Parallel Indices -- Still Relevant?

[Pelagic]

Originally Posted by Dockhead

Well, stabilized Course-up works too. But this is an excellent point, for those of us relying on the offset EBL rather than drawing it on the chart.

Just to clarify...what are you using to stabilize your Relative motion, course up presentation?

[Uricanejack]

Originally Posted by Lodesman

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.

Ahh you giving away your age there

[Dockhead]

Originally Posted by Pelagic

Just to clarify...what are you using to stabilize your Relative motion, course up presentation?

Course up on my radar is oriented to the track from the nav system. So it looks like Head up, but doesn't swing around when the boat's head swings.


But after this discussion, I'm going to try to use North up, at least when I'm near any land masses. I can see the benefit of this presentation.

[Uricanejack]

Originally Posted by VanIsle

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.

What gives you this idea.
A PI works just fine on a heads up display. In many ways it’s more simple.

[ameehan]

Originally Posted by KJThomas

Unfortunately no. If they had been on a course parallel to the rock, as they were on previous occasions, the PI would have worked fine.

In fact, they were altering course as they approached the rock, and underestimated the bodily advance/sideways transfer, on the turn radius.

Did I really need to add “properly executed “ to original statement? Using a properly executed parallel index the unfortunate CC would not have come close to aforementioned rock, the clue is in the word parallel.
APWM

[Uricanejack]

Originally Posted by Dockhead

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.

I would give you a word of caution about using the offset EBL to PI.
Be careful.

A fundamental principle of a PI. The line you are using must be a tangent to a known range( ring or VRM).
A common error when using a off set EBL is to adjust the EBL to get a required course or course change.
As soon as you adjust the bearing it’s no longer a tangent.

The point of origin for the offset EBL has to be moved to make it a tangent again.

Pi as better done using a separate parallel line. Unfortunately a lot of manufactures do not make this feature easy to use.

Some radars it’s tied to an electronic curser. Which has limited variations. With the parallel lines set to specific range rings.
They do have the advantage of always being tangents.

A common error

[Dockhead]

Originally Posted by Uricanejack

I would give you a word of caution about using the offset EBL to PI.
Be careful.

A fundamental principle of a PI. The line you are using must be a tangent to a known range( ring or VRM).
A common error when using a off set EBL is to adjust the EBL to get a required course or course change.
As soon as you adjust the bearing it’s no longer a tangent.

The point of origin for the offset EBL has to be moved to make it a tangent again.

Pi as better done using a separate parallel line. Unfortunately a lot of manufactures do not make this feature easy to use.

Some radars it’s tied to an electronic curser. Which has limited variations. With the parallel lines set to specific range rings.
They do have the advantage of always being tangents.

A common error

I don't quite understand what you're saying. Maybe I haven't had enough coffee yet this morning.



When I use the offset EBL as a PI, I start by setting the VRM at the desired range from the PI. Then, using the cursor, I set the offset EBL to touch the VRM (making it a tangent to the VRM), and then align it with my course. Then, I observe the progress of the VRM along the offset EBL.



If I change course away from the dangers, I don't worry about changing the EBL -- I just observe with satisfaction how I'm getting further away.


If I change course towards them, forcing me to get closer to them, I start all over again, using a different desired range from the PI.


Am I missing something, or doing something wrong?

[Uricanejack]

Originally Posted by Dockhead

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.

Back to your original question. Is worth while.

The proper use of a PI is one the best and most accurate methods of observation for navigation there is.
Not quite as accurate a good visible transits but very good and with advantage it works very well in pooor visibility.
Read some MAIB, TSB ,NTSB reports on Groundings a common theme will be the lack of use of Parrallel indexing.

Properly using parallel indexing uses your radar to its best effect for navigation.

Plotting your position using radar gives a very good position. The position is where you were.
A PI gives very good position information for where you will be.

The comparison with GPS and plotters or electronics.
The GPS May be more precise.
It’s still just an electronic positition. In the virtual world.
The radar will give you a position relative to the real world.
The PI improves the radar information.

Personally I put observation even with it’s less precision in a higher priority over electronic.
In today’s world with todays tech. The observation should be used to confirm the reliability of the electronics. The electronics can inporove the overlay precision of the information. The electronics can confirm the observations are correct.

If they do not agree you have a problem. More importantly the disagreement informs yo you have a problem.
Reliance on one or the other may mean you miss an error.

While I personally prioritize observation. By Eyes Radar Electronic.
I have been made aware of a problem with a Radar by the electronics.
Which I might otherwise not have noticed due to poor visibility.

[Uricanejack]

Originally Posted by Dockhead

I don't quite understand what you're saying. Maybe I haven't had enough coffee yet this morning.



When I use the offset EBL as a PI, I start by setting the VRM at the desired range from the PI. Then, using the cursor, I set the offset EBL to touch the VRM (making it a tangent to the VRM), and then align it with my course. Then, I observe the progress of the VRM along the offset EBL.



If I change course away from the dangers, I don't worry about changing the EBL -- I just observe with satisfaction how I'm getting further away.


If I change course towards them, forcing me to get closer to them, I start all over again, using a different desired range from the PI.


Am I missing something, or doing something wrong?

A purist might argue you are doing something wrong. It is a practice which has been known to lead to navigation error and grounding.
Most radar instructor will advise against ussing the off set EBL for a PI.

My opinion what you are doing is just fine and dandy. Particularly when you reset and start over. On occasions I have used an offset. EBL as a PI.
I ussualy have used this technique because the particular radar I was ussing had an awkwardly method for adjusting a dedidacted PI line.

Quite simple moving and adjusting the Off Set EBL was a much simpler fewervstep process.

I personally believe it is ok to do so. Provided it is reset or as you say start again.

It is a practice.
I watch very closely if I observe someone ussing the EBL innthis way. My experience even many professional navigators will miss this. Which I treat as an error in principle.
If they re adjust. I will still ask tougher questions. To see if they understand why it might be a problem.

The possible error or hazard. Is the temptation to say this isn’t working.
Adjust the EBL then use the EBL adjustment to adjust the course steered.
The problem.
The unadjusted EBL is a tangent to the desired range.
The adjusted EBL is not.

When you are being set away from the danger as you described not a problem.
Adjust the EBL the adjust course you will still be set away from the danger.

If being set towards the danger. If you do not start again but are tempted to just adjust the EBL. It is no longer a tangent to the range you have it set for.
The possible result is an adjustment if course to pass the danger or hazard at a specific distance or range. Will now possible be lees than intended or perceived due to the EBL no longer being a tangent to the desired range.

[Dockhead]

Originally Posted by Uricanejack

Back to your original question. Is worth while.

The proper use of a PI is one the best and most accurate methods of observation for navigation there is.
Not quite as accurate a good visible transits but very good and with advantage it works very well in pooor visibility.
Read some MAIB, TSB ,NTSB reports on Groundings a common theme will be the lack of use of Parrallel indexing.

Properly using parallel indexing uses your radar to its best effect for navigation.

Plotting your position using radar gives a very good position. The position is where you were.
A PI gives very good position information for where you will be.

The comparison with GPS and plotters or electronics.
The GPS May be more precise.
It’s still just an electronic positition. In the virtual world.
The radar will give you a position relative to the real world.
The PI improves the radar information.

Personally I put observation even with it’s less precision in a higher priority over electronic.
In today’s world with todays tech. The observation should be used to confirm the reliability of the electronics. The electronics can inporove the overlay precision of the information. The electronics can confirm the observations are correct.

If they do not agree you have a problem. More importantly the disagreement informs yo you have a problem.
Reliance on one or the other may mean you miss an error.

While I personally prioritize observation. By Eyes Radar Electronic.
I have been made aware of a problem with a Radar by the electronics.
Which I might otherwise not have noticed due to poor visibility.

Maybe I don't really have the right to an opinion here -- unlike you and others in this thread, I am not a pro, and most of what I know is self taught.


But for whatever little it may be worth -- I deeply and fundamentally agree with all of what you wrote above, and it goes to the heart of why radar is important, and why PI's are still relevant (original topic of this thread).


Scientists say something like -- observations are gold; hypotheses are silver; conclusions -- bronze.



GNSS position data is pretty reliable and much more accurate than position data we can get another way. But charts may not be, and in any case, your position as displayed on a chart plotter is all a conclusion, subject to different kinds of possible errors. It's a fantastic tool, but it is not an observation -- which a radar picture IS.


For that reason, I consider radar navigation to be extremely relevant, and I, for one, power up the radar before hauling the anchor or leaving the dock, and shut it down only if I'm leaving the boat for a while. I ALWAYS have the radar overlay on the chart plotter display (when I'm using the chart plotter display) to verify what the plotter tells me, and in critical situations I try to back up the electronic solutions of the nav system (like a plotted track on a safe path through hazards) with PI's, or use other radar nav techniques. Offshore, I keep the helm display on the radar screen, not the plotter screen.


In the Arctic this summer, these techniques really saved the day for me. In the absence of accurate charts, GPS position was irrelevant, so radar nav became primary. Far above the Arctic Circle, we experienced a little darkness only in August, but we did have some days of rain and poor viz. Without radar, in the absence of even vaguely accurate charts, we would have been absolutely stuffed.

[Uricanejack]

Every one is entitled to an opinion.
Opinions are like a certain well known orrafice every one has one.
Asking questions and having an open mind are the important
The ability to change an opinion is the important skill.

If you have a room full of professionals you will get as many opinions as there are professionals in the room.
The validity and spectrum of those opinions will vary just the same as any other groups.

Keep asking the questions. You will get a broad variety of opinions as answers.
I am just an other old guy with a computer on the internet. With a lot of opinions

Your questions and opinions are ussualy very good I’m sure a lot of people learn useful points from your threads.

[Pelagic]

One of the safety checks I learned to do on Radar VRMs probably stemmed from early model problems, but has become a ritual habit at the start of each watch.

The VRM can misread as a result of rough operator use.
I have actually seen that problem on demo models at marine equipment shows.

The only way to test is to see if the VRM matches to the fixed ring at different ranges.
Could really screw up your PI day[emoji57]

[requiem]

Originally Posted by Uricanejack

What gives you this idea.
A PI works just fine on a heads up display. In many ways it’s more simple.

Since reading this thread I've been trying to make sure I have a good handle on the concept. As best I can make out, it really depends on what happens to the PI when you change heading, as the PI can stay tangent to the selected VRM and still get you into trouble. OTOH, if the center of rotation of the PI is around your own vessel, and it rotates to maintain the same true bearing, then it might work in Heading Up mode. As I haven't played with these yet, I'm not sure what's common these days.

Here's a diagram I made to show what I believe VanIsle was describing; assume the red marker is a buoy you are using to set the PI, the rectangle indicates a hazard (reef, submerged rocks, etc.), and you are being set southeast by the current.



The top row is a simple North Up display; the proper course correction keeps the marker and the hazard outside the PI and sliding nicely along it.

The bottom row is a Heading Up display; a smaller heading adjustment places the marker back on the correct side of the PI but only for a short time and another heading correction is soon needed. Note that after a change in heading the PI no longer describes the true bearing between the marker and the hazard: the hazard is appears to rotate around the marker with each correction made, while in reality the boat slides slowly towards it.

Now if you correctly took the current into account with the first correction, as with the top row, then you would still pass safely by, but it would appear on screen that the marker was quite far from the index line. If you tried to keep the marker along the index line, you'd end up making a series of smaller corrections that could eventually result in drifting into the hazard.

[IslandHopper]

Originally Posted by Uricanejack

The proper use of a PI is one the best and most accurate methods of observation for navigation there is.
Not quite as accurate a good visible transits but very good and with advantage it works very well in pooor visibility.
Read some MAIB, TSB ,NTSB reports on Groundings a common theme will be the lack of use of Parrallel indexing.

Properly using parallel indexing uses your radar to its best effect for navigation.

If a junior officer handed me a passage plan without PI's for entering/leaving port, or any other areas where safe clearance is required on passage, he/she will be getting it straight back.....and they all better be laid out neatly on the relevant charts....a passage plan without PI's is not a passage plan

[Uricanejack]

Originally Posted by requiem

Since reading this thread I've been trying to make sure I have a good handle on the concept. As best I can make out, it really depends on what happens to the PI when you change heading, as the PI can stay tangent to the selected VRM and still get you into trouble. OTOH, if the center of rotation of the PI is around your own vessel, and it rotates to maintain the same true bearing, then it might work in Heading Up mode. As I haven't played with these yet, I'm not sure what's common these days.

Here's a diagram I made to show what I believe VanIsle was describing; assume the red marker is a buoy you are using to set the PI, the rectangle indicates a hazard (reef, submerged rocks, etc.), and you are being set southeast by the current.



The top row is a simple North Up display; the proper course correction keeps the marker and the hazard outside the PI and sliding nicely along it.

The bottom row is a Heading Up display; a smaller heading adjustment places the marker back on the correct side of the PI but only for a short time and another heading correction is soon needed. Note that after a change in heading the PI no longer describes the true bearing between the marker and the hazard: the hazard is appears to rotate around the marker with each correction made, while in reality the boat slides slowly towards it.

Now if you correctly took the current into account with the first correction, as with the top row, then you would still pass safely by, but it would appear on screen that the marker was quite far from the index line. If you tried to keep the marker along the index line, you'd end up making a series of smaller corrections that could eventually result in drifting into the hazard.

Thanks.
A good job trying to figure out Van Isles point. On reflection my reply wasn’t very good. I usually try not to be negative. Sometimes I post something I later regret.

I’m not sure what Van Isle meant or why he believes you can’t PI while head up.

A picture is worth a thousand words. Unfortunately I haven’t figured out how to post pictures.
I
Good illustration of a PI on a North up stabilized display. As a general rule of thumb. I would recommend not ussing buoys to PI. They tend to move. Sometimes they move out of position. Any fixed object would be better.

I will try and explain how to PI on an Unstabalised Heads Up Radar display.
First point is everything is relative. Even though you will be steering a compas course. You should know your compas course.

Everything you observe or measure on the display is relative.
Most conventions.
Refer relative bearing to “Red or Green” some may use left or right. Port or starboard can be used. To avoid confusion often left to the vessel and visual.

If you turn the boat to port the head on the radar stays the same, the land and other targets appear to move in the opposite direction to starboard.

This will appear on the display as an increase in the relative bearing to starboard by an amount equal to the port alteration.

The green relative bearing will increase.

The problem. Your PI is no longer valid if it is a fixed line.

If you use a curser.

In the old days of yore when I was a lad the curser was mechanical.

Today my typical small boat Radar has an Electronic curser. It has options of ither a full screen or half screen of parallel lines which match up to the range rings.

I can initially set it parallel to my heading. As in your first heads up picture Diagram.

My desired safe distance 1 mile from the buoy (or beacon) to clear a rock 0.5 miles from the buoy.

I detected a set and drift towards the beacon using my PI. As per diagram No 2

Now it gets a bit more complex.
Obviously I need to alter courses to
A at least counter the set and drift accepting I will now be less than my desired distance of 1 mile no longer clearing the rock by 0.5 miles

If I had plotted the buoy I could figure out the actual set and drift. But I didn’t bother. So I am going to estimate the angle.
To make it easy 5 degrees.

So I alter to port 5 degrees.

My PI parallel to my heading marker will no longer work. My course has now changed but my heading marker on my display will stay the same.

I need to turn my curser 5 degrees green. Now I can watch the buoy appear to travel along parallel not to my heading but the curser.

If it does I have successfully countered the 5 degree set

Clearly I will now pass less than my desired distance from the buoy and closer to the rock but at least it’s not getting worse.

It would be better if I had altered 10 degrees to port to regain my desired course.

Again my heading stays the same on my display. The relative bearing of the buoy now has now increased by 10 degrees green. So I rotate my curser to 10 degrees green.

If I estimated my set correctly I should now be regaining my distance the buoy should appear to track back along my display getting closer to my PI curser line.

When I regain my desired distance. I can return to only 5 degrees port of my required course To pass the buoy at 1 mile.
When I do this I have to rotate my curser 5 degree red from 10 green or reduce to 5 green.

Now if my estimation was good the buoy will apearntomtravel along the curser to where it touches the range ring.

The curser is a tangent to my 1 mile range ring. My CPA with the buoy will be 1 mile.
I will have passed the rock at the desired distance.

Hope this helps.

After all most of us on small boats only have basic heads up unstabelised radars.

The curser is very helpful.