confined waters nobody should rely on the GPS only.
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Old 18-02-2019, 07:10   #6
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Re: Parallel Indices -- Still Relevant?

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Originally Posted by Lodesman View Post
. . . IMO, XTE is useless when sailing; I tend to use waypoints to sketch out the rough track, then let the wind decide the actual track - moving WPs as we go along.

I agree completely -- navigation when under sail does present problems which normal GPS navigation can't easily solve.


Another situation is when you're doing CTS navigation (subject of another recent thread) in strongly tidal waters -- your actual track has nothing to do with the rhumb line, and you need to be really careful not to get yourself into a dangerous place. I don't use PI for this, though -- I set danger waypoints (using the skull & crossbones icon ), and keep an eye on the plotter. I work out safety bearings for any particular hazard.



Quote:
Originally Posted by Lodesman View Post
If then coming up on a fixed object (ODAS, oil platform, anchored vessel, etc) I will drop the offset EBL on it (based on GPS-derived CMG; I prefer N-up orientation), and monitor progress off that.



Interesting -- why do you prefer N-up on the radar?



I'm very stuck in my habits and never use it like that. I like the vividly and totally relative POV from the Head-up radar screen, which is complementary to the N-up plotter screen. Probably I'm missing some advantage to N-up. I would never the plotter any other way than N-up.
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Old 18-02-2019, 07:43   #7
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Re: Parallel Indices -- Still Relevant?

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Originally Posted by Dockhead View Post
Interesting -- why do you prefer N-up on the radar?

I'm very stuck in my habits and never use it like that. I like the vividly and totally relative POV from the Head-up radar screen, which is complementary to the N-up plotter screen. Probably I'm missing some advantage to N-up. I would never the plotter any other way than N-up.
More than anything else, I'm used to N-up as that is what I learned, and used professionally. I find it easier to reconcile the radar picture with the plotter and/or paper charts, which are also N-up.
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Old 18-02-2019, 07:50   #8
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Re: Parallel Indices -- Still Relevant?

I should have also said, I'm comfortable switching to H-up, and have done so in narrow channels and canals. I've also been forced to by equipment faults, which makes PIs even more valuable.
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Old 18-02-2019, 08:37   #9
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Re: Parallel Indices -- Still Relevant?

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Originally Posted by Lodesman View Post
More than anything else, I'm used to N-up as that is what I learned, and used professionally. I find it easier to reconcile the radar picture with the plotter and/or paper charts, which are also N-up.

OK, I'll have to try it.



I think the orientation of these views is of the essence of what kind of information you get from them. H-up on the plotter is a perversion in my opinion -- that gives you a simply perverted view of the world in orientation to your bow. Sometimes you can't even recognize the land masses and objects you see. I would much rather have an earth-centered view of the geography, with N as a constant. It allows you to "learn" charts and understand areas, and then, on the plotter, you see your place in that context, not vice versa. With the plotter picture reconciled with the picture of the area in my mind, which comes from learning it, then it's easy for me to flip that around and understand it in relation to what I see with my eyes (or radar).



Looking at the radar screen, I like to see relative motion, and the position of objects relative to me. The transition from N-up on the plotter to H-up on the radar actually helps me flip the view around in my mind.



But now you've made me curious, so I'll try N-up next time I'm out. Do you ever use True Motion?
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Old 18-02-2019, 09:07   #10
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Re: Parallel Indices -- Still Relevant?

Reference that provide background guidance on this subject.


Parallel Indexing with Radar



www.learnmarinenavigation.com/parallel-indexing-with-radar



How To Use Parallel Indexing Techniques For Ship Navigation?

https://www.marineinsight.com/marine...ip-navigation/
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Old 18-02-2019, 09:16   #11
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Re: Parallel Indices -- Still Relevant?

Quote:
Originally Posted by Dockhead View Post
OK, I'll have to try it.

I think the orientation of these views is of the essence of what kind of information you get from them. H-up on the plotter is a perversion in my opinion -- that gives you a simply perverted view of the world in orientation to your bow. Sometimes you can't even recognize the land masses and objects you see. I would much rather have an earth-centered view of the geography, with N as a constant. It allows you to "learn" charts and understand areas, and then, on the plotter, you see your place in that context, not vice versa. With the plotter picture reconciled with the picture of the area in my mind, which comes from learning it, then it's easy for me to flip that around and understand it in relation to what I see with my eyes (or radar).



Looking at the radar screen, I like to see relative motion, and the position of objects relative to me. The transition from N-up on the plotter to H-up on the radar actually helps me flip the view around in my mind.



But now you've made me curious, so I'll try N-up next time I'm out. Do you ever use True Motion?
Or you can use South Up orientation which simplifies matters when your course is toward 180. Reference chart below:

The simplest scheme is to just place your chart on a lazy suzanne and spin it to align with your heading and the heading up view of the radar.
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Old 18-02-2019, 09:44   #12
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Re: Parallel Indices -- Still Relevant?

Quote:
Originally Posted by Gruz View Post
Parallel Indexing is a valuable tool to judge the drift but it should always be used together with bearings or other means of position fixing.

In confined waters nobody should rely on the GPS only.

In order to see set and drift is precisely why I will not have the autopilot slaved to the plotter. I like to set a course and observe where the five NM runs take me, so to speak, and to calculate the presence or absence of tidal and/or current effects thereby.

Also helps sail trim issues, too, if the boat is not set up correctly.
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Old 18-02-2019, 10:07   #13
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Re: Parallel Indices -- Still Relevant?

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Do you ever use True Motion?
Yes, but not a fan of it. Probably due to my RM upbringing . I like to toggle TM vectors now and then to make the confirmation between radar and visual aspect assessments.
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Old 18-02-2019, 10:08   #14
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Re: Parallel Indices -- Still Relevant?

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In order to see set and drift is precisely why I will not have the autopilot slaved to the plotter. I like to set a course and observe where the five NM runs take me, so to speak, and to calculate the presence or absence of tidal and/or current effects thereby.

Also helps sail trim issues, too, if the boat is not set up correctly.

I do find nav mode useful for some situations, but your last point is a really good one. I don't use it much when sailing. The sometimes sudden course changes, the pilot makes to get back on the course line, can really play hell with sail trim.
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Old 18-02-2019, 10:13   #15
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Exclamation Re: Parallel Indices -- Still Relevant?

Parallel indices should only be used in N-up mode on the radar screen. They are not reliable in the Head-up mode.

Imagine you are headed North, passing an island on its west side. There is a submerged rock 1/2 mile to the south and west of the island. In N-up mode, you draw or place your parallel index line to keep your track 1 mile west of the island and 1/2 mile west of the hidden rock. Any drift to the east will be immediately apparent as the index line moves off the edge of the island. A change in course to the west brings the index line back on the edge of the island and puts you back into safe water, clear of the rock.

But, in Head-up mode, if you drift to the east, a change in course to the west brings the index line back on to the edge of the island, but not clear of the rock. Or at least not 1/2 mile clear.
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