Relative reference direction

[Kitiara]

Hello, is there an option to change the reference direction to relative instead true motion ?

[nohal]

What exactly do you mean?

[Kitiara]

Motion of targets relative to the motion of the observing ship.

[nohal]

That is taken into account in the CPA calculations, what exactly would you like to see?

[Kitiara]

An option to change default motion mode and vector time, just like how you do in radars. Details explained below;

In RM, Relative Motion, own ship is displayed at a fixed point and all target trails are shown relative to own ship's movement. This means that stationary targets will have trails if own ship is moving.

In TM, True Motion, stationary targets don't produce any trails as own ship moves.

Especially In RM, vectors are shown to indicate the velocity(speed and direction) of own ship and moving targets. The length of the vector indicates speed and it's bearing indicates direction.

About the vector time; There must be a vector time selection box. The 'vector time' selected will determine the length of the vectors shown. The length of a vector represents the distance the ship/target will travel in the 'vector time'. The vector time should be adjustable between 1 and 60 minutes.

[nohal]

Yes, I understand and use this on a radar, but how would this work exactly while being plotted on a chart?

[Kitiara]

The same interactive way as the radar. It will use the information provided by AIS. Please see the video below which demonstrates what I'm talking about.

https://youtu.be/zESi-W1xrXg?si=yB4wUujd4A6I-mL5&t=707

[Hakansv]

@Kitiara
How would we show this in another way? I think the CPA for both is clear.
Course and speed are used for this calculation.

[Kitiara]

There is no other vessel in the video unfortunately.

About the image; As far as i understand the green lines are in true vector form. Is there a way to make OpenCPN to draw them relative to own ship's movement ? Also i don't see any option to set the vector time.

[nohal]

Toolbox -> Ships -> AIS targets for the COG/SOG predictor length. And again, why would drawing the AIS target predictors relative would be any useful in a picture where everything is true, all the true accurate data, unlike with radar, is known and you have the (T)CPA perfectly showing the situation including the point of encounter?

[nohal]

And question two: How do you draw relative trail on a chart, which is true, in a way that is not confusing or outright dangerous?

[Kitiara]

Displaying relative vectors in RM mode provides a dynamic representation of how the ship's motion affects nearby targets, as you already know. It enhances situational awareness by showing how target trails change based on own ship's movement, offering a different perspective compared to fixed true vectors. That's the only reason, having a different perspective.

Relative trails in RM mode doesn't alter the accuracy of the chart data. About displaying, i think it's better if i show you a working example. I'll let you know if i find one.

[nohal]

Of course it does not alter the chart data, what I want to see, as you seem to already understand, is how that display showing two completely incompatible things that are impossible to show right at the same time does it in a way that is not confusing.

[Hakansv]

But this leads me to a question - Is the relative mode available in the radar plugin?

Yes. You can switch between relative and true targets trails in radar_pi. Sometimes valuable in dense fog to see if you catch up a target in front of you. (Can be more reliable than a calculated (guessed) (M)ARPA projection?)
But I don't understand that feature's connection to OCPN chart plotter if we're talking about AIS targets. Here all AIS target's speed and course are know so we can draw actual trails, based on real positions, and speed/course time prediction vector based on data from the target's own GNSS sensor compared to own prediction vector also based on real positions? When also using our CPA calculation everything is as clear as possible?

[Kitiara]

I didn't know that the radar_pi has a relative option. In that case, radar_pi alone can be sufficient however it doesn't generate any echo on the chart right ? Unlike gxRadar ?

[Hakansv]

Of course radar_pi can show radar overlay.
More here: https://opencpn-manuals.github.io/main/radar/0.1/index.html