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Author Topic:   RADAR Overlays
David Pendleton posted 08-26-2009 01:49 AM ET (US)   Profile for David Pendleton   Send Email to David Pendleton  
My Raymarine display supports chartplotter/RADAR overlay. I don't use it as objects on a chart rarely have a corresponding RADAR image.
Peter posted 08-26-2009 09:32 AM ET (US)     Profile for Peter  Send Email to Peter     
"My Raymarine display supports chartplotter/RADAR overlay. I don't use it as objects on a chart rarely have a corresponding RADAR image."

Is this so only for Raymarine? Just as a comparable datapoint, I installed a Garmin 4208 with the 18 inch Garmin HD radome this season on my Whaler 27 to replace an aging stand alone Apelco (a former Raytheon brand) CRT radar. I went with Garmin based on past experience with their chart plotters and their easy to use user interface. My experience is that the chart plotter/RADAR overlay function shows radar returns for day markers, lighted buoys, nuns, lighthouses (all configured to have radar reflectors) and coastline features including wharfs, for example, all of which show up on the chart, obviously. It will even show a radar overlay for channel markers which don't have a radar reflector incorporated. In other words, anything that produces a radar return in the straight radar plotting mode shows up on the chart in the overlay mode. The only difference between the OVERLAY mode and the straight RADAR mode is that the color is monochromatic (default is orange) in the overlay mode whereas in the RADAR display, the returns can be multicolor (typically a boat moving will produce a yellow and blue with the blue trailing return to show what looks like a boat wake) versus a stationary object producing only a yellow return). It will even present a RADAR overlay on the 3D chart display. I can't imagine that a Raymarine display would not do the same.

David Pendleton posted 08-26-2009 10:14 AM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Peter, this is true of any RADAR.

My comment was mainly in reference to features of the coastline where the RADAR image will rarely look exactly as it does on a chart.

SJUAE posted 08-26-2009 11:00 AM ET (US)     Profile for SJUAE    

My comment was mainly in reference to features of the coastline where the RADAR image will rarely look exactly as it does on a chart

Very true, your more likely to get some relections back off buildings than the coastline.

Also to get Radar/Chart overlay you need to have a gyro compass

The overlays are less important if like you I presume you have seperat units to display chart/sonar/radar

However on boats with single combo display it becomes more relevant.


Peter posted 08-26-2009 12:59 PM ET (US)     Profile for Peter  Send Email to Peter     
David -- Thanks for the clarification. I agree that how the coastline gets represented in the overlay depends on the makeup of the coastline. If the coastline is made up of hard structures like rock cliffs which return strong radar returns which line up fairly well with the chart image of the coastline. If the coastline is softer, such as gently rising beach slope with trees in the background, it won't line up as well.

The overlay is nice in that it takes some of the guesswork out of what the RADAR returns are on the RADAR screen when you are not out in open water. The downside of the overlay is that there is a lot of information to look at and the contrast is not as good the dedicated RADAR screen so as to allow picking out small non-permanent targets. Think analogously like looking for stars in a twilight background versus pitch dark backround.

jimh posted 08-26-2009 01:42 PM ET (US)     Profile for jimh  Send Email to jimh     
[Separated from another discussion.]
Peter posted 08-27-2009 08:26 AM ET (US)     Profile for Peter  Send Email to Peter     
Steve -- I don't believe a gyro compass is needed for radar overlay. I don't believe my Garmin 4208 has a gyro compass built-in and I certainly don't have one otherwise yet the radar returns overlay onto the chart in near perfect correspondence with the structure on the chart. Even when the chart twitches somewhat on the screen because the boat is moving slow and rocking (my GPS antenna is mounted too high), the radar overlays move with the chart.

I find that I switch back and forth between overlay and straight radar imaging. Also, the Garmin 4208, like most other multi-function displays, can do a two way split screen showing radar and pure navigation chart, for example. Lot's of flexibility and fun to play with.

SJUAE posted 08-27-2009 08:07 PM ET (US)     Profile for SJUAE    

You are correct not having gyro compass does not stop you overlaying the two, however it's accuracy/validity varies on conditions and speed.

As you have found, if you were turning on your own axis the chart position (from the gps) would likely to remain stationary but the radar picture would rotate with the boat so you could be up to 180 degrees out or as the GPS position fluctuates within it's own accuracy, this can add to misalignment on the overlay.

Interesting that you note that you don't normally see a great misalignment.

FYI from Garmin:

When interfacing this three-axis compass heading sensor to Garmin’s marine lineup of MFDs and radars, mariners can expect unparalleled radar overlay performance and accuracy along with excellent MARPA capabilities. This heading sensor (based on Airmar H2183) uses a rate gyro, so it can maintain an exceptional 2° heading accuracy under heavy dynamic conditions, with up to 30° of pitch and roll

Garmin's one

Maretron's one

As you say lots to play with:)


Peter posted 08-28-2009 12:41 AM ET (US)     Profile for Peter  Send Email to Peter     
Without the compass heading sensor, I don't have MARPA. The compass heading sensor would probably help damp out the image rotation at slow speeds when rocking. I'm not ready to drop $900 on one. Don't need it for what I do with the 4208.
David Pendleton posted 08-28-2009 01:39 AM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
For my Raymarine RL70CRC+, you don't need a heading sensor for RADAR/chart overlay.

You do need a heading sensor for MARPA, however (when using an RL70CRC+).

20dauntless posted 08-31-2009 05:01 PM ET (US)     Profile for 20dauntless    
I have a Raymarine C80 with an RD218 radome antenna. I normally have use the split screen mode. The left side of the screen has the charts at 6 miles and the right side has them at 1 mile. The radar is almost always running (so I'm familiar with it when I need it) on 3 mile sync and it overlays on both charts. If I'm in really limited visibility I sometimes toggle between my normal screen and just the radar screen where the contrast is greater.

Regarding the heading sensor, I don't know if the C80 requires one or not. I have a KVH Azimuth 1000 fluxgate compass that feeds info to the C80 via NMEA 0183. This works well and was much cheaper than the Raymarine heading sensor.

swist posted 08-31-2009 07:20 PM ET (US)     Profile for swist  Send Email to swist     
Educate me here please. I used radar a lot in my earlier boating days, but now on a Montauk I only use chartplotting GPS. So I've never used them together.

Theoretically, should not the radar return line up (within the GPS accuracy, normally very good) with the fixed items on the chart (shoreline, buoys, etc)?

But practically speaking, you are getting redundant data - how does the display technology resolve this?

pglein posted 08-31-2009 07:38 PM ET (US)     Profile for pglein  Send Email to pglein     
I use Raymarine's radar overlay feature extensively on my trawler. I find that it works pretty darn good 99% of the time. The only time they radar and charted shoreline don't line up prety good is when the compass has become mis-calibarated. To recalibrate, I simply do a Swedish circle and voila', it's all lined up. This only happens about once a year or so.

When I say they line up "pretty good" I mean that they are close enough to one another that the difference is irreleveant given the distances. The closer I zoom in, the more accurate the radar becomes. I figure the radar is probably more accurate as to the location of the shoreline, but only at high tide. At low tide, obviously, the shoreline is closer, and that isn't necessarily picked up by the radar. Truthfully, the radar isn't for finding shoreline; it's for finding other boats. What I like about the radar overlay is that I have a better feel for the distribution of boats and space. For example, if I'm in a relatively narrow passage, I can have a better feel for whether or not I'm crowding another vessel in the passage, or vice-versa. Although I have dual displays at the helm, I prefer to have one displaying the fishinder. So, the radar overlay allows me to see both the radar and chartplotter at the same time. Most importantly, I can quickly get a three-dimensional feel for my surroundings with a simple glance; even if visibility out the windows is zero. This allows me to focus my attention on the horizon, rather than spending more time staring at the instrumentation, reconciling seperate displays to build the image in my head.

Also, without the radar and gps linked, you can't get accurate, reliable SOG, COG, and TTCPA feedback out of the ARPA functions, which I find very helpful on those rainy northwest nights when you can barely see past the anchor roller.

20dauntless posted 08-31-2009 09:56 PM ET (US)     Profile for 20dauntless    

Yes, some information is redundant. Buoys, shoreline, etc show up on top of chart things, but you can still see what's underneath the overlay.

One thing I didn't mention before is the value of MARPA. I often transit fairly crowded areas (Guemes Channel and Rosario Strait to Thatcher Pass) in the dark or fog. I love being able to select a radar target and have the computer calculate bearing, speed and CPA.

SJUAE posted 09-01-2009 05:28 PM ET (US)     Profile for SJUAE    

Let's assume the scenario that you are using your radar for very poor visibility.

Navigating purely by GPS/chart plotter is a higher risk as

1) Charts are never 100% accurate
2) Buoys move
3) GPS knows where your were, where you are but not where you are heading
4)GPS has inaccuracies, these seem smoothed out when travelling at speed as the distance travelled is often further than the inaccuracy factor, when looking at the scale shown on your plotter, but not always so at very slow speeds.
5) Chart data does not show real time obstructions e.g. other boats

By having radar with overlay on you are minimising the risk and getting visual real time confirmation on markers that confirm with the chart plotter and other obstructions

By having a heading sensor/flux gate/gyro compass you are minimising the possible miss alignment of the radar and chart plotter when travelling at slow speeds due to the GPS not being able to always accurately determine heading.

How important it is too minimise these risk/inaccuracies depends on the individual’s circumstances/requirements


handn posted 09-08-2009 09:27 AM ET (US)     Profile for handn  Send Email to handn     
The radar overlay works great on both my 1934 and 1734 chartplotters.
Where we boat (in Mexico) the charts are as much as two miles off because in remote locations the latest cartography is British Admiralty 19th century.
What is especially useful is that when radar overlay is used, the chartplotter automatically corrects the chart to the actual location shown by the radar. Thus the overlay is useful in even bright sunlight and abosolutely essential at night or in fog.
The local topography helps because much of it is mountains and rock formations which make perfect radar targets.

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