Unfamiliar landfall at night

[bobwebster]

Originally Posted by sailaway21

If I'm reading your display correctly, and there is some doubt in my mind as to that, the error is on the order of minutes not seconds, ie.. miles off. Are there more islands off screen that I'm not seeing and what is the nature of the topography of the islands. I'm having trouble resolving the two images I'm seeing as being the same-but then, I guess that's the whole point, eh? (g)

In the photo the minutes are a little hard to read, but it's set to dd°mm'.mmm. You can see (upper right) that the distance from the vessel to the waypoint is .933 nm, confirming the image scale.

The islands on the screen are the only ones there, but you're right. The shape on radar does not precisely agree with the map. However, the shape of the islands in Rains' Mexico guide conforms more to the radar than those of the charts. It was hard to tell the shape of the islands visually, but the angles we could see seemed to match the radar.

[sailaway21]

It's difficult to take an image of essentially a radar snapshot and make much out of it. Much depends on the topography of the island, the state of the tide, and weather conditions.

This might be another example of a reason to not overlay radar returns over a chartplotter display as well. It creates a needless confusion. It's also interesting that most people, if the radar picture is less than perfect are reluctant to rely on it-which is probably wise-but will willingly rely on the GPS picture due to it's precise clarity. Just the opposite should be the case.

[billyruffn]

a couple of posts above bobwebster wrote:

"The islands on the screen are the only ones there, but you're right. The shape on radar does not precisely agree with the map. "

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The "shape" of an island on a chart will hardly ever match the shape of the island on radar because the chart is a vertical depiction and the radar is much more of a line-of-sight or horizontal view of the geography. When intrepreting a radar return you have to remember that it's reflected energy you're seeing. Energy goes out and reflects back only from solid surfaces in the direct path of the radar beam. If an island has a higher shoreline (hills or mountains) on the side from which you're approaching that's probably all you will see on the radar -- the shoreline. You can't "see" on radar any of the land mass that's 'behind' the hill, just as you can't see it with your eyes.
It is true that the general shape of a shoreline shown on radar will match fairly closely the shape of the near shore on the chart (i.e. where land meets water), but don't expect the overall shapes of charted land masses to look similar to their radar returns.

The fact that the returns shown on Bob's chart plotter (copied above) look like several islands rather than two is most likely due to the contours of the two islands. High elevations on the near shore with an even higher elevation on the other side of the island with a valley in between will appear on the radar screen as two distinct returns. Looking at it with an eyeball from a distance it will appear as one land mass, while on radar it appears as two -- because at a distance you eyes can't discern the difference in ranges of the two high elevations while the radar can.

That said, the returns shown on Bob's chart plotter do look a little strange. It' probably has to do with the way the signal is refined and presented in the display. I have no experience with these "combined" displays and like one poster above, I'm not a fan of this integration of technology. When intrepreting radar I much prefer to use a tuneable CRT screen designed specifically for displaying radar returns. I also find that the land contours shown on many old fashion paper charts are very helpful in intrepreting what the radar's showing you.

[JiffyLube]

I noticed one day while watching a submarine passing us within 1/2 miles with my eyes, and on our plotter/radar with overlay, that the sub was much more visible on radar only than on overlay...in fact I could hardly make out the sub on overlay.

[hellosailor]

I guess this doesn't tell us where the error really is coming from, just (as usual) that error will be present in any navigation.

"The datum (WGS84 vs. NAD27, for example) usually only makes a couple hundred feet difference, and the electronic chart "informs" the chart plotter of its datum." I've personally been in well-surveyed areas where large items (anampitheatre) were over 1/2 mile out of position in NAD27, where they were last surveyed on a USGS topo, versus their actual position under GPS. The difference depends on many things, among them the location and the quality of the old survey--some of the old baselines just weren't good but pre-GPS simply accepted as 'good enough' for many years.

"Also, the land was where the GPS claimed it was before and after visiting the Marietas....If you compare the charts (electronic and otherwise) with Rains's Mexico Guide, you can see that there is a difference in the lat/lon between the two." So, the GPS and radar and guide book agree, but the marine charts (from what source?) were off?

That wouldn't surprise me either. I've seen most of the commercial private mapping (Navteq, etc.) out by 1/3 to 1/4 mile in too many places. Daylight, depth sounders, local knowledge, and the occassional slam tack and about aren't dead yet.<G>

[Plumper]

It looks to me like there may a bearing error in the radar. Slew the radar picture counter clockwise by 45 degrees and things match up more closely. The compass feeding the radar should be compared to the Chart overlay before relying on it.

[bobwebster]

Originally Posted by hellosailor

So, the GPS and radar and guide book agree, but the marine charts (from what source?) were off?

The paper chart was an Admiralty chart, incorrect. The electronic chart was Navionics, incorrect. I wrote Navionics and they said they'd check on it and asked where I found the correct data. Charlie's Chart guide is incorrect, but I think they mentioned that charts in the area are off. The radar bearing is correct. Rain's guide is correct, agrees with the radar, and mentions that charts in the area are slightly fubarred. I suspect someone towed the island a mile or two just to keep wayward sailors on their toes.

[hellosailor]

Bob-
"Admiralty charts to be transferred to WGS 84 datum, says U.K. Hydro Office
Sea Technology, Mar 2000
E-mail Print Link The United Kingdom Hydrographic Office (UKHO) will start a three-year programme this month to transfer 310 Admiralty charts of Great Britain to a horizontal datum compatible with WGS 84 Datum.

The transfer of the remaining charts of the British Isles will follow. Currently, Admiralty charts of the British Isles are referred to several regional datums, for example Ordnance Survey of Great Britain 1936 (OSGB 36) Datum. However, navigational positions obtained from satellite positioning systems, such as the global positioning system (GPS), which are now used for navigation, are normally referred to World Geodetic System 1984 (WGS 84) Datum. "
Admiralty charts to be transferred to WGS 84 datum, says U.K. Hydro Office | Sea Technology | Find Articles at BNET.com

So apparently Admiralty Chart Datum (ACD) is known to be offset from WGS84 and should not be expected to agree with GPS positioning. Did Navionics ever give you an answer?

[hellosailor]

Bob-
"Admiralty charts to be transferred to WGS 84 datum, says U.K. Hydro Office
Sea Technology, Mar 2000
E-mail Print Link The United Kingdom Hydrographic Office (UKHO) will start a three-year programme this month to transfer 310 Admiralty charts of Great Britain to a horizontal datum compatible with WGS 84 Datum.

The transfer of the remaining charts of the British Isles will follow. Currently, Admiralty charts of the British Isles are referred to several regional datums, for example Ordnance Survey of Great Britain 1936 (OSGB 36) Datum. However, navigational positions obtained from satellite positioning systems, such as the global positioning system (GPS), which are now used for navigation, are normally referred to World Geodetic System 1984 (WGS 84) Datum. "
Admiralty charts to be transferred to WGS 84 datum, says U.K. Hydro Office | Sea Technology | Find Articles at BNET.com

So apparently Admiralty Chart Datum (ACD) is known to be offset from WGS84 and should not be expected to agree with GPS positioning. Did Navionics ever give you an answer?

[Plumper]

Most plotters and GPS units allow the user to select the appropriate datum. Confirming that the datum in use and the datum on the electronic/paper chart are the same should be routine.