radar overlay

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radar overlay

Postby dpiano on Sat Feb 09, 2019 11:56 am

Is there a way to adjust the radar overlay image on our 2013 R-29 Garmin so it agrees with the GPS image? Perhaps it’s normal, but ours always shows detected images (buoys, boats land) somewhat left of where they really are.
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Re: radar overlay

Postby bob daily on Sat Feb 09, 2019 12:35 pm

Radar overlay and chart data alignment can be found on the internet. I can't check out the unit on my boat to see if it's in the menu due to about a foot of snow
Just remember GPS data indicates the direction the boat is moving, not the way it's pointing, that said in bad visibility I trust the radar to let me know where the shore line is as well as other objects.
Good luck Bob
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Re: radar overlay

Postby Red Raven on Sat Feb 09, 2019 1:07 pm

dpiano,

Run the Sea Trial Wizard found in the auto pilot controller menu. You have to get into dealer mode to get to it which requires a special key sequence I can’t remember at the moment. You can find it in your manual or on the Internet. This wizard will align your electronic compass (part of the CCU, the round black ball mounted somewhere in your boat) with the boat. You will need to take the boat out on a clam day to run it.

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Re: radar overlay

Postby BradOwens on Sat Feb 09, 2019 1:47 pm

In the radar settings menu there is an adjustment for radar offset and I think there is one for adjusting image rotation. That may work for you.
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Re: radar overlay

Postby JamesTXSD on Sat Feb 09, 2019 8:07 pm

Before adjusting any offset on the radar, I would make sure that the amount of offset is ALWAYS the same. The issue isn't likely your radar, but rather the data on the mapping being fed through the GPS. If you have the radar on in clear weather, you can see what the radar sees... is it exactly the same as what's on your chartplotter? Likely, not.

I have seen people complain that their "GPS isn't working, because it shows us on land in a narrow channel." More likely, the GPS is working just fine, but the mapping data in that area isn't perfect. Many places haven't been resurveyed in decades.

The radar overlay is great to be able to tell if there is an obstruction ahead, or if that is a nav aid. Something HUGE on the water or the shoreline. You need to be able to trust your navigation equipment, but it is very important to understand the limitations of the mapping. Run the radar as you pass a nav marker: does it show up at the proper distance to starboard, then directly abeam, then quartering behind you? Turn around and make a pass the other direction and see if it is the same on port. If so, the radar is accurate. Run that same path with the GPS/chartplotter and see if it is accurate. Of course, to be accurate, you'd have to do that every place you cruise.

I have not run into a radar situation where there was an issue with an offset, but have frequently seen inaccuracies with the mapping. Nature of the beast. For me, being able to see the radar return in the approximate area where a marker is on the chart lets me know that it is that marker and not some other obstruction, like a boat. In low visibility situations, I am not going to be running so close to anything that shows up on radar that the discrepancy between the radar and the charting would make a difference.

Running two separate chartplotters, I have also seen different mapping representation in a given area. Most often, the mapping information will be "good enough" to guide you through an area... but when the visibility goes down, I want to know that I can trust the radar.
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Re: radar overlay

Postby knotflying on Sun Feb 10, 2019 8:59 am

Keep in mind that the charts on the chartplotter may not be as accurate as reality. You will notice that in several instances the placement of your vessel on the chartplotter as compared to where a channel marker is can be significantly off. This is possible either through charting error or GPS error. What you see outside or on your radar is more reliable than how your vessel is indicated in relationship to the chart. Hence, when vision is limited and you are relying on your radar, always trust your radar targets and how they relate to your vessel. Personally, I prefer to use a split screen when in actual conditions and avoid confusion with other clutter on the pure radar screen. It is important to make sure that your radar offset has been adjusted properly in order to have accurate target location as it relates to your vessel. When things are calm go to radar and pick a large vessel and proceed towards it. Place your bow pointed to the center of the vessel and see that it also shows the vessel target in the same position on your radar screen. If they are not aligned make your adjustment accordingly. Once set you are good to go in limited visual conditions always rely on the radar for target avoidance and channel markers, not the chartplotter.
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Re: radar overlay

Postby BB marine on Sun Feb 10, 2019 9:45 am

If you have autopilot onboard you may need to do Sea trial wizard. The information of boat heading is coming from the GHP Reactor. The GPS location is correct but boats heading is not. If the auto pilot is not properly set up the effects that you are experiencing may be the related. Easy test while cruising look at your compass heading compare it to your autopilot heading. If they are different then this is your issue. Run a Sea Trial wizard and it should correct the issue. Has stated the Radar is right If you are seeing 10 degrees difference from compass reading and auto pilot it will change the overlay to not align with the chart plotter. I had this issue for two years. I posted a similar question to this on TugNuts this is the advise I was given and it worked.
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Re: radar overlay

Postby dpiano on Sun Feb 10, 2019 12:28 pm

Thanks everyone for your input. I am sure the issue is not mapping, since the difference I see between radar and GPS is consistent no matter where I am. On my last run I went down the Columbia, and back up the next day. The nav markers showed on radar off to one side of the GPS image going down, and then an equal amount on the OTHER side of the GPS image coming back up. So I assume to correct it, I should run the sea trial wizard? However the GPS heading and position themselves always seem quite accurate, except when there is a mapping issue.

When I ran down the Columbia the other day it was in a blinding snow shower, almost dark. Had the radar on, and it showed a tug coming right up the channel toward me long before I could see it. To me, the main thing (other than not running dumb conditions like that in the first place) is to trust the radar and GPS within their own limits. If there is something big out there, it may not be exactly where the screen says, but by golly it's THERE somewhere, and beware.
I also like the fact that, since rear vision is not good on these boats, the radar is a great rear view mirror.
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Re: radar overlay

Postby Red Raven on Sun Feb 10, 2019 1:28 pm

Before you run the Sea Trial Wizard check to see if there is anything metal or magnetic that has been placed near the CCU (black plastic ball). We carry folding electric bikes on board in the cave and they mess up the CCU compass. The symptom is exactly as you state. If the metal or magnetic item is fixed and will never be moved the Sea Trial Wizard will likely calibrate it out. If the item moves it will mess up the heading each time it moves. I wouldn’t change the radar settings until you have verified the CCU compass heading is correct by completing the Sea Trial Wizard.

I moved the CCU on our boat to resolve (or at least improve) the issue and still allow the bike storage there.

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