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Old 26-09-2011, 05:24   #1
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Radar and Chartplotter

So yesterday the fog rolled in and I turned on the radar and had the radar overlaid on the chart screen.

Now I can accept that bouys on the chart and not necassay were they really are and that the radar is probably showing were it really is. But thought it a little werid that a big rock that is basically a small island is not is the same place according to the radar and chartplotter.

Maybe that rock island has drifted out of place a little
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Old 26-09-2011, 06:00   #2
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Re: Radar and Chartplotter

When I add my AIS to the picture the big ships do the same thing as the rock.
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Old 26-09-2011, 06:39   #3
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Re: Radar and Chartplotter

It sounds like your chart plotter is not set to the same chart datum as your chart.

Fabbian
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Old 26-09-2011, 06:41   #4
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Re: Radar and Chartplotter

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Originally Posted by fgd3 View Post
It sounds like your chart plotter is not set to the same chart datum as your chart.

Fabbian
It is, there is only 1 datum setting for the combined system.
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Old 26-09-2011, 06:52   #5
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Re: Radar and Chartplotter

This goes back to many discussions which point out that nautical charts are notorious for being "off" in their "registration." This is the alignment of the features of the chart with the actual latitude/longitude system used by GPS's.
- - Back in the before-GPS days, the accuracy of Lat/long was not that good as it is today with the various GPS systems. So a little "offset" between the chart derived lat/long and the real lat/long was within the normal "margin of error" of navigation techniques.
- - Some electronic charting systems allow you to manually "re-register" a chart and bring its Lat/long's into alignment with the GPS lat/long's. But many, especially chart plotters don't let you do that. But the better integrated radar/chart display plotters do allow you to manually align the radar with the map. A nice feature.
- - So you just have to realize that radar is displaying the accurate distance and bearing to a geographical feature from your boat while a "charting system" frequently does not. Which it the reason I personally prefer to have separate displays for the radar and the e-chart. Sometimes the "offset" between radar and chart just makes things to complicated to deal with and/or distracting.
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Old 26-09-2011, 08:30   #6
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Re: Radar and Chartplotter

Yep. Seen this countless times. Things are where they seem to be ... on the plotter. The radar errors come from:
- radar antenna misalignment,
- radar heading sensor misaligned and/or not calibrated,
- slow network while the boat is changing course.

Much less of an issue if HQ equipment is used.

b.
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Old 26-09-2011, 12:54   #7
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Re: Radar and Chartplotter

Quote:
Originally Posted by osirissail View Post
This goes back to many discussions which point out that nautical charts are notorious for being "off" in their "registration." This is the alignment of the features of the chart with the actual latitude/longitude system used by GPS's.
- - Back in the before-GPS days, the accuracy of Lat/long was not that good as it is today with the various GPS systems. So a little "offset" between the chart derived lat/long and the real lat/long was within the normal "margin of error" of navigation techniques.
- - Some electronic charting systems allow you to manually "re-register" a chart and bring its Lat/long's into alignment with the GPS lat/long's. But many, especially chart plotters don't let you do that. But the better integrated radar/chart display plotters do allow you to manually align the radar with the map. A nice feature.
- - So you just have to realize that radar is displaying the accurate distance and bearing to a geographical feature from your boat while a "charting system" frequently does not. Which it the reason I personally prefer to have separate displays for the radar and the e-chart. Sometimes the "offset" between radar and chart just makes things to complicated to deal with and/or distracting.
Quote:
Originally Posted by barnakiel View Post
Yep. Seen this countless times. Things are where they seem to be ... on the plotter. The radar errors come from:
- radar antenna misalignment,
- radar heading sensor misaligned and/or not calibrated,
- slow network while the boat is changing course.

Much less of an issue if HQ equipment is used.

b.
Combine these two posts and you have a good starting place for what errors might be creeping into the mix...

-dan
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Old 04-10-2011, 21:25   #8
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Re: Radar and Chartplotter

I experienced the same problem when first using the radar/ chart overlay. My Northstar system has the ability to rotate the radar signal to align with a fixed chart plotter position, such as a fixed channel marker, rock or island.
Once aligned with a known mark, all other signals from the radar will be accurate.
I believe most combined radar/ chartplotter products would have the ability to align the return signal.

Peter
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Old 04-10-2011, 21:42   #9
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Re: Radar and Chartplotter

The NOAA charts for the New England coast are generally dead-on accurate. There are exceptions, but they are very rare, and usually take the form of an uncharted rock, or a reported imaginary rock. I am aware of only one place in Maine where the charted shoreline is off by more than a few feet. The alignment errors that you see are most likely due to radar calibration issues, or possibly propagation issues (maybe multipath?)

How big was the discrepancy that you saw?
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Old 10-10-2011, 06:52   #10
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Re: Radar and Chartplotter

Don,

Barnakiel and dacust/Dan are right. My thought is the very first thing to check is your radome alignment. There's a high probability that the radome mount is not perfectly aligned with the fore/aft axis of your boat. Heck, even your fluxgate compass and possibly even your main magnetic compass aren't either.

If virtually every radar return on your integrated MFD is a one or two degrees port or starboard of where your MFD plots the landmark/navaid, then you can be almost certain that the alignment is off. On our Raymarine E-120 Classic, it's simple to go into the radar configuration and adjust the 0-degree bearing mark. Our radome, mounted up the mizzen mast, is off to starboard by about 1 degree, so I adjusted it accordingly via the configuration menu.

One thing I've found in the few years we've been using radar is that having the radar overlain on the chart is a nice feature to have, but as osirissail basically puts it, to really get the most out of your radar, you have display it without the chart background so that you can really pick out the details.

On our E-120, when overlaid on the charting page, all returns are the same magenta color with no details. On the dedicated radar page, the returns are color-coded much like the weather radar you see on TV. You can actually see where the rain in that squall really is - and the big freighter in it - rather than seeing just a magenta blob on the chart. But then again, you'd be amazed at the amount of detail you can pick out on an older analog CRT-based radar system. Bottom line is that many "older" vintage multifunction displays like our 2006 E-120 simply don't have the processing power nor the graphics memory and internal bus bandwidth to be able to plot a detailed chart and overlay it with detailed radar imagery in real-time.
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Old 10-10-2011, 08:06   #11
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Re: Radar and Chartplotter

I agree with the above. In the design of our "electronic nav station", I have concluded that a MFD is a mistake capable of deceiving the operator should problems like alignment not be considered. So, apart from fixing that permanently, I believe a stand-alone RADAR display has merit to "focus the mind" on what *is* over what is *represented*.

Would I like a RADAR overlay on the plotter? Hell, yes. It's a handy thing to have, but it should be left on the plotter, because I want to refer to it for bearings and nav aids and not actually look at it constantly as I see on some boats. At night, in fog, approaching a coast or harbour, I want RADAR and I want AIS. The plotter I consult if we are within 1/2 mile of something solid. The rest of the time, it's Eyeball Mark 1 for stuff in the water that the RADAR can't pick out.

Frankly, offshore, the plotter is useless if you have a compass or the most simple form of handheld GPS to note XTE and COG. Not so the RADAR and AIS. That's handy to have for weather reasons and because some enormous ships are surprisingly stealthy!
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