Plotting Questions; Radar ranges; magnetic compass bearings; positions on Open CPN...

[wingssail]

Raymond Pincott, I'm sorry, I just got lost on this compass side track and I never understood the difficulty of your original question, it seems so obvious to me.

You want to use a bearing and radar range of an object while looking at a GPS position on an OpenCPN display to verify the GPS position. Right?

From my view you don't need a plug-in or any complicated technique.

If you can see or have a range (any kind of range) and bearing of anything on the chart (visual or otherwise):

1. right click anywhere on the chart on OpenCPN
2. Left click "Measure"
3. Place your cursor on the chosen object on the OpenCPN chart
4. Left click
5. Move the cursor (to extend the range and bearing) to agree with the radar range and bearing. Note you will use the reciprocal of the bearing.
6. When the measure display box (on the cursor) shows the range and bearing you right click and then left click on "drop mark"

At his point you will see if the mark you dropped agrees with the ship's position which was determined by the GPS.

Have I missed something?

[RaymondR]

Originally Posted by Raymond Pincott

How to put a position on the chart using ranges and bearings to confirm satelite position is correct.

I think you may have a problem with that unless it's a fairly large error and you have some fixed marks on a chart and revert to cross bearing coastal navigation methods.

[Raymond Pincott]

Good news i finally have the answer to my original question. "How to put a position on the chart using ranges and bearings to confirm satellite position are correct." in OpenCPN. Also if the vessels positioning system has failed or is unreliable how to use ranges and bearings to put positions on the electronic chart.
The method to to add on Ocpn_Draw 1.6 to OpenCPN. Read and follow the instructions. The method of getting the EBL's (Electronic Bearing Lines) and VRM's (Variable Range Markers) is a bit clunky and not really user friendly but with a bit of perseverance and repetition it works. A position can be placed on the electronic chart using two or three ranges or two or three bearings (radar or visual) or a mixture of ranges and bearings.
It would be great asset to OpenCPN to have the system of using ranges and bearings made a bit more friendly to use.
Thank you for all you help.
Raymond

[Dockhead]

Originally Posted by requiem

Does a fluxgate count as stabilized? (I wouldn't think so, even if they tend to be far better than the traditional magnetic ones at the binnacle.) Satellite compasses still seem to be a rare and fancy bit of kit.

Some fluxgate compasses are stabilized. E.g. the Airmar H2183. Most are not.

[Dockhead]

Originally Posted by Raymond Pincott

Good news i finally have the answer to my original question. "How to put a position on the chart using ranges and bearings to confirm satellite position are correct." in OpenCPN. Also if the vessels positioning system has failed or is unreliable how to use ranges and bearings to put positions on the electronic chart.
The method to to add on Ocpn_Draw 1.6 to OpenCPN. Read and follow the instructions. The method of getting the EBL's (Electronic Bearing Lines) and VRM's (Variable Range Markers) is a bit clunky and not really user friendly but with a bit of perseverance and repetition it works. A position can be placed on the electronic chart using two or three ranges or two or three bearings (radar or visual) or a mixture of ranges and bearings.
It would be great asset to OpenCPN to have the system of using ranges and bearings made a bit more friendly to use.
Thank you for all you help.
Raymond

Why would you go to all that trouble?

Just put the cursor on the landmark and you get instantaneous range and bearing. No need to draw anything at all.

The OpenCPN radar plugin has EBL/VRM built in. But it also has radar overlay -- that's much better than messing with the EBL/VRM.

In my opinion, OpenCPN doesn't need anything more than this.

[Dockhead]

Originally Posted by RaymondR

I think you may have a problem with that unless it's a fairly large error and you have some fixed marks on a chart and revert to cross bearing coastal navigation methods.

That's what he's proposing to do.


But you don't need a complete three point fix -- you can verify range and bearing to one good charted landmark and that's already pretty good.


Much better however is to use the radar overlay -- then the radar image and chart are both visible and you instantly see any problem.


My own approach to this is to run the radar all the time and leave the radar overlay on. Then I'm continuously verifying position -- and also the accuracy of the chart -- with the radar.

[bcn]

Originally Posted by Dockhead

My own approach to this is to run the radar all the time and leave the radar overlay on. Then I'm continuously verifying position -- and also the accuracy of the chart -- with the radar.

The split screen is quite handy for that.

[Raymond Pincott]

Why have the knowledge and ability to be able to put a position on an electronic chart?
I have had the vessels positioning system (GPS) for a period of time change hemispheres putting my vessel from the Southern hemisphere to the Northern hemisphere as you appreciate the Radar overlay was of no use to me when that happened. I knew I was in the Southern hemisphere and where I was on the earths surface and I need to put my position on the electronic chart.

[Dockhead]

Originally Posted by Raymond Pincott

Why have the knowledge and ability to be able to put a position on an electronic chart?
I have had the vessels positioning system (GPS) for a period of time change hemispheres putting my vessel from the Southern hemisphere to the Northern hemisphere as you appreciate the Radar overlay was of no use to me when that happened. I knew I was in the Southern hemisphere and where I was on the earths surface and I need to put my position on the electronic chart.

Of course everyone should have the ability to plot a position on a chart, and figure out that position in multiple ways in the absence of electronics. No disagreement there at all.


But radar overlay instantly tells you if there is something wrong with the electronic position. It solves without any effort, without any drawing, or anything, that problem which you stated in the OP.


If the radar overlay warns you that there is a problem, then you go on to other methods.


In our age people freak out at the idea of losing electronic position data. We have become extremely dependent on GPS.



I was sailing for a lot of years (dating myself) before I had my first GPS in the early 90's. Those of us who grew up with that are more relaxed. We used LORAN, RDF, whatever (I threw away an old LORAN set not that long ago), but even with nothing at all, we got back home.

[bdbcat]

I concur with Dockhead.
In the old days, before AIS, radar was (aside from eyeball) the primary collision avoidance tool. ARPA was, and still is, magic.
These days, radar overlay is for me mainly a "confirmation of situational awareness." If the overlay looks wrong, time to stop, look out the window, and figure it out.


IMHO
Dave

[requiem]

By coincidence I was also following a discussion on gCaptain (professional mariners' forum) about the UKHO moving away from paper and it was interesting to see how the attitudes differ.

There it's well recognized that GPS input and the charting system are two separate things, or to paraphrase one poster, "there's nothing you can do on a paper chart that you can't do with an electronic chart", whereas on the recreational side people can't seem to separate the concept of GPS from electronic charts.

As part of my "old man yells at clouds" routine, I'll repeat my belief that, part from the safety implications, omitting proper nav functions from recreational systems will contribute to a further loss of navigational skills.

[Dockhead]

Originally Posted by requiem

By coincidence I was also following a discussion on gCaptain (professional mariners' forum) about the UKHO moving away from paper and it was interesting to see how the attitudes differ.

There it's well recognized that GPS input and the charting system are two separate things, or to paraphrase one poster, "there's nothing you can do on a paper chart that you can't do with an electronic chart", whereas on the recreational side people can't seem to separate the concept of GPS and electronic charts.

As part of my "old man yells at clouds" routine, I'll repeat my belief that, part from the safety implications, omitting proper nav functions from recreational systems will contribute to a further loss of navigational skills.

I agree.

I have probably 50kg of paper charts on my boat, but I rarely use them any more and should probably give them away. My paper chart substitute is OpenCPN and, wherever possible, raster charts. On a large 4k screen. It's not quite as good as real paper, but the storage, updating, and handling of paper charts is just a crazy work load. The electronic charts are the way to go.

It's just a bonus that you can also get your position (and other data) onto that chart. Losing that would not be a big problem.

[merrimac]

Originally Posted by Dockhead

However -- the compass antennae are a good 6 meters away from the radar antenna, and this is a problem -- this gives a parallax error. Slight, but if you're trying to produce bearings of better than 1 degree accuracy, it makes a difference. There is no way in my system to compensate for this, so what I've done is align for targets which are far away and directly ahead where there is no parallax, and take into consideration that there will be a slight error with targets which are closer and more to beam.

Ahem, there is a GPS offset that you can enter in the radar_pi plugin? See the attached screenshot of the Advanced > Installation menu. You can thank Douwe who was concerned about this as well and so added it to the plugin...

My Simrad NSX also does this, by the way.

[st599]

Originally Posted by wholybee

Just use the "create route" function to draw lines on the chart exactly as you would on a paper chart.

In the case of a range and bearing from a radar, start the route at the landmark, and draw it in the inverse direction from the bearing, at the correct range. If you have bearings of several landmarks, then do the same, but from the multiple landmarks. Using similar techniques you can also do running fixes as well.

It is worth noting, "paper charts" are not being discontinued. "Raster" charts are being discontinued. You can still print paper charts from vector charts, and as long as people want to buy paper charts, the chart printer on demand companies will print them from vectors. Or, you can print them yourself.

I think the OP is running in to the same problem that we're having in the UK. Whilst you can get charts printed, they don't count for "coded" vessels (small commercial/sailing schools etc.). They need to be printed by UKHO, Imray or similar chart agencies OR they need to have an electronic navigation system on to which compass fixes, distance off, radar distances etc. can be plotted easily and that is then used by the software as a position for further calculations. I.e. you need to be able to plot a 3 point fix on screen, click on the location and assign this as the boat's position at such and such a time.


Most plotters don't do this. There are mini-Ecdis systems that cost thousands that do, but most can't afford the upgrade.

[El Pinguino]

I'm afraid anyone with a typical 'consumer grade' radar who thinks their radar will give them sub one degree bearing accuracy is kidding themselves.
This is from Furuno
'Azimuth Resolution
Suppose that two separate objects are in the same direction, at the same distance, from the boat from which you are looking. The resolution of a radar depends mainly on the horizontal beam width, so the narrower the beam width of an antenna, the higher the resolution of the radar. To increase the resolution of a Radar, we need to reduce the horizontal beam width, which can be accomplished with a larger antenna. There are X-band radars with antennas up to 3 meters wide, and in this case the angular beam width is 0.75 degrees, which is very important for accuracy. On the other hand, Radomes, with an antenna width of about 40cm, have an average horizontal beam width of 5.7°.
https://www.furuno.com/en/technology/radar/basic/

Combine this with the nature of most things you may choose to take a bearing from - with the exception of beacons - and radar bearings are a sub par way of fixing your position.
This is from Panbo- a Navico Halo 24 display.

https://panbo.com/wp-content/uploads...ry-768x461.jpg

https://panbo.com/navico-halo24-raym...doppler-radar/